Phycocapture of CO 2 as an option to reduce greenhouse gases in cities: Carbon sinks in urban spaces

Climate change is a shift in the average weather patterns, which could stand for a long-term period. This phenomenon is related to greenhouse gas emissions generated by anthropogenic and non-anthropogenic activities. The most notable climate change effects are the rise of sea levels, changes in the water pH, apparition or increased transmission of diseases, changes in the water cycle, loss of marine ecosystems, and several negative impacts on human health. Due to the adverse effects occasioned by climate change, global initiatives have been taken to mitigate its impact, one of these is the reduction of greenhouse gases such as CO 2 . Some microorganisms such as photosynthetic bacteria and microalgae can capture CO 2 and use it as a carbon source for growth. The outstanding CO 2 bio-capture or CO 2 phycocapture capacity shown by microalgae make them excellent candidates for reduction of atmospheric CO 2 in cities. CO 2 phyco-capture equivalent CO 2 emissions in Mexico City Metropolitan Area (MCMA) was determined as a case study, considering greenhouse gas emissions in this city. It was estimated that 94,847 tons of microalgae biomass must be produced daily to equal the amount of CO 2 emissions (170,726 CO 2 -eq per day), thus obtaining a zero balance of emissions. For the above, CO 2 phyco-capture implementation can be possible in cities and also in open spaces and that even its production can work as the carbon credits nowadays implemented, the space required, and the high capture rate led us to consider that the microalgae production on a larger scale may have a faster effect on the concentration of CO 2 globally, which can help with greater urgency to the aims established by 2030.


Introduction
Climate change is the variation in the usual weather pattern in some regions; this atypical variation can be determined by analyzing of the historic climate reports, which show changes in the average temperature over an extended time-lapse (decades or longer).This phenomenon, known as global warming, is mainly related to anthropogenic emissions of greenhouse gases (CO 2 , CH 4 , and N 2 O), producing an increase of 0.85 • C in the global temperature from 1880 to 2012 [1].Besides disruption on biotic and abiotic systems, this temperature variation impacts socioeconomic development globally [2,3].The anthropogenic activities that produce the most greenhouse gases emission per decade (94,340 thousand metric tons of CO 2 ) are industrial activities, deforestation, urbanization, burning of fossil fuels, and cement manufacture [4].
CO 2 emission from fossil fuels combustion is the major contributor in global warming, 65 % of global greenhouse gas emissions (74 % of total U.S. anthropogenic CO 2 emissions).The countries that contribute the most to CO 2 global emissions from fossil fuel combustion are China, U.S., European Union and India.This is related specially to energy production (25 % of 2010 global greenhouse gas emissions); petroleum burning is the most used source for energy production (2057 million metric tons of carbon dioxide in 2019 produced by U.S) and to electricity production the most used source is coal (782 million metric tons of carbon dioxide in 2019 produced by U.S) [5,6].
Rising global temperatures may expose the population to severe heat waves.Regarding healthy problems triggered by climate change, infection risk is one of the most concerning.For instance, a 0.5 • C rise in temperature can increase 2 times the dengue infection rate and produce an elongation of dengue's season.These temperature changes can also trigger some other dangerous diseases such as malaria and Chagas disease [7][8][9][10].Also, effects on fetal health produced by climate change's effects are well documented [11,12].For instance, temperatures below to -4 • C and superior than 30 • C are associated with a reduction in birth weight [13].Environmentally, global climate change produces several risks such as an increase of wildfires occurrence, erosion rate, the vulnerability of mangrove areas, mass extinction of species, an increase of vulnerability of species, species migration to different ecosystems, and invasion of alien species in natural protected areas [14][15][16].Furthermore, some other significant effects are the sea-level rise (4.77 mm/year at Uruguayan coast), changes in water pH by the absorption of CO 2 in the ocean, and salinization of groundwater by a salt-water intrusion [1,17,18].
Different studies have shown climate change effects in the hydrologic cycle specifically in rainfall events, and also in infiltration and evapotranspiration rates [19,20].Due to these, food production (crop yields) has been reduced, producing food security problems [21][22][23].For example, some grains' production as wheat can be reduced by 32 % due to the rainfall trend changes [24].Another food production sector affected by climate change is fishing, due to the loss of marine ecosystems and the biodiversity of marine species [25].In addition to the food sector, there are other economic sectors that may be affected for climate change such as tourism.The tourism sector has been affected by the loss of natural areas as flora and fauna of attractive places, increased sea levels in coastal areas and degradation of ecotourism attractions [26].Due to all the global concerns that produce climate change, this phenomenon has been taking importance in different global strategies and programs such as the Kyoto Protocol, the Paris Agreement, and recently, the United Nations' Sustainable Development Goals [27,28].These actions seek to reduce the climate change, reduce the concentration of greenhouse gases and reduce anthropogenic emissions to these gases into the environment [29,30].
Among the strategies to reduce greenhouse gases emissions from anthropogenic activities are found renewable fuels use and sustainable energy development such as hydrogen fuel use [31].Nuclear power and geothermal energy have been adopted to dissociate atmospheric CO 2 to hydrocarbon fuels, these CO 2 conversion methods including aqueous electrolysis, thermochemical cycles and electrolysis via molten salts where the formation of CH 4 , C 2 H 4 , C 4 H 8 , and C 7 H 16 take place [32,33].The use of biomass to produce energy also is a way to reduce environmental impact from anthropogenic activities because it is used renewable materials for its production.The efficiency of biomass can be enhanced subjecting biomass to pre-treatments methods before its use such as dewatering, drying, torrefaction, densification, size reduction and pelletization [34].Also, CO 2 capture by chemical processes is a good strategy to reduce greenhouse gases in the atmosphere.For instance, zeolite 13 can efficiently capture CO 2 (95-99 % purity), an approach useful in cement plants [35,36].Moreover, biological strategies have been followed to CO 2 capture.Generally, these approaches are known as bio-capture or bio-sequestration techniques [37].Fig. 1 shows the principal bio-capture approaches that have generated interest in academia and among policymakers.
In the present review, CO 2 bio-capture are explored with special insight on phyco-capture and their metabolic mechanisms for CO 2 bio- capture.Furthermore, strategies to enhance their bio-capture potential and possible technologies applied in urban zones are also discussed.Finally, a phyco-capture's case of study in the Mexico City metropolitan area is presented.

