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Abstract:
Sulphide-driven anoxygenic photosynthesis is an ancient microbial
metabolism that contributes significantly to inorganic carbon fixation
in stratified, sulphidic water bodies. Methods commonly applied to
quantify inorganic carbon fixation by anoxygenic phototrophs, however,
cannot resolve the contributions of distinct microbial populations to
the overall process. We implemented a straightforward workflow,
consisting of radioisotope labelling and flow cytometric cell sorting
based on the distinct autofluorescence of bacterial photopigments, to
discriminate and quantify contributions of co-occurring anoxygenic
phototrophic populations to in situ inorganic carbon fixation in
environmental samples. This allowed us to assign 89.3% +/- 7.6% of
daytime inorganic carbon fixation by anoxygenic phototrophs in Lake
Rogoznica (Croatia) to an abundant chemocline-dwelling population of
green sulphur bacteria (dominated by Chlorobium phaeobacteroides),
whereas the co-occurring purple sulphur bacteria (Halochromatium sp.)
contributed only 1.8% +/- 1.4%. Furthermore, we obtained two metagenome
assembled genomes of green sulphur bacteria and one of a purple sulphur
bacterium which provides the first genomic insights into the genus
Halochromatium, confirming its high metabolic flexibility and
physiological potential for mixo- and heterotrophic growth.
