N. E. Levin

Seasonal changes in light and physicochemical conditions have strong impacts on cyanobacteria, but how they affect community structure, metabolism, and biogeochemistry of cyanobacterial mats remains unclear. Light may be particularly influential for cyanobacterial mats exposed to sulphide by altering the balance of oxygenic photosynthesis and sulphide‐driven anoxygenic photosynthesis. We studied temporal shifts in irradiance, water chemistry, and community structure and function of microbial mats in the Middle Island Sinkhole (MIS), where anoxic and sulphate‐rich groundwater provides habitat for cyanobacteria that conduct both oxygenic and anoxygenic photosynthesis. Seasonal changes in light and groundwater chemistry were accompanied by shifts in bacterial community composition, with a succession of dominant cyanobacteria from Phormidium to Planktothrix , and an increase in diatoms, sulphur‐oxidizing bacteria, and sulphate‐reducing bacteria from summer to autumn. Differential abundance of cyanobacterial light‐harvesting proteins likely reflects a physiological response of cyanobacteria to light level. Beggiatoa sulphur oxidation proteins were more abundant in autumn. Correlated abundances of taxa through time suggest interactions between sulphur oxidizers and sulphate reducers, sulphate reducers and heterotrophs, and cyanobacteria and heterotrophs. These results support the conclusion that seasonal change, including light availability, has a strong influence on community composition and biogeochemical cycling of sulphur and O 2 in cyanobacterial mats.

Triple oxygen isotope ratios (∆′ 17 O) offer new opportunities to improve reconstructions of past climate by quantifying evaporation, relative humidity, and diagenesis in geologic archives. However, the utility of ∆′ 17 O in paleoclimate applications is hampered by a limited understanding of how precipitation ∆′ 17 O values vary across time and space. To improve applications of ∆′ 17 O, we present δ 18 O, d ‐excess, and ∆′ 17 O data from 26 precipitation sites in the western and central United States and three streams from the Willamette River Basin in western Oregon. In this data set, we find that precipitation ∆′ 17 O tracks evaporation but appears insensitive to many controls that govern variation in δ 18 O, including Rayleigh distillation, elevation, latitude, longitude, and local precipitation amount. Seasonality has a large effect on ∆′ 17 O variation in the data set and we observe higher seasonally amount‐weighted average precipitation ∆′ 17 O values in the winter (40 ± 15 per meg [± standard deviation]) than in the summer (18 ± 18 per meg). This seasonal precipitation ∆′ 17 O variability likely arises from a combination of sub‐cloud evaporation, atmospheric mixing, moisture recycling, sublimation, and/or relative humidity, but the data set is not well suited to quantitatively assess isotopic variability associated with each of these processes. The seasonal ∆′ 17 O pattern, which is absent in d ‐excess and opposite in sign from δ 18 O, appears in other data sets globally; it showcases the influence of seasonality on ∆′ 17 O values of precipitation and highlights the need for further systematic studies to understand variation in ∆′ 17 O values of precipitation.