Bio-capture of CO 2 by natural ecosystems
There are some ecosystems that can capture CO 2 .Mangrove forests bio-capture processes can annually capture 654 tons of C per km 2 [38].Other coastal ecosystems that are excellent CO 2 sinks are seagrass meadows (e.g., Cymodocea nodosa) and salt marshes, commonly known as blue C [39,40].Antarctic coastal ecosystems and the Gulf of Alaska coastal ocean are considered essentials CO 2 sinks, especially during austral summer, storing from 14 to 34 Tg C/yr [41,42].Also, some studies have shown that the soil and ground biomass in the semi-arid mulga lands of eastern Australia has a CO 2 fixation potential rate of 1.1 t CO 2 -e/ha/year [43].However, due to the loss of some ecosystems, there has been a decrease in the world's C storage, which has caused the world carbon balance to be more negative, encouraging the acceleration in the increase in world temperature.For example, in the Tiantong National Forest Park (China), the forestland decreases its C storage at a rate of 4.38 % annually [44].It is essential to highlight that these ecosystems' carbon storage is affected by stand age [45], climate, and stand age [46].
Some human activities such as using smart farming techniques, constructing wetlands (well operated), and using some microorganisms in specialized devices can reduce atmospheric CO 2 concentration [47][48][49].For instance, some crops can capture CO 2 and can be used for other purposes; the sugarcane can sink 1.4657 Kg C/Kg of vegetable raw material [50].Constructed wetlands are used as CO 2 sinks.In these constructed ecosystems, its carbon density of vegetation is 1255 g C/m 2 , and soil carbon is 2669 g C/m 2 [51].The restoration of ecosystems is also a useful option to increase carbon storage and its conservation, among these are found both marine and terrestrial ecosystems.A restored mangrove forest has C stock of 42 Mg/ha [52].The salt marsh ecosystem restoration enhances C capture in these ecosystems, coming to capture 213 g C/m 2 /yr [53].Afforestation initiatives are helpful for carbon sequestration.The program Grain for Green Program in Henan Province enhances low yield sloped cropland, barren hills, and wasteland into forest and grasslands.Due to its activities, the total carbon sequestration rate is 2.47 Tg C/yr, and the total storage in 2012 was 51.73 Tg C [54].These initiatives promote carbon sequestration, however, they are not enough to equalize carbon emissions in the world.
The international program Reducing Emissions from Deforestation and Forest Degradation and conservation, Sustainable Management of Forests and Enhance of Forest Carbon Stocks (REDD+) enhance sustainable forest management (agroforestry and landscape approaches)in developing countries with actions like conservation and enhancement of forest carbon stocks [55].Forest ecosystem management can enhance carbon storage by clearcutting and optimizing trees' rotation age, especially in forest plantations [56].Additionally, these ecosystems' growth positively effects the economy because of the high global demand for harvested wood products.In this way, forest products' trend demand increases the harvest by 17 % in the EU (from 518 Mm 3 in 2015 to 605 Mm 3 in 2030), whence, implementing forest carbon sinks as forest plantation is a good option to obtain an economic income [57].It is essential to highlight that, although there is a large carbon capture by forest, the harvested wood products manufacture emits CO 2 and methane by combustion [58].It is important to establish that these systems are limited by the geographic spaces available to establish the initiatives as well as the capture capacity of the biological systems.However, biological carbon capture produces considerable benefits that chemical capture cannot achieve such as the production of organic matter to obtain subproducts and in the case of carbon capture in natural ecosystems, the protection and care of natural areas is crucial for wildlife and endangered species.

