PNNL

Abstract

Microbial mats inhabiting extreme environments have been studied as modern analogs of stromatolites. Mats in Octopus Spring and Mushroom Spring, Yellowstone National Park are predominated by unicellular photoautotrophic cyanobacteria (Synechococcus spp.), which are thought to cross-feed filamentous photoheterothrophic bacteria (mainly Roseiflexus spp.), except under early morning anoxic conditions when Roseiflexus spp. have been shown to fix dissolved inorganic carbon (DIC). Transcription patterns, however, suggest that Roseiflexus spp. may perform photomixotrophy, in which DIC is incorporated together with organic compounds, during the daytime. We investigated the roles played by Synechococcus spp. and Roseiflexus spp. in DIC and organic matter uptake in mid-day light and oxic mats. Mass spectrometry was used to show that 13C-bicarbonate uptake under infrared (IR) light (utilized by anoxygenic phototrophs) or visible-minus-blue light (V-B) (used by cyanobacteria) was about two thirds and one-third, respectively of that incorporated in full light. Laser-ablation mass spectrometry analysis demonstrated that 13C incorporation under V-B light was restricted to the uppermost portion of the mat, whereas 13C incorporation under IR light was maximal in deeper mat layers. 13C-acetate, -propionate, -lactate, -and -glycolate were incorporated to an equal or greater extent under IR and full light. Incorporation of 13C into peptides showed that both Synechococcus spp. and Roseiflexus spp. were active in DIC uptake, whereas Roseiflexus spp. exhibited greater uptake of 13C-organic acids, especially glycolate and lactate into peptides. Peptides of proteins of the 3-hydroxypropionate pathway were labeled. Thus, Roseiflexus spp. appears to exhibit photomixotrophy throughout the day.