Emerging prospects of mixotrophic microalgae: Way forward to sustainable bioprocess for environmental remediation and cost-effective biofuels

Highlights

• Mixotrophy opens new avenue for effective treatment of organic waste by microalgae.

• Mixotrophic algae cultivation runs two pathways thus effective for bioremediation.

• Organic wastes treatment combines CCU technology by mixotrophic algae cultivation.

• Mixotrophic cultivation offers better energy recovery and carbon footprint.

• Mixotrophy also need design improvement for better exploitations and economics.

Abstract

Algal bioremediation becoming most fascinating to produce biomass as biofuels feedstock while remediating wastes, also improving carbon-footprint through carbon capturing and utilization (CCU) technology. Non-algae process however offers effective treatment but metabolic CO₂ emission is major drawback towards sustainable bioprocess. Mixotrophic cultivation strategy (MCS) enables to treat organic and inorganic wastes which broadly extend microalgae application towards cleaner and sustainable bioeconomy. Latest focus of global think-tanks to encourage bioprocess holding promise of sustainability via CCU ability as important trait. Several high CO₂ emitting industries forced to improve their carbon-footprints. MCS driven microalgae treatment could be best solution for those industries. This review covers recent updates on MCS applications for waste-to-value (biofuels) and environment remediation. Moreover, recommendations to fill knowledge gaps, and commercial algal biofuel could be cost-effectiveness and sustainable technology for biocircular economy if fuelled by waste streams from other industries.

Introduction

Worldwide annual generation of liquid, gaseous, and solid wastes has significantly been increased, thus posing a serious challenge for proper waste management due to their potential impacts on environment. The transformation of these waste streams into valuable bioproducts and biofuels, via biological process has been considered as the most suitable and sustainable route. If the adopted bioprocess emitting greenhouse gases (GHGs) itself that would certainly dilute the impact of remediation process and carbon footprint credit. In this context, linking the bioremediation of waste streams with microalgae platform would offer solutions to rising environmental pollution and energy demand. Moreover, other benefits which may not be achieved by non-algae bioprocesses such as biomass feedstock for various biofuels, better carbon footprint, and a renewable and sustainable bioprocess for circular bioeconomy.

Here algal platform does not limit to photoautotrophic cultivation only. Under which algae able to utilize inorganic carbon mainly enriched in gaseous phase (mainly as CO₂) and unable to consume organic carbon which are abundantly available in liquid and solid waste streams. Thus, new cultivation strategy and microalgae potential to grow in mixotrophic mode is highly important to exploit them effectively for organic waste remediation. Moreover, considering the CCU technology, algae platform is most promising among others especially for high CO₂ mitigation rate due to its higher productivity than other terrestrial plants (Chisti, 2007, Choi et al., 2019). Microalgae described as a promising third-generation renewable and sustainable source of bioenergy. It can offer immense biomass yield approx. 7–20 times more than soy or corn per land unit and bio-oil 23 and 115 times greater than palm oil and canola in l. ha⁻¹ respectively (Chisti, 2007, Bharathiraja et al., 2015). The conventional crops have lesser environmental impacts than the algae in terms of energy use, GHGs emissions, and water regardless of the cultivation site (Clarens et al., 2010). Bioremediation and biofuel production from waste resources by microalgae platform is mainly important to utilize abundantly available solar energy which also implies in MCS.

Microalgae are unicellular photosynthetic eukaryotes, they require energy and carbon sources to support their growth and accumulate lipid as biofuel precursor. For energy and carbon sources, they are mainly dependent on light and inorganic carbon during photoautotrophic growth whereas growing on mixotrophic mode, both above needs can be obtained solely from organic source. However light and inorganic carbons are additional energy and carbon sources respectively for their growth (Perez-Garcia and Bashan, 2015, Zhan et al., 2017). The microalgae-based biofuel production process mainly comprising upstream and downstream processes, in which upstream only includes microalgal cultivation. Whereas downstream process includes harvesting and/or dewatering processes to obtain a concentrated biomass and biofuel refinement by which extraction of lipids or other value-added products can be produced from biomass (Zhan et al., 2017).

Mixotrophic microalgae able to metabolize organic and inorganic carbon concurrently and provide reasonably wider and promising applications in environmental remediation by treating flue gases and organic wastewaters together. Compared to the expensive heterotrophic cultivation (due to high capital and operating costs) as well as lower biomass yield of autotrophic cultivation (subject to high extraction cost), the MCS could be an ideal cultivation method for microalgae to cope up these challenges (Perez-Garcia and Bashan, 2015, Zhan et al., 2017). Since mixotrophy comprises both heterotrophic and autotrophic parts thus able to grow constantly under limitation of either source (light or organic/inorganic carbon) to bring the process economically viable for large-scale production. Moreover, it also provides a new insight of environmental and economical sustainability via benefits of environmental bioremediation, carbon credits and biofuel production.

The most important point for establishment of any viable bioprocess is economy. The economics of microalgae process lies on its cultivation, harvesting/dewatering, drying, cell disruption, extraction and final product purification. These expenses can be overcome by higher productivity. To attain adequate productivity and lipid content at the industrial scale, adequate and cost-effective feedstock’s, nutrients, water and photobioreactor are essential. All of these added into the cost of microalgal biodiesel production and can be overcome with adequate lipid yield. Hence the combination of wastewater and flue gas treatment could bring microalgae based biodiesel production platform as more viable and sustainable technology. Though the MCS has a great promise for economic microalgae biofuel production, also there are numerous challenges to overcome for achieving its industrial scale advancement. For example, utilizing wastewater as feed also has few drawbacks: (a) some wastewater could be highly toxic and unable to support microalgal growth without pretreatment (b) efficiency of microalgal growth associated wastewater treatment may greatly affect by seasonal variation; and (c) competition with other fast-growing microbes in the wastewater may cause slow growth of algae. Hence, exploiting microalgae to treat wastewater may be problematic for high biomass productivity when carried out without strategic treatment plan (Perez-Garcia and Bashan, 2015).

Current review aims to compile the recent developments carried out during last decades on mixotrophic cultivation of microalgae for effective waste remediation and cost-effective bioprocess development for lipid production utilizing industrial wastes, including wastewater; waste or CO₂-enriched gas (flue gas and biogas); and other waste organics. Review also includes the primary common operational challenges, limiting factors and disadvantage associated with MCS and their possible mitigation strategies in order to meet the commercial potential to establish this technology a future most promising and breakthrough technology in both aspects: bioremediation and biofuel production in a sustainable way.