Possible technologies for CO 2 bio-capture in urban spaces
In cities, the urban trees can be used as an option for carbon capture, but they can be limited because they increase and lack space in cities.For example, in Singapore, its tropical urban trees' potential to balance anthropogenic emissions of CO 2 is limited and insufficient [59].The high CO 2 concentration in air and other pollutants can also enhance stomatal closure and decrease city trees' density, such as Ligustrum lucidum in Mexico city [60].Due to the high population rate in cities, the space for the establishment of green areas is limited, therefore, other technologies need to be implemented in order to develop cities with a zero-carbon balance.
Alternatively to vegetal cultures, microorganisms' growth in bioreactors to carbon capture has brought attention [61].For instance, the bacteria Halomonas stevensii can remove 100 % of CO 2 (15 % of CO 2 v/v); in the process, the pH in the experiment changed from 10 to 6.05 in 24 h with a production of fatty alcohols (dodecanol, tetradecanol and pentadecanol) [62].Microalgae's bioreactors have a great CO 2 mitigating potential.Bio-capture or phyco-capture of CO 2 (phyco-capture is the use of algae to capture into its biomass CO 2 and other contaminants present in the air, soil, and water, this term is related to phyco-remediation, meaning that is defined as the use of algae to treat wastewaters) [63].
Microalgae represent a superior option for carbon fixation than terrestrial plants for higher growth and faster biomass production, doubling their biomass in less than 24 h for most species [64,65].Microalgae use carbon dioxide for energy conversion while producing approximately half of the atmospheric oxygen [66].It is estimated that these microorganisms can capture a maximum of 2.35 GtCO 2 in 100, 000 km 2 culture area [67].It is suggested that microalgae carbon fixation could be a significant portion of the remaining unidentified carbon sink, which shows the potential of using these organisms to CO 2 capture [68].The advantages and disadvantages of the application of these CO bio-capture methods are described in Table 1.

CO 2 phyco-capture mechanism
Through photosynthesis (mechanism used by most forms of carbon capture), microalgae can fixate CO 2 by what is known as phyco-capture process.In this process, inorganic compounds and light energy are converted into organic matter by photoautotrophs [69].Microalgae use light-dependent and light-independent reactions to produce energy (Fig. 2).The first reaction occurring in the thylakoid membranes, and the second reaction in the stroma, both inside the chloroplasts.Through non-cyclic photophosphorylation, electrons are transferred from water through photosystems I and II (light-dependent reactions), where complexes of proteins and pigments harvest light and perform the reactions to obtain NADPH and ATP.The captured energy is then used in the Calvin cycle (light-independent reactions) to produce carbohydrates, such as glucose (C 6 H 12 O 6 ), by using CO 2 , and therefore, performing carbon fixation [69][70][71].Some microalgae strains can tolerate high CO   For example, Scenedesmus sp. has tolerated high concentrations of CO 2 ; it was reached 2.75 g of biomass per liter at 70 % CO 2 , which is a high growth even in optimal conditions (0.03 % CO 2 ) [73].Also, each microalga has a carbon content (Table 2) that can guide to determine the potentiality of microalgae to fix CO 2 .Broadly, it has been determined that per 1 g of microalgae biomass production, 1.8 g CO 2 is needed [78,79].Among the microalgae acclimatization processes to high CO 2 conditions is found the increase of synthesis and accumulation of lipids, and also the increase of pigments synthesis especially Chl a and polysaccharide production [80,81].The use of microalgae for CO 2 biocapture implies other advantages, such as the production of lipids, carbohydrates, proteins, and some other nutrients found in microalgal biomass growth [66,[82][83][84][85].These high-value molecules find a transcendental use producing biofuels and food/feed products [86].

