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The majority of anaerobic biogeochemical cycling occurs within marine sediments. To understand these processes, quantifying the distribution of active cells and gross metabolic activity is essential. We present an isotope model rooted in thermodynamics to draw quantitative links between cell-specific sulfate reduction rates and active sedimentary cell abundances. This model is calibrated using data from a series of continuous culture experiments with two strains of sulfate reducing bacteria (freshwater bacterium Desulfovibrio vulgaris strain Hildenborough, and marine bacterium Desulfovibrio alaskensis strain G-20) grown on lactate across a range of metabolic rates and ambient sulfate concentrations. We use a combination of experimental sulfate oxygen isotope data and nonlinear regression fitting tools to solve for unknown kinetic, step-specific oxygen isotope effects. This approach enables identification of key isotopic reactions within the metabolic pathway, and defines a new, calibrated framework for understanding oxygen isotope variability in sulfate. This approach is then combined with porewater sulfate/sulfide concentration data and diagenetic modeling to reproduce measured 18 O/ 16 O in porewater sulfate. From here, we infer cell-specific sulfate reduction rates and predict abundance of active cells of sulfate reducing bacteria, the result of which is consistent with direct biological measurements.

46Algal blooms in lakes are often associated with anthropogenic eutrophication; however, they can 47 occur without the human introduction of nutrients to a lake. A rare bloom of the 48 alga Picocystis strain ML occurred in Spring of 2016 at Mono Lake, a hyperalkaline lake in 49California, which was also at the apex of a multi-year long drought. These conditions presented a 50 unique sampling opportunity to investigate microbiological dynamics and potential metabolic 51 function during an intense natural algal bloom. We conducted a comprehensive molecular 52 analysis along a depth transect near the center of the lake from surface to 25 m depth in June, 53 2016. Across sampled depths, rRNA gene sequencing revealed that Picocystis associated 54 chloroplast were found at 40-50 % relative abundance, greater than values recorded previously. 55Despite high relative abundances of the photosynthetic oxygenic algal genus Picocystis, oxygen 56 declined below detectible limits below 15 m depth, corresponding with an increase in 57 microorganisms known to be anaerobic. In contrast to previously sampled years, both 58 metagenomic and metatranscriptomic data suggested a depletion of anaerobic sulfate-reducing 59 microorganisms throughout the lake's water column. Transcripts associated with Photosystem I 60 and II were expressed at both 2 m and 25 m, suggesting that limited oxygen production could 61 occur at extremely low light levels at depth within the lake. Blooms of Picocystis appear to 62 correspond with a loss of microbial activity such as sulfate reduction within Mono Lake, yet 63 microorganisms may survive within the sediment to repopulate the lake water column as the 64 bloom subsides. Mono Lake, California provides habitat to a unique ecological community that is heavily stressed 68 due to recent human water diversions and a period of extended drought. To date, no baseline 69 information exists from Mono Lake to understand how the microbial community responds to 70 human-influenced drought, algal bloom, or what metabolisms are lost in the water column as a 71 consequence of such environmental pressures. While previously identified anaerobic members of 72 the microbial community disappear from the water column during drought and bloom, sediment 73 samples suggest these microorganisms survive at lake bottom or in the subsurface. Thus, the 74 sediments may represent a type of 'seed bank' that could restore the microbial community as a 75 bloom subsides. Our work sheds light on the potential photosynthetic activity of the halotolerant 76 alga Picocystis strain ML and how the function and activity of the remainder of the microbial 77 community responds during a bloom at Mono Lake. 78 79