Applying ‘-omics’ to phosphorus cycling: Comparative metatranscriptomics of microbial phosphate release from aquatic sediments

Background/Question/Methods

Polyphosphate (polyP) metabolism by benthic microbial communities represents a dynamic component in the global phosphorus (P) cycle. By synthesizing and later hydrolyzing these P reserves, polyphosphate-accumulating organisms can affect P availability and potentially alter the saturation state of sedimentary porewaters with respect to phosphate minerals. In particular, marine polyphosphate-accumulating bacteria such as the sulfur-oxidizing genera Thiomargarita and Beggiatoahave been implicated in phosphogenesis at modern upwelling zones and in the formation of ancient phosphorite deposits. However, phosphogenesis may involve more than large sulfur bacteria: sediment microbial communities are diverse, and likely contain numerous poly-P metabolizing taxa. Therefore, we used incubation experiments of sediment-hosted microbial communities followed by comparative metatranscriptomics to identify important polyphosphate metabolizing organisms in different aquatic environments and compare their activity under distinct environmental conditions.

Results/Conclusions

We collected and incubated sediments from three locations: (i) marine cold-seep sediments near Barbados that contain abundant sulfur-oxidizing bacteria, including Thiomargarita-like organisms; (ii) sulfidic marine sediments from Santa Barbara Basin (SBB) with no microscope-observable sulfur bacteria; and (iii) freshwater sediments from Spring Lake, MN, a eutrophic lake with seasonally anoxic bottom waters. In all incubations, P was consumed under oxygenated conditions and liberated under anoxic conditions, with and without sulfide and acetate additions. Incubation sediments were preserved for RNA extraction, and metatranscriptome libraries were generated from Barbados and SBB experiments. Preliminary results from Barbados indicate that sulfur oxidizing bacteria are likely important poly-P metabolizers at this site: only certain gamma- and epsilonproteobacteria were expressing polyP kinases and exopolyP-ases, even though rRNA and rpoB transcripts show that active microbial populations also included representatives of the Methanosarcinales, Bacteroidetes, and Deltaproteobacteria. Comparative analyses of lacustrine and marine sediments holds promise for gaining a better understanding of the microbial communities and processes responsible for biogeochemical P cycling in these comparable but distinctive regimes.