CO 2 phyco-capture potential
Kassim and Meng (2017) showed the CO 2 bio-capture capacity of Chlorella sp. and T. suecica (CO 2 uptake rate is 95 and 104 mg/L/day by Chlorella sp. and T. suecica, respectively).Also, it was determined that these microalgae strains could endure high CO 2 concentrations (0.04 %-15 %), being the optimal 5 % for Chlorella sp. and 15 % for T. suecica.Also, studies have analyzed the cell growth of different microalgae under an increase of CO 2 content (up to 50 %) in the air injected.The results showed that cell growth was given at a rate 3-5 times faster than cultures using normal air [88].Another study showed that Chlorella sp.TISTR 8263, marine Chlorella sp. and C. protothecoides TISTR 8243.Scenedesmus obliquus and Chlorella pyrenoidosa grew on media with CO 2 concentrations of up to 50 %.This study also found that S. obliquus removed a maximum of 0.288 g L − 1 day − 1 , and C. pirenoidosa a total of 0.260 g L − 1 day − 1 in the presence of 10 % CO 2 [89].
Also, Grasiella sp. has been found to capture 0.26 g L − 1 per day or 18.9 g m -2 day − 1 at conditions of pH 8.0-9.0 (Wang et al., 2018).The microalgae Arthrospira platensis (Spirulina platensis) have revealed a linear relationship between cell growth rates and CO 2 removal from biogas with an efficiency of carbon fixation for biomass production of almost 95 % [91].Euglena gracilis can remove 64.8 % of CO 2 at an initial CO 2 concentration of 10 %, pH of 3.5, and temperature of 27 • C [92].Chlorella sp. has been shown to remove high CO 2 levels when grown at different CO 2 concentrations.After analyzing different studies, the relationship between the CO 2 concentration in the medium and the CO 2 removal rate seems to be inversely proportional.For instance, for CO 2 concentration of 2% the removal rate was 58 %, for 5% was 27 %, for 10 % was 20 %, and for 15 % was 16 % [93].This trend is also seen for Spirulina sp., achieving a CO 2 capture of 53.29 % and 45.61 % when is grown in 6% CO 2 and 12 % CO 2 , respectively [94].Nannochloropsis oculate grown also at different CO 2 concentrations of 2%, 5%, 10 %, and 15 % achieved a CO 2 removal of 47 %, 20 %, 15 %, and 15 % respectively [95].Scenedesmus obliquus removed 28.8 % of CO 2 in a culture media with 6% CO 2 , and 13.56 % in 12 % CO 2 [94].Other examples of CO 2 phyco-capture are expressed in Table 2.
The microalgae strains Dunaliella tertiolecta, Chlorella vulgaris, Thalassiosira weissflogii, and Isochrysis galbana can grow and capture CO 2 from emissions of the cement industry.However, in this process, dust concentrations of cement gases may inhibit microalgae growth.Therefore, it must be removed before being injected into microalgae culture [96].Some of the CO 2 phycocapture studies were done with wastewater as a culture media, making a complete phycoremediation process [97][98][99].Also, Chlorella sorokiniana UKM2 strain was grown in palm oil mill effluent (10 % v/v) aerated with 1% (v/v) CO 2 , and it was determined a CO 2 uptake rate of 567 mg/L/day, obtaining a microalgae growth of 1 g/L on day 5 of culture.Also, 100 %, 65 %, and 56 % of NH 4 + , total nitrogen (TN), and total phosphate (TP) were removed, respectively [100].Similarly, treated chemical wastewater (purified terephthalic acid wastewater) can be used as culture media, obtaining an algal CO 2 capture rate of 91.59 % with a 10 % CO 2 supply [101].
Kitchen wastewater and sewage wastewater have also been treated with microalgae.For example, Chlorella reached biomass productivity of 0.6 g/L using sewage wastewater as a culture medium with a flue gas supply (6% CO 2 , SOx180 ppm, and NOx250 ppm) [102].Also, there have also been studies where the removal of emerging pollutants in wastewater has been demonstrated by microalgae [103].For instance, Cheng et al. (2018) found that Chlorella PY-ZU1 removes 95 % of ethinylestradiol (Initial concentration: 5 mg/L) from wastewater with a 15 % CO 2 supply.Also, antibiotics such as amoxicillin and cefradine in wastewater can be removed by microalgae with 10-30 % CO 2 supply [105].Microalgae also are used in bioremediation of wastewater with heavy metals (As, B, Cu, Mn, Zn, among others).However, this process, particularly the CO 2 concentration supply, can affect heavy metals removal [106].
The process of phyco-capture of CO 2 in wastewater is an example of a circular economy by obtaining by-products from waste.For example, Scenedesmus sp.culture in domestic wastewater supplemented with 2.5 % CO 2 produced 196 mg of biomass/L/day with lipid content of 33 % and achieved a CO 2 consumption rate capture of 368 mg/L/d.Among the biomass's lipid composition, FAME (saturated and unsaturated fatty acids) were found and determined to be potentially applicable for biodiesel production [107][108][109].Another critical highlight from these studies is the importance of maintaining the N:P ratio in 10:1 to generate high amounts of microalgae biomass and lipids [110].Also, biofuel production can be pursued since many microalgae increase their lipid accumulation when the carbon to nitrogen ratio is increased [91].For example, high CO 2 levels (30− 50%) resulted in the best carbon dioxide concentration for the production of total lipids and polyunsaturated fatty acids in Scenedesmus obliquus SJTU-3 and Chlorella pyrenoidosa SJTU-2 [89].Chlamydomonas sp.JSC4 cultivated with 8% CO 2 supply may have a lipid composition of 46 % of C16:O, 21 % of C18:0, and 13 % of C18:1 [111].
Chlorella vulgaris growth in treated wastewater with 15 % CO 2 supply reached lipid productivity of 0.164 g lipids/g cell/day with 44 % of oleic acid content [112].In addition to lipids' production, some pigments can be extracted from microalgae biomass obtained from CO 2 phycocapture process, for example, Thermosynechococcus sp.CL-1 can produce 0.074 mg/L/h and 0.39 mg/L/h of zeaxanthin and β-carotene, respectively [113].Also, from CO 2 phycocapture process, it can be obtained an income of $440-1028/ton of microalgae biomass produced [114].Some studies, where besides CO 2 capture by microalgae may be removing the heavy metals such as Al and Fe presented in flue gas [115].Furthermore, using microalgae biomass for heavy H 2 S, SO 2 , and NO removal from coal combustion has been applied [116][117][118].Demonstrating that microalgae for CO 2 bio-capture can also provide cleaner air to the atmosphere without heavy metals [115].

