(627a) Using Microbial Consortia to Increase the Productivity of Algal Open Ponds

Authors: 
Gomez, J. A., Massachusetts Institute of Technology
Höffner, K., Massachusetts Institute of Technology
Barton, P. I., Massachusetts Institute of Technology

In March 2015, the United States pledged to cut its carbon emissions 26-28% by 2025 [1]. To achieve this goal, the development of cheap biofuels is necessary. First generation biofuels are obtained from food crops. The realization that these biofuels are only slightly better than traditional fossil fuels regarding environmental impact combined with their competition for food resources prompted research on second-generation biofuels which are obtained from waste biomass [2]. The most promising technologies for second-generation biofuels production are based on microbial conversion of biomass into lipids. Preferred characteristics of the microorganisms are high specific growth rate, high lipids to biomass yield, high cell density, ability to use complex substrates, affinity to substrate, and low nutrient requirements [3]. These characteristics are found in some strains of microalgae.

Microalgae are attractive because some strains accumulate up to 50% dry weight in lipids [4]. Also, algae do not compete for food resources, they can be grown on wastewater and/or seawater, and they are up to one order of magnitude more efficient than higher-order terrestrial plants in capturing sunlight [4], [5]. Despite their huge promise in reducing greenhouse gas emissions, algal biofuels are not yet economically feasible as closed photobioreactors incur high capital and operating costs, and cheap open pond systems have low biomass and lipids productivities. Open ponds have low productivities because they are CO2 limited and they are susceptible to invasion by undesirable microorganisms. In this paper we introduce the idea of using microbial consortia to counteract these limitations.

Traditionally, CO2 limitations have been addressed by sparging flue gas [6], [7], and no real solution has been provided to invasion. Microbial consortia can address both limitations. First, microbial consortia provide resilience by utilizing resources more efficiently compared to an algal monoculture, making pond invasion by undesirable species more difficult [8]. Second, symbiotic relationships can be established between microalgae and other microbes in which the microbes convert carbon sources into CO2 and algae absorbs the CO2 into biomass. Therefore, algae ponds can become feasible in locations far away from flue gas sources.

The lack of predictive and reliable models of dynamic bioprocesses has halted the exploration of novel strategies to improve productivities, such as microbial consortia. Dynamic flux balance analysis (DFBA) provides a way of exploiting equipment process models and genome-scale metabolic networks to make predictions of novel system behavior [9], [10], [3]. Recently published simulators enable reliable and efficient simulation of DFBA systems [11], [12]. In this paper we use the High-Rate Algal Pond model [13], [14] and DFBA to model an algae/yeast coculture and explore its benefits. Results show that microbial consortia can make algae ponds economically feasible in locations far away from flue gas sources, but with available agricultural waste.

Keywords:Algae pond, microbial consortia, raceway pond, dynamic flux balance analysis, synthetic ecology.

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