Micron-scale mapping of sulfur cycling across the oxycline of a cyanobacterial mat: a paired nanoSIMS and CARD-FISH approach

Abstract

The metabolic activities of microbial mats have likely regulated biogeochemical cycling over most of Earth's history. However, the relationship between metabolic activity and the establishment of isotopic geochemical gradients in these mats remains poorly constrained. Here we present a parallel microgeochemical and microbiological study of micron-scale sulfur cycling within hypersaline microbial mats from Guerrero Negro, Baja California Sur, Mexico. Dissolved sulfide within the mats was captured on silver discs and analyzed for its abundance and d 34 S isotopic composition using high-resolution secondary ion mass spectrometry (nanoSIMS). These results were compared to sulfide and oxygen microelectrode profiles. Two-dimensional microgeochemical mapping revealed well-defined laminations in sulfide concentration (on scales from 1 to 200 lm), trending toward increased sulfide concentrations at depth. Sulfide d 34 S decreased from B þ 10% to À20% in the uppermost 3 mm and oscillated repeatedly between À10% and À30% down to a depth of 8 mm. These variations are attributed to spatially variable bacterial sulfate reduction within the mat. A parallel examination of the spatial distribution of known sulfate-reducing bacteria within the family Desulfobacteraceae was conducted using catalyzed reporter deposition fluorescence in situ hybridization. Significant concentrations of Desulfobacteraceae were observed in both oxic and anoxic zones of the mat and occurred in several distinct layers, in large aggregates and heterogeneously dispersed as single cells throughout. The spatial distribution of these microorganisms is consistent with the variation in sulfide concentration and isotopic composition we observed. The parallel application of the methodologies developed here can shed light on micronscale sulfur cycling within microbially dominated sedimentary environments.

Sulfur isotopes of organic matter preserved in 3.45-billion-year-old stromatolites reveal microbial metabolism

“…The exact cause of this fine-scale isotopic variability is less clear. Previous ion probe studies targeting pyrite observed microscale isotopic variability and interpreted this to indicate microbial processes (15,27,32). It is known that these metabolic processes-and consequently their isotopic fractionations-vary significantly in space and time and as a function of mass flux within the system (i.e., open versus closed system) (14,27).…”

Diversity and Stratification of Archaea in a Hypersaline Microbial Mat

“…Studies of archaea in the pond 4 mat have usually been related to methanogenesis, which is generally considered a minor metabolic theme of the mat (3,47). Consequently, the aims of this study were to describe the archaeal diversity in the pond 4 mat and to determine how that diversity is distributed with respect to depth in the mat and previously measured chemical gradients (16,28,29). The results uncover substantial unrecognized archaeal diversity and very likely indicate the occurrence of so-far-undetected chemical gradients in the mat.…”

Spatially resolved capture of hydrogen sulfide from the water column and sedimentary pore waters for abundance and stable isotopic analysis

“…To minimize photochemical production of Ag (s) , the films were mounted ahead of time in the dark and covered during transit to the field and prior to deployment. Metallic silver still reacts with aqueous sulfide (e.g., Fike et al 2008Fike et al , 2009, although the kinetics are likely to be different than that observed during the laboratory experiments described here. As can be seen in Figure 5a, the upper boundary of the chemocline is fairly abrupt, spanning a distance of at most a few cm.…”

“…Previous work in the microbial mats of Guerrero Negro documented spatially coherent isotopic variations (~20‰) extending from the chemocline more than 1 cm into the mat. These isotopic variations were attributed to reflect corresponding variations in microbial sulfate reduction (Fike et al, 2008;Fike et al, 2009), likely due to variable cell-specific sulfate reduction rates (Sim et al, 2011) resulting from the differential availability of electron donors. However, the sulfide capture method previously employed (on 1" metallic silver discs) precluded a robust estimate of variable sulfide concentrations or documenting the spatial extent of these patterns.…”