Strategies to enhance CO 2 phyco-capture potential
There are some strategies to ensure good results in the CO 2 phycocapture process, such as using a tolerant CO 2 strain, implement a combined chemical-biological CO 2 capture process such as using polyethylene glycol 200, diethanolamine, pentoses (xylose and arabinose), adding CO 2 absorbents such as potassium carbonate to promote microalgae growth, modifying the growing temperature, superficial gas velocity, gas flow rate, and CO 2 inlet concentration [88,[119][120][121][122][123][124][125][126].Highlighting the use of NaHCO 3 , which improves CO 2 fixation efficiency in a range of 82%-99% [127].It is essential to consider the light intensity because the light penetration in the bioreactor will be reduced with the microalgae growth process, decreasing microalgae cell growth [128,129].Moreover, the literature recommends maintaining the light intensity in 4500 lx (1200-1600 μmol m − 1 s − 1 ).These values must also be evaluated for each microalga [88,130].For instance, Chlorella PY-ZU1, by increasing light intensity from 4500 to 6000 lx, increased its growth and CO 2 fixation efficiency [131].
Furthermore, control the aeration's bubble size is necessary to increase biomass production and CO 2 dissolution [132].Also, the use of some instruments to increase mass-transfer CO 2 to culture medium can be useful.For example, the use of a single helical baffle in microalgae culture increases 23 % algal biomass; also this promoted homogeneity distribution of nutrients because of the spiral flow generated [133].The use of micro-bubbles aerator can also enhance CO 2 mass-transfer to the culture medium.Excellent results have been seen using a round gas distributor with holes with an inner diameter of 0.5 mm and spacing of 1.5 mm [134].
It is possible to explore some nanotechnology tendencies applied to CO 2 bio-capture (Fig. 3).Nanoparticles used as additive in microalgae cultivation can improve the CO 2 bio-capture and make more efficient photoconversion.The most limiting factor to apply nanoparticle additives is related to the microalgae toxicity which must be evaluated for each specific set of microalgae and nanoparticles.Toxicity factors include the nanoparticle characteristics such as size, material, oxidation state and crystal structure, the concentration, media interaction and microalgae strain.Other studies were carried out using different strains and nanoparticles presented elsewhere [135].Here it is discussed the successful cases the combination of Chlamydomonas reinhardti with Cr 2 O 3 nanoparticles at 100 mg/L [136].Other with the same strain and CuO at three concentrations 0.1, 1 and 10 mg/L [137], similarly the use of AgNP (30− 50 nm) between the concentrations of 0.01 to 0.15 mg/L [138].This includes the nanoparticle type and material used, additionally to the nanostructure applied.In general, the nanoparticles can positive or negatively affect the microalgae culture depending on the concentration and degree of freedom for their direct interaction.A successful example is the use of nanofibers containing nanoparticles with CO 2 capture potential (e.g., nanofibers with NPsFe 2 O 3 that has a CO 2 adsorption capacity of 164.2 mg/g), also the use of these nanofibers can increase lipid production, helping by this way the production of by-products of interest [139,140].
It is important to know that the CO 2 supply in microalgae cultures affects pH; this reduction affects the nutrient uptake and, consequently, the microalgae growth.Also, the pH reduction can allow the contamination of the microalgae culture with bacteria and fungi.Consequently, monitoring pH is essential in the CO 2 bio-capture process by microalgae [87].It is important to highlight that the highest conversion of CO 2 into carbon biomass was observed at pH 7 [141].Regarding industrial flue gas use, this can be directly introduced into the microalgal cultures without any pretreatment, producing a pH decreases in the culture medium, an increase in the temperature, and dissolution of nitrogen oxides and sulfur oxides present.Although extremely low pH and acidic gases have fatal effects on higher plants, some microalgae survive these conditions and capture CO 2 [142].The acidification of culture medium by the effect of the CO 2 supplied may have a chain reaction on the microalgae physiology.For example, CO 2 acidification on Tetraselmis sp. may induce a lower conversion of light to chemical energy because it affects the microalgae photosynthetic metabolism [143].A successful alternative is to capture and purify the industrial flue gas to be administered to microalgal cultures at desired concentrations [64], making tolerable conditions for most microalgae.

