Contribution of allochthonous carbon to the microbial community in an acidic geothermal lake

Microorganisms can live in environments where macroscopic organisms cannot survive, such as in the extremely acidic geothermal Boiling Spring Lake (BSL) in Lassen Volcanic National Park, CA. BSL is an oligotrophic lake with thriving microbial populations whose carbon (C) sources are still poorly understood. In this study, I examined the possibility that allochthonous leaf litter (mostly pine needles) observed falling in the lake can fuel microbial production in BSL. Coniferous needles are known to be particularly difficult to colonize by microbes and have low decay rates, but no studies have examined the combined effects of low pH and high temperature on pine needle leaching and decomposition. In both lab- and field-based experiments, I found coniferous leaves had high decay rates during leaching and decomposition in BSL compared to those reported in the literature for other neutral pH and mesophilic lakes and streams. Thus, the acid and high temperatures appear to help condition the leaves and make their C more biologically available. I also found little difference in decay between fresh and dry leaves, but when needles were cut, I observed faster leaching rates and slower decomposition rates, suggesting the effects of mechanical weathering are important, possibly by degrading the waxy cuticle barrier. I also observed that leaves decayed more slowly in 0.2 μm-filtered BSL water, suggesting both native and introduced microbes contribute to leaf breakdown. Based upon bacterial growth experiments and recovery of an ascomycete fungus, both prokaryotes and eukaryotes seem to contribute to leaf decay. The addition of a nitrogen (N) source (yeast extract) enhanced prokaryote growth more than adding a simple carbon such as glucose, suggesting BSL is both C and nitrogen limited. I also used radioisotopes to assess primary and secondary production rates, and observed autotrophic growth was low, and dominated by chemoautotrophy, while heterotrophic uptake rates varied depending on the organic substrate used, and were highest for acetate. These results suggest that allochthonous carbon is important in fueling microbial production in the lake. I constructed a preliminary C budget for the lake by measuring leaf deposition and estimating other dissolved organic carbon (DOC) sources from BLS’s hydrologic budget.