Using microalgal communities for high CO2-tolerant strain selection
Graphical abstract
Introduction
Carbon dioxide (CO2) accounts for nearly 80% of the total greenhouse gas (GHG) emissions worldwide, and most members of the United Nations have committed themselves to significantly reduce their GHG emissions [1]. Exhaust gases from power plants attribute to ca. 40% of the U.S. annual CO2 emission in 2010, and the concentration of CO2 in power plant exhausts varies from 10 to 15% depending on the source of fuels [2]. Phototrophic algae fix CO2 and incorporate to biomass. Phototrophic carbon fixation through microalgae cultivation has been proposed as a biological way to mitigate CO2 pollution, especially for the sequestration of CO2 from industrial exhaust gases such as flue gases [[3], [4], [5], [6], [7]]. Therefore, the ideal microalgae candidates for sequestering CO2 in flue gases should be able to grow under high CO2 concentration (e.g. ≥10%).
Photosynthesis performed by microorganisms (including cyanobacteria and eukaryotic microalgae) is an ancient process [8,9]. Photosynthetic eukaryotes inhabited coastal waters ca. 1.4–1.9 billion years (Gyr) ago [[10], [11], [12]]. Green algae such as Chlorophytes (for example, Scenedesmus and Chlamydomonas), are thought to be the dominant phytoplankton in the Mesoproterozoic ocean (0.9–1.6 Gyr ago) and have become much less abundant in the Paleozoic period (0.25–0.54 Gyr ago) [13]. Kasting [14] suggested that the concentration of atmospheric CO2 decreased from 10% (v/v) at 2.5 Gyr ago to 1% (v/v) at 1 Gyr ago, and continued to decrease. The geological records on algal species and atmospheric CO2 level suggest that some Chlorophytes species thrived in the high CO2 environment during the Mesoproterozoic period, and the concentration of atmospheric CO2 has decreased gradually by CO2 fixation of Chlorophytes over the geological time. While atmospheric CO2 decreased, other types of microalgae that can use CO2 more efficiently emerged [15]. It is possible some current Chlorophytes species still retain the ability or strategy to grow under high CO2 concentrations. The key mechanisms governing the microalgal tolerance to high CO2 concentrations could involve the photosynthetic apparatus state transitions, rapid shutdown of CO2-concentrating mechanisms, or adjustment of fatty acid composition of membranes [[16], [17], [18], [19]].
Some microalgal isolates are able to grow when bubbled with high CO2 concentration [[20], [21], [22], [23], [24]]. One green algal strain, Chlorella sp., was able to grow under 100% CO2 and flue gas, although the maximum growth rate occurring at 10% CO2 concentration [22]. Another Chlorella strain was found to grow faster in 10% CO2 [25]. Hanagata et al. reported that the green alga Scenedesmus sp. could grow under 80% CO2 conditions, but the maximum cell mass was observed in 10–20% CO2 concentrations [19]. Another Scenedesmus strain was able to grow in a large photobioreactor (500 L) bubbled with flue gas that contains 10–12% CO2 [23]. Desmodesmus spp. could grow under 100% unfiltered flue gas from coal combustion [24]. Other non-green algae can also grow in high CO2 environment. For example, red algae like some Cyanidium species can grow in pure CO2 [26,27]. Growth of mixotrophic algae like Euglena gracilis was enhanced under elevated concentrations of CO2 (5–45%) [28]. Therefore, it is evident that different microalgae in diverse algal lineages are able to thrive in high CO2 conditions.
Early studies mainly relied on available algal cultures to test their capability to grow in high CO2 levels. The approach for screening and selecting CO2-tolerant strains includes bioprospecting (the search for specific microalgae species from local habitats which are CO2-tolerant) and acclimation of natural microalgae to high CO2 concentrations [29]. A high-throughput screening method has been used to discover algal monocultures that can grow in various concentrations of CO2 [23]. While the culture-based method has been widely used to select desirable algal strains for different purposes, the limitations of this method are multiple: 1) Only a limited number of algal cultures can be tested; 2) maintaining, growing and monitoring of many algal cultures is very time consuming; 3) selected algal strains may not be ideal for the local applications (i.e. use of local water); 4) microalgae grow poorly on the microplates compared to large bioreactors [30]. A recent study shows that a community-based method can be used to enrich and isolate CO2-tolerant microalgae [24], suggesting that exposing natural phytoplankton communities to the desirable test conditions enables expedited selection of target algal strains from a whole community of microalgae in a particular aquatic ecosystem.