Possible technologies for phyco-capture of CO 2 in urban spaces
Phyco-capture of CO 2 is an option to improve air quality in urban spaces and reduce greenhouse gas concentration in the atmosphere, creating a CO 2 sink type in areas where it produces a large percentage of these gases in the world by anthropogenic activities.This proposal is based on microalgae's potential to improve air quality by capture CO and other air contaminants.Also, the main objective is creating carbon neutrality between CO 2 emissions from everyday activities in a city and the CO 2 phyco-capture process.As we mentioned in the previous section, microalgae have great potential in greenhouse gas capture especially in CO 2 capture.However, critical infrastructure is needed to achieve this goal.Among these needs, a properly bioreactor design to ensure the most CO 2 phyco-capture in the smallest possible space where the bioreactors are spotted.Regarding the possible microalgae strain to use, it is recommended to use one with high CO 2 tolerance and CO capture capacity, as shown in Table 4.
On the other hand, the bioreactor design has several issues that must be considered, such as the space it occupies, the volume of culture, and the ease of operation.Currently, there are some studies where microalgae are grown on large scale.In some cases, these projects aim has been CO 2 phyco-capture; however, this potential is not studied in depth.Among the bioreactors types for microalgae culture are found open pounds, airlift bioreactors, manifolds reactors, and serpentine bioreactors.The microalgae cultivation can be done, depending on whether or not they are directly exposed to the atmosphere, in open pond bioreactor and closed bioreactor.In open-pond systems, the most outstanding characteristics include simple construction, low cost, and easy operation.However, some disadvantages are high evaporation loss, low light utilization by the cells, difficulties for operation control, high risk of contamination, low biomass density and low CO 2 sequestration rate.The open systems include ponds, lagoons, deep channels, shallow circulating units, and others [144,145].
Raceway open pounds are bioreactors that can be used for microalgae cultivation.They commonly are systems that generate low microalgae biomass, but they have low capital and operating costs for large-scale cultures.These systems are found in many dimensions depending on the space available for construction.When is needed to supply CO 2 in these systems is done by pipeline gas system ensured gas delivery in aeration zones sometimes equipped with a gas diffuser.Most of the wastewater treatment with microalgae is made in these systems, is possible that the supply CO 2 enhances microalgal biomass production [146,147].High-rate ponds (HRPs) also are used for microalgae cultivation.For instance, a pilot-scale with HRPs for wastewater treatment by microalgae was implemented.The HRPs were made of fiberglass with six-blade steel paddles; they had the following characteristics: width of 1.28 m, length of 2.86 m, total depth of 0.5 m, the surface area of 3.3 m 2 , and sound volume of 1 m 3 .The cultures were supplied with 99 % and 5% CO 2 .The CO 2 supply was carried out in the lower part of the HRPs, through a carbonation column as a gas diffuser, and allowed a more significant contact time of the gaseous CO 2 with the culture.These systems was monitored pH, temperature, dissolved oxygen, photosynthetically active radiation, and Escherichia coli presence [148].Open raceway ponds are usually used for microalgae biomass production.For instance, Spirulina platensis has been cultivated in this type of reactor with a capacity of 14,000 L for biomass, proteins, and pigments production (phycocyanin).It was demonstrated that CO 2 supply provides a carbon source and can be used to control culture pH [149].
Other options to use as microalgae bioreactor closed systems have many advantages, including easy control, insufficient space required, high CO 2 sequestration rate, and no contamination risk, nearby all microalgae species may be cultivated, and high biomass density.Nevertheless, closed systems' major limitations are high investment cost, high operation cost, and scalability difficulty.The commonly used closed photobioreactors include tubular, flat panel, cylindrical airlift photobioreactors, stirred fermenters, and bag reactors [144,150].Another option to use as a microalgae bioreactor is to use of conical transparent polyethylene bags with bubble columns with a capacity of 25 L, a microalga consortium was growing in this system for heavy metals capture from coal-fired flue gas with 5.5 % of CO 2 .It was found that B, Mn, and Zn were found in the microalgae cells during the cultivation period, concluding that microalgae may offer a solution for the heavy metal pollution in the air [115].In this system also it was determined that high CO 2 concentration supply (>5% CO 2 ) showed lower saturated fatty acids unsaturated fatty acids present, which can reduce the viability of using this microalgae biomass for biodiesel production [151].
Cylindrical photobioreactors act as an airlift bioreactor also can be used for CO 2 phyco-capture purposes, where the bottom of the bio-rector can supply CO 2 .For example, for the cultivation of Chlorella vulgaris MSU-AGM 14, it was used cylindrical photobioreactors of 30 L with a 4% and 8% CO 2 supply, it was determined that CO 2 supply promoted biomass and lipid productivity [152].In the use of this type of bioreactors, it has been shown that it is essential to control superficial gas velocity, as the CO 2 removal efficiency capacity can be increased at a gas velocity of 1.88 × 10 − 3 m/s, and also can enhance biomass productivity [120].It was analyzed that airlift bioreactor design required a more significant amount of operating energy than raceway pond.In the case of the first it is needed 3.21 MJ/Kg of biomass and in the raceway pond is needed 1.15 MJ/Kg of biomass, which means that twice as much energy is used in airlift bioreactor, however in this, a more outstanding biomass amount was produced, and therefore it is expected a greater CO 2 phyco-capture capacity [153].
Horizontal tubular photobioreactors can also be used for CO 2 phycocapture; remarkably, these systems consist of thin tubular placed horizontally to allow a greater incidence of light in the culture.These systems are similar manifolds reactors and serpentine bioreactors.In this kind of system, the use of CO 2 microbubbles dissolvers at the gas inlet to enhance the CO 2 dissolved and biomass production in the culture medium, reducing bubble formation diameter [154].Other types of bioreactors used for microalgae cultivation are the laminar photobioreactors or microalgae biofilm photobioreactors.They consist of a vertical chamber covered on both sides with fabric (polypropylene geotextile) sheets of 1.5 × 2 × 0.10 m size, a water recirculation system, and a mixing tank.This type of bioreactors is based on some microalgae strains' capacity to grow as biofilm; species such as Chlorella spp.and Scenedesmus spp.were used in these systems.CO 2 can also be supplied to these cultures, it can be supplied inside the chamber, and in this bioreactor, the moistened fabric prevents CO 2 leakage from the system.Among these systems' advantages are the constant exposure of the microalgae to incident radiation and the high biomass production [155].
Microalgae biofilm photobioreactors can also be used to Scenedesmus obliquus culture; the roughness of substratum surface for biofilm adhesion was used and improved in 28 % biomass density, showing the importance of substrate adhesion capacity in these systems.With these properties, 65 % CO 2 removal efficiency can be achieved with a gas inlet of 10 % CO 2 [156].
It is recommended that bioreactors must have a monitoring system for some variables such as light, pH, and dissolved CO 2 .As it was mentioned in the previous section, pH is a variable essential in CO 2 phyco-capture process.For example, a microalga consortium (Chlamydomonas sp., Nannochloris sp. and Chlorella stigmatophora) grown in a high-rate algal pond of 4.46 m 3 (surface area of 10 m 2 ) reaches the highest conversion of CO 2 into microalga biomass at pH 7 in that case.Especially in open bioreactors, is needed to consider the possible presence of microalgae depredator in the culture such as protozoans and  metazoans species.This presence can affect microalgae growth and, in some cases meaning the total loss of culture [141].Recently, studies have developed the processes and technologies to apply microalgae on an industrial scale.However, the transition from pilot-scale operations to industrial-scale operations often exposes microalgae cells to harsh circumstances, resulting in reduced product production.Thus, several innovative technological solutions are still needed to exceed their satisfactory performance to achieve commercial viability.In this context, photobioreactors' use to perform microalgae-based processes prompted the development of different reactor configurations to achieve scale characteristics proportional to industrial demands [157].Table 3 shows some examples of CO 2 phyco-capture on a significant scale.

