The role of microbial sulfate reduction on organic matter oxidation was studied quantitatively in temperate intertidal surface sediments of the German Wadden Sea (southern North Sea) on a seasonal base in the years 1998-2007. The sampling sites represent the range of sediments found in the back-barrier tidal area of Spiekeroog Island: sands, mixed and muddy flats. The correspondingly different contents in organic matter, metals, and porosities lead to significant differences in the activity of sulfatereducing bacteria with volumetric sulfate reduction rates (SRR) in the top 15 cm of up to 1.4 μmol cm −3 day −1 .Depth-integrated areal SRR ranged between 0.9 and 106 mmol m −2 day −1 , with the highest values found in the mudflat sediments and lower rates measured in sands at the same time, demonstrating the impact of both temperature and organic matter load. According to a modeling approach for a 154-km 2 large tidal area, about 39, 122, and 285 tons of sulfate are reduced per day, during winter, spring/ autumn, and summer, respectively. Hence, the importance of areal benthic organic matter mineralization by microbial sulfate reduction increases during spring/autumn and summer by factors of about 2 and 7, respectively, when compared to winter time. The combined results correspond to an estimated benthic organic carbon mineralization rate via sulfate reduction of 78 g C m −2 year −1 .

The Um Alhool area in Qatar is a dynamic evaporative ecosystem that receives seawater from below as it is surrounded by sand dunes. We investigated the chemical composition, the microbial activity and biodiversity of the four main layers (L1–L4) in the photosynthetic mats. Chlorophyll a (Chl a) concentration and distribution (measured by HPLC and hyperspectral imaging, respectively), the phycocyanin distribution (scanned with hyperspectral imaging), oxygenic photosynthesis (determined by microsensor), and the abundance of photosynthetic microorganisms (from 16S and 18S rRNA sequencing) decreased with depth in the euphotic layer (L1). Incident irradiance exponentially attenuated in the same zone reaching 1% at 1.7-mm depth. Proteobacteria dominated all layers of the mat (24%–42% of the identified bacteria). Anoxygenic photosynthetic bacteria (dominated by Chloroflexus) were most abundant in the third red layer of the mat (L3), evidenced by the spectral signature of Bacteriochlorophyll as well as by sequencing. The deep, black layer (L4) was dominated by sulfate reducing bacteria belonging to the Deltaproteobacteria, which were responsible for high sulfate reduction rates (measured using 35S tracer). Members of Halobacteria were the dominant Archaea in all layers of the mat (92%–97%), whereas Nematodes were the main Eukaryotes (up to 87%). Primary productivity rates of Um Alhool mat were similar to those of other hypersaline microbial mats. However, sulfate reduction rates were relatively low, indicating that oxygenic respiration contributes more to organic material degradation than sulfate reduction, because of bioturbation. Although Um Alhool hypersaline mat is a nutrient-limited ecosystem, it is interestingly dynamic and phylogenetically highly diverse. All its components work in a highly efficient and synchronized way to compensate for the lack of nutrient supply provided during regular inundation periods.

The effect of sulfate addition on the stability of, and microbial community behavior in, low-temperature anaerobic expanded granular sludge bed-based bioreactors was investigated at 15°C. Efficient bioreactor performance was observed, with chemical oxygen demand (COD) removal efficiencies of >90%, and a mean SO2−4 removal rate of 98.3%. In situ methanogensis appeared unaffected at a COD: SO2−4 influent ratio of 8:1, and subsequently of 3:1, and was impacted marginally only when the COD: SO2−4 ratio was 1:2. Specific methanogenic activity assays indicated a complex set of interactions between sulfate-reducing bacteria (SRB), methanogens and homoacetogenic bacteria. SO2−4 addition resulted in predominantly acetoclastic, rather than hydrogenotrophic, methanogenesis until >600 days of SO2−4-influenced bioreactor operation. Temporal microbial community development was monitored by denaturation gradient gel electrophoresis (DGGE) of 16S rRNA genes. Fluorescence in situ hybridizations (FISH), qPCR and microsensor analysis were combined to investigate the distribution of microbial groups, and particularly SRB and methanogens, along the structure of granular biofilms. qPCR data indicated that sulfidogenic genes were present in methanogenic and sulfidogenic biofilms, indicating the potential for sulfate reduction even in bioreactors not exposed to SO2−4. Although the architecture of methanogenic and sulfidogenic granules was similar, indicating the presence of SRB even in methanogenic systems, FISH with rRNA targets found that the SRB were more abundant in the sulfidogenic biofilms. Methanosaeta species were the predominant, keystone members of the archaeal community, with the complete absence of the Methanosarcina species in the experimental bioreactor by trial conclusion. Microsensor data suggested the ordered distribution of sulfate reduction and sulfide accumulation, even in methanogenic granules.

The mineralization of organic matter in marine sediments by microbial activity was studied in Umm Alhool sabkha. In intertidal surface sediments, the development of steep compositional and physico-chemical gradients was a common phenomenon. Rapidly, oxygen is consumed within the upper few mm of marine mats and sediments. In permeable sediments, however, oxygenated bottom waters may have flew through the upper part of the surface sediments leading to enhanced participation of oxygen in element cycling. Whereas in microbial mats, the surface sediments are locally formed, indicating a disturbance in the balance of the biogeochemical processes. Umm Alhool sabkha, situated between Umm Sa'id (Mesaieed) and Al Wakrah, drew our attention to study the biogeochemical cycling because both microbial mats and mangroves ecosystems affect its biogeochemistry. In the present study, the chemistry of pore water below mats surfaces of intertidal sandy sediments was investigated in winter 2011 using a number of different techniques. Pore water was sampled down to 20 cm below surface using pore water lances, diffusion samplers, and centrifugation of sediment core sections. Microsensor measurements of sulfide and pH were also performed on the upper 2 cm. Specifically, we measured salinity, dissolved O2, pH, SO4²¯, H2S, Cl¯, TN, TOC, PO4³¯, NO3¯, NH4+, H4SiO4, and microbial sulfate reduction rates have been analyzed using intact sediment cores. Sulfidic sediments were characterized by high sulfate reduction rates exhibiting maxima between about 5-15 cm, associated with decreased oxygen penetration depths, and proton activities. Anaerobic metabolic activity in pore waters below mat surface lead to significantly enhanced concentrations of sulfide, ammonium, dissolved inorganic carbon, phosphate, silica (steep gradients), and a net consumption of sulfate. They acted as windows for the liberation of reduced substances into the bottom water or the atmosphere. This study represents the first comprehensive investigation of the chemical composition and sulfate reduction rates in Umm Alhool microbial mat ecosystem. It shows how dynamic and self-fueling the system is.