Microalgae are very diverse in the natural environment. It has been estimated that more than one million algal species exist in nature, names for 44,000 of which have probably been published, and 33,248 names have been processed by AlgaeBase (http://www.algaebase.org) [31,32]. Many studies have contributed to better understand the impacts of abiotic or biotic factors to natural environment community shifting. With the use of molecular sequencing technology, the change of prokaryotic and eukaryotic communities can be monitored simultaneously [[33], [34], [35], [36]]. Co-monitoring the variation of different microbial communities in response to a specific change or event (i.e. algal bloom) has become a powerful tool to study the interaction between organisms in the natural environment [37]. It would be interesting to know how microbial communities in natural aquatic bodies respond to high CO2 exposure, nutrient enrichment, or both. By exposing the natural microalgal communities to high CO2 condition, we also want to know which kind of microalgal populations will emerge to dominate the community at the end of the experiment.
In this study, we exposed a water sample collected from the Back River, Baltimore to 10% CO2. Because the concentration of CO2 in power plant exhausts varies from 10 to 15% depending on the source of fuels. The 10% CO2 concentration was chosen in this study as we intended to isolate algal strains that are suitable for sequestering CO2 from the flue gas of power plant. The goal here is to understand how bacterial and microalgal communities change when the natural water is exposed to a high CO2 concentration. The bacterial and algal community will be analyzed by sequencing the partial 16S and 18S rRNA genes, respectively. Also, we identified and counted the cell density of major microalgae taxa. Ultimately, we isolated CO2-tolerant algal strains following the CO2 enrichment experiment.
Section snippets
Experimental design
The environmental water sample was taken from Back River, Baltimore, Maryland (latitude: 39.300°N, longitude: 76.489°W) on December 12, 2016, which was located close to the Baltimore Back River Wastewater Treatment Plant. Four treatments were set up to expose the environmental sample to 1) 10% CO2 with high nutrient (BG11 medium); 2) 10% CO2 without high nutrient; 3) air with high nutrient and 4) air only. All the CO2 gas was purchased from Airgas USA, LLC (CD USP50), and it was delivered
Physiological responses of high CO2 and nutrients
When the water samples were enriched with nutrients, their color turned dark green gradually from day 6 to day 17, while the samples that did not receive nutrients remained relatively clear throughout the 17 days' incubation period. The growth of phytoplankton and bacteria in the nutrient-enriched samples was evident based on the OD600 reading, phytoplankton and bacterioplankton counts (Fig. 1). On day 17, phytoplankton and bacteria cell densities in the nutrient-enriched samples were
Conclusion
Selection of optimal microalgal species is vital to sequester CO2 using a phototrophic carbon fixation system. Here we present a comprehensive study showing how eukaryotic and prokaryotic communities respond to nutrient enrichment and high concentration of CO2. We also demonstrate that desired algal strains can be obtained by exposing a natural community to the selected conditions. By exposing aquatic communities to high concentrations of CO2 and nutrients, green algae (Chlorophyta) became the
Acknowledgements
We acknowledge the funding supports (NoA#4701, NoA#5214, and NoA#5417) from the Maryland Industrial Partnership Program, the Maryland Department of Natural Resource and HY-TEK Bio, LLC. We also thank Robert Mroz and Paul Rosenberger for reviewing the entire manuscript. Hualong Wang wishes to thank the China Scholarship Council for a Visiting Student Scholarship.
Declaration of authors' contribution
Feng Chen and Hualong Wang designed the research. Hualong Wang and Fru Azinwi Nche-Fambo conducted the experiments. Feng Chen and Hualong Wang analyzed the data. Hualong Wang, Feng Chen and Zhigang Yu wrote the paper. All authors read and approved the final manuscript.
Declaration of conflict of interest
The authors declare that they have no conflict of interest.
Declaration of consent and animal use
No conflicts, informed consent, human or animal rights applicable.
Declaration of authorship and submission
The authors declare that they agree to authorship and submission of the manuscript for peer review.
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