Stable isotope labeling confirms mixotrophic nature of streamer biofilm communities at alkaline hot springs
Streamer biofilm communities (SBC) are often observed within chemosynthetic zones of Yellowstone hot spring outflow channels, where temperatures exceed those conducive to photosynthesis. Nearest the hydrothermal source (75–88°C) SBC comprise thermophilic Archaea and Bacteria, often mixed communities including Desulfurococcales and uncultured Crenarchaeota, as well as Aquificae and Thermus, each carrying diagnostic membrane lipid biomarkers. We tested the hypothesis that SBC can alternate their metabolism between autotrophy and heterotrophy depending on substrate availability. Feeding experiments were performed at two alkaline hot springs in Yellowstone National Park: Octopus Spring and “Bison Pool,” using various [superscript 13]C-labeled substrates (bicarbonate, formate, acetate, and glucose) to determine the relative uptake of these different carbon sources. Highest [superscript 13]C uptake, at both sites, was from acetate into almost all bacterial fatty acids, particularly into methyl-branched C[subscript 15], C[subscript 17] and C[subscript 19] fatty acids that are diagnostic for Thermus/Meiothermus, and some Firmicutes as well as into universally common C[subscript 16:0] and C[subscript 18:0] fatty acids. [superscript 13]C-glucose showed a similar, but a 10–30 times lower uptake across most fatty acids. [superscript 13]C-bicarbonate uptake, signifying the presence of autotrophic communities was only significant at “Bison Pool” and was observed predominantly in non-specific saturated C[subscript 16], C[subscript 18], C[subscript 20], and C[subscript 22] fatty acids. Incorporation of [superscript 13]C-formate occurred only at very low rates at “Bison Pool” and was almost undetectable at Octopus Spring, suggesting that formate is not an important carbon source for SBC. [superscript 13]C-uptake into archaeal lipids occurred predominantly with [superscript 13]C-acetate, suggesting also that archaeal communities at both springs have primarily heterotrophic carbon assimilation pathways. We hypothesize that these communities are energy-limited and predominantly nurtured by input of exogenous organic material, with only a small fraction being sustained by autotrophic growth.