Case of study: CO 2 phyco-capture potential in Mexico city Metropolitan Area
Regarding the microalgae potential to CO 2 capture, it was determined the CO 2 phyco-capture potential in Mexico City Metropolitan Area (MCMA) as a case of study to show the most significant opportunity that have the implementation of phyco-capture processes in cities, under the premise of capturing the same amount of emissions (CO 2 -eq) that a city generates.MCMA is integrated by 3 states of Mexico: Mexico City (CDMX), Estado de Mexico (59 municipalities) and Hidalgo (Tizayuca municipality) (Fig. 4).MCMA is the most populated geographic zone of Mexico, with 21.4 million people.It was determined by Secretaría del Medio Ambiente de la Ciudad de México (SEDEMA) the total emissionrelated with greenhouse gases of MCMA, taking into account different anthropogenic activities as mobility, economic activities, among others, and it was considered punctual and non-punctual emissions.It was determined that the total emission of CO 2 is of 52,439,503 Tons/year, and the total emission of CO 2 -eq is of 62,315,082 Tons/year in MCMA (Fig. 5).These emissions are mostly because of its vehicle fleet made up of 5,742,152 vehicles, which provide 32,323,570 Tons/Year/CO 2 -eq, representing at least 50 % of total emissions in this geographical area [158].Other significant sources of greenhouse emissions are landfills use, industrial and home combustion processes, generation, transmission and distribution of energy and generation of goods and services.The CO 2 phyco-capture necessary to equal the equivalent CO 2 emissions in MCMA was determined, considering greenhouse gas emissions.It is estimated that 94,847 tons of microalgae biomass need to be produced daily to equal the amount of CO 2 emissions (170,726 CO 2 -eq per day), thus obtaining a zero balance of emissions cultivation volume of 0.22 km 3 (Fig. 6).
Mexico has a microalgae culture potential of 526,672 km 2 (26.8 % of the country), considering land characteristics and resources (land use, topographic slope, temperature, evaporation, solar radiation, vegetation, water, and CO2 sources.Microalgae cultivation areas in different locations of Mexico are as follows: 21,243 ha in Mexico City, 563,521 in Estado de México, and 567,830 in Hidalgo, the spaces determined by this model and other spaces that can be used increased the possibility to use this CO 2 capture method [159].One of the biggest concerns to produce biomass is the land use for this purpose.Thus, one of the ways to reduce land use is the construction of reactors that allow producing more biomass in less space, such as optimized open pounds, airlift bioreactors (vertical column photobioreactors), sequential bioreactors, and manifold bioreactors [131,144,160,161].These reactors can be implemented in avenues that have a space for green areas, in green areas previously established, such as parks, some known as the city's lungs, in under-exploited spaces, and wastewater treatment plants [148].Its installation in future infrastructure projects and land use plans can also be considered (Fig. 7).Also, microalgae strain with high carbon content and accelerated microalga growth can enhance CO 2 phyco-capture in less space, improve the transfer of CO 2 to the culture medium by regulating flows, and reducing the size of bubbles and optimize the light intensity according to the microalga strain used.One of the strategies that can be implemented is to increase CO 2 inlet concentration by used

