Metabolite Cross-Feeding Drives the Symbiotic Growth of Chlorella vulgaris and Heterotrophic Microbes

Climate change due to rising atmospheric greenhouse gases is one of the most threatening problems facing humanity. Methods for capturing greenhouse gasses and producing biofuels have gained much attention. Microalgae and cyanobacteria can be used to capture carbon dioxide from point-source emitters and produce biofuels at high levels. Open bioreactors are needed to economically grow these photoautotrophs; however, heterotrophs often contaminate open bioreactors. The presence of heterotrophs can be advantageous in some cases, but frequently they are detrimental to the growth of photoautotrophs.

In this work, we show that Chlorella vulgaris UTEX #395 (a unicellular, non-motile, photosynthetic microalga) releases significant amounts of polysaccharides during photoautotrophic growth, termed “photosynthate”. Using C. vulgaris photosynthate, we isolated heterotrophic microorganisms from soil samples that can grow on the released sugars and used ¹³C-labeled photosynthate to confirm that soil microbes efficiently consumed it. In a co-culture with C. vulgaris the soil microbes significantly improved overall biomass production. Using pulse-chase tracer experiments in a co-culture of C. vulgaris and soil microbes, we demonstrated that soil microbes efficiently consumed photosynthate while C. vulgaris received carbon dioxide in return. The symbiotic exchange between the organisms where each partner benefited from the other was quantified using ¹³C-tracers. The increased understanding of photoautotroph-heterotroph interactions obtained in this study may help to improve the possibility of economically viable large scale biofuel production from waste carbon dioxide sources.