Particle-Associated Biogeochemical Processes in Fayetteville Green Lake, a High Sulfur, Permanently Anoxic Lake

In low O₂ environments, communities of autotrophs are organized along redox gradients by the vertical succession of available terminal electron acceptors used to derive energy and the tolerance of their carbon fixation enzymes to O₂. Their carbon fixation pathways fractionate carbon to differing extents while simultaneously fractionating their terminal electron acceptors. Within these redox zones, they are further partitioned into particle-associated (PA) and free-living (FL) assemblages. So, there should be a predictable relationship along redox gradients and among PA and FL particulate matter between δ¹³C_POC, stable isotope measurements relevant to the terminal electron acceptors (e.g., δ¹⁵N, δ³⁴S), genes that encode for carbon fixation enzymes and for those that mediate transformations of terminal electron acceptors.

There have been many projects that have studied the geochemistry of size-fractionated particulate matter and PA versus FL microbiology, but few have studied both in parallel. Here I present δ¹³C_POC, δ¹⁵N and the molecular biology data of particulate matter operationally defined as PA (> 2.7 μ)and FL (0.2 – 2.7 μ) from Fayetteville Green Lake, NY- a meromictic lake with similar concentrations of dissolved inorganic carbon and sulfate to marine anoxic basins.

PA δ¹³C_POC ranged from -27 to -43 with a minimum at 20.5m (upper monimolimnion). FL δ¹³C_POC reached a minimum of -41 at the lower redoxcline boundary, followed by a positive excursion to -34 in the upper monimolimnion. The greatest difference between PA and FL δ¹³C_POC occurred in the upper monimolimnion, where δ¹⁵N reached a minimum of -6, consistent with complete denitrification. These features coincided with the maximum concentration of RuBisCO-II, which was predominantly PA. This was unexpected, as carbon fixation via RuBisCO-II results in δ¹³C_POC between 9 and -13. The most abundant OTUs at these depths were Sulfuricurvum kujiense, a sulfur-oxidizing denitrifier, Desulfobulbaceaand purple sulfur bacteria (PSB). TheDesulfobulbaceaare known ectosymbionts of the PSB, and both are typically particle-associated. Sulfuricurvum kujiensefixes carbon through the reverse citric acid (rTCA) cycle and is common in free-living assemblages.