Elucidating the symbiotic interactions between a locally isolated microalga Chlorella vulgaris and its co-occurring bacterium Rhizobium sp. in synthetic municipal wastewater
Co-cultivation of microalgae and bacteria during municipal wastewater treatment can boost carbon and nutrient recovery as a result of their synergistic interactions. The symbiotic relationships between the locally isolated microalga Chlorella vulgaris and the bacterium Rhizobium sp., co-isolated from municipal wastewater, were investigated batchwise under photoautotrophic, heterotrophic, and mixotrophic conditions in a synthetic municipal wastewater medium. During photoautotrophic growth in BG11 medium, photosynthetic algal oxygenation and organic carbon production supported bacterial activity but no significant beneficial effects on microalgal growth were observed. In synthetic wastewater, a twofold higher biomass concentration was achieved in the axenic algal culture compared with the co-culture under heterotrophic conditions, suggesting a competition for nutrients. A comparable carbon removal was observed in all cultures (83–79% TOC), but a faster nitrogen consumption (59% TN) and complete phosphate assimilation (100% TP) was only achieved in the co-culture. A positive synergistic relationship was found under mixotrophic conditions, clearly supported by an in situ O2/CO2 exchange between the microorganisms. This mutualism led to a threefold higher biomass production with a 13-fold higher fatty acid content compared with the axenic algal culture, as well as a superior wastewater treatment performance (+ 58% TOC, + 41% TN and + 44% TP). The co-cultivation of C. vulgaris and Rhizobium is therefore suggested as a potential microbial consortium for a cost-efficient biomass generation during municipal wastewater reclamation, especially under mixotrophic conditions.
KeywordsChlorella vulgaris Rhizobium Symbiosis Mixotrophy Heterotrophy Wastewater treatment
The authors would like to thank Dr. Francesco Gentili at Swedish University of Agricultural Sciences (SLU) for isolating and providing the microalgal strain. The Vibrational Spectroscopy Core Facility at Umeå University is also acknowledged for the technical support in the FTIR analysis.
Lorenza Ferro, Raúl Muñoz, Christiane Funk, and Esther Posadas designed the experiments. Lorenza Ferro performed the experimental work, data collection, and analysis under the supervision of Esther Posadas and Raúl Muñoz. Michela Colombo analyzed the microalgae and bacteria cell concentrations. All authors were actively involved in the interpretation of the results and in the preparation of this manuscript. All authors approved the manuscript and this submission.
This research was funded by the Regional Government of Castilla y León and the FEDER Funding Program (CLU 2017-09), the Swedish Energy Agency (grant number 2013-006259, project: 476 38239-1), the Swedish Research Council FORMAS (grant numbers 942-2015-92 and 213-477 2014-1504), Umeå University, and the JC Kempe Memorial Scholarship Foundation. The COST Action ES1408 (European network for algal bioproducts-EUALGAE) is gratefully acknowledged for the STMS grant given to Lorenza Ferro. Michela Colombo was supported by Fondazione Italiana per la Ricerca sul Cancro (post-doctoral fellowship 18013).
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