Cost-effective aspects
It was estimated that per m 3 of culture installed in a CO 2 phycocapture plant, it needed an investment of 3350 $USD (817 $USD per tCO 2 -eq/year: Considering 5 years of operation) [165].In the case of the use of mangrove blue for CO 2 capture is needed 9 $USD per tCO 2 -eq/year [166], however, the areas of application are limited and are not enough to capture all the CO 2 generated in the world.In the case of implementation of plantations of timber species the costs oscillate between 10 $USD to 68 $USD tCO 2 -eq and 25 $USD tCO 2 -eq in reforestation implementation [167], although the implementation costs are low, the space required and CO 2 capture rate makes this a difficult option to apply it, more considering the projected global climate change scenarios that propose to look for different greenhouses gases capture methods.These methods have to be more effective and faster, for the little time estimated that exist to change this situation without affecting the current lifestyle of the human being.
For phyco-capture process, the most effective option to reduce production cost is to use the land adjacent to wastewater treatment plants, where this waste can generate biomass as a culture medium.Besides, wastewater can reduce the amount of clean water needed for microalgae production [86].Also, it can be used strains with more CO 2 capture potential, obtaining more CO 2 capture in less space and reduce the input costs.Also, biomass production can be used to produce subproducts such as pigments production, bioplastics, biofertilizers, biodiesel, and animal nutritional supplements, generating a circular economy process and an economic income [86].

Perspectives and challenges
Climate change has on world agendas, and its consequences make it a priority issue for their attention.The search for methods to capture greenhouse gases is of vital importance to the environment and human lifestyle.The bio-capture of CO 2 by microalgae is an excellent candidate to be implemented in the world.On large scales, these systems can function like the current carbon credits implemented in forest zones, where the systems capture the CO 2 equivalent to the emissions of human activities not precisely carried out in the broadcast area.This way of capture can guarantee a greater capture efficiency than that carried out with higher plants, where this system can capture more than 15 times what is captured by an average tree.Also, because these systems must be implemented in bioreactors, these can be established in different areas.In cities, CO 2 phycocapture process can be implemented as bioreactor parks, using land dedicated to green areas, neighboring lands to wastewater treatment plants or be included in new land utilization plans.Although carbon credits implemented in forest spaces are so useful, the space required and the low capture rate led us to consider that the growth of microalgae on a larger scale may have a faster impact on the concentration of CO 2 globally, which can help with greater urgency to the aims established by 2030.Also, the CO 2 captured by microalgae could be considered for inclusion in carbon markets.

Fig. 5 .
Fig. 5.Total emission related with greenhouse gases of Mexico City Metropolitan Area by zone.Data source: [158,168].

Fig. 6 .
Fig. 6.Phyco-capture needed in Mexico City Metropolitan Area considering total emission related with greenhouse gases by zone.It was considered that to produce 1 kg of microalgae biomass it is needed 1.8 kg of CO 2 .Data source: [158,168].

Fig. 7 .
Fig. 7. Possible ways to implement bioreactors for CO 2 phyco-capture process in cities. a) Implementation of bioreactors in avenues with green areas, b) Implementation of bioreactors in green areas such as parks and boardwalks, c) Implementation of bioreactors in WWTP.

Table 1
Comparative evaluation CO 2 bio-capturing attributes of different methods.
f kg/day.