The Guaymas Basin (Gulf of California) is a hydrothermal vent site where thermal alteration of deposited planktonic and terrestrial organic matter forms petroliferous material which supports diverse sulfate-reducing bacteria. We explored the phylogenetic and functional diversity of the sulfate-reducing bacteria by characterizing PCR-amplified dissimilatory sulfite reductase (dsrAB) and 16S rRNA genes from the upper 4 cm of the Guaymas sediment. The dsrAB sequences revealed that there was a major clade closely related to the acetateoxidizing delta-proteobacterial genus Desulfobacter and a clade of novel, deeply branching dsr sequences related to environmental dsr sequences from marine sediments in Aarhus Bay and Kysing Fjord (Denmark). Other dsr clones were affiliated with gram-positive thermophilic sulfate reducers (genus Desulfotomaculum) and the delta-proteobacterial species Desulforhabdus amnigena and Thermodesulforhabdus norvegica. Phylogenetic analysis of 16S rRNAs from the same environmental samples resulted in identification of four clones affiliated with Desulfobacterium niacini, a member of the acetate-oxidizing, nutritionally versatile genus Desulfobacterium, and one clone related to Desulfobacula toluolica and Desulfotignum balticum. Other bacterial 16S rRNA bacterial phylotypes were represented by non-sulfate reducers and uncultured lineages with unknown physiology, like OP9, OP8, as well as a group with no clear affiliation. In summary, analyses of both 16S rRNA and dsrAB clone libraries resulted in identification of members of the Desulfobacteriales in the Guaymas sediments. In addition, the dsrAB sequencing approach revealed a novel group of sulfate-reducing prokaryotes that could not be identified by 16S rRNA sequencing.

Abstract. The UV-screening pigment scytonemin is found in many species of ensheathed cyanobacteria. Past work has shown that the pigment is synthesized in response to exposure to UV-A irradiance. This study investigated the effect of other correlated stress factors including heat, osmotic and oxidative stress on the synthesis of scytonemin in a clonal cyanobacterial isolate ( Chroococcidiopsis sp.) from an epilithic desert crust. Stress experiments were carried out both in conjunction with UV-A irradiance and in isolation. Increases in both temperature and photooxidative conditions in conjunction with UV-A caused a synergistic increase in the rate of scytonemin production. In contrast, increased salt concentration under UV-A irradiance inhibited scytonemin synthesis. However, unlike the responses to temperature and oxidative stress, cells synthesized low levels of scytonemin under osmotic stress in the absence of scytonemin-inducing irradiance. These results suggest that scytonemin induction may be regulated as a part of a complex stress response pathway in which multiple environmental signals affect its synthesis.

The goal of this study was to use environmental sequencing of 16S rRNA and bop genes to compare the diversity of planktonic bacteria and archaea across ponds with increasing salinity in the Exportadora de Sal (ESSA) evaporative saltern in Guerrero Negro, Baja CA S., Mexico. We hypothesized that diverse communities of heterotrophic bacteria and archaea would be found in the ESSA ponds, but that bacterial diversity would decrease relative to archaea at the highest salinities. Archaeal 16S rRNA diversity was higher in Ponds 11 and 12 (370 and 380 g l−1 total salts, respectively) compared to Pond 9 (180 g l−1 total salts). Both Pond 11 and 12 communities had high representation (47 and 45% of clones, respectively) by Haloquadratum walsbyi-like (99% similarity) lineages. The archaeal community in Pond 9 was dominated (79%) by a single uncultured phylotype with 99% similarity to sequences recovered from the Sfax saltern in Tunisia. This pattern was mirrored in bop gene diversity with greater numbers of highly supported phylotypes including many Haloquadratum-like sequences from the two highest salinity ponds. In Pond 9, most bop sequences, were not closely related to sequences in databases. Bacterial 16S rRNA diversity was higher than archaeal in both Pond 9 and Pond 12 samples, but not Pond 11, where a non-Salinibacter lineage within the Bacteroidetes >98% similar to environmental clones recovered from Lake Tuz in Turkey and a saltern in Chula Vista, CA was most abundant (69% of community). This OTU was also the most abundant in Pond 12, but only represented 14% of clones in the more diverse pond. The most abundant OTU in Pond 9 (33% of community) was 99% similar to an uncultured gammaproteobacterial clone from the Salton Sea. Results suggest that the communities of saltern bacteria and archaea vary even in ponds with similar salinity and further investigation into the ecology of diverse, uncultured halophile communities is warranted.

During the Precambrian, ultraviolet (UV) radiation reaching the Earth's surface, including UVC wavelengths (190-280 nm), was considerably higher than present because of the lack of absorbing gases (e.g. O 2 and O 3 ) in the atmosphere. High UV flux would have been damaging to photosynthetic organisms exposed to solar radiation. Nevertheless, fossil evidence indicates that cyanobacteria-like ancestors may have evolved as early as 3.5 ؋ 10 9 yr ago, and were common in shallow marine habitats by 2.5 ؋ 10 9 years ago. Scytonemin, a cyanobacterial extracellular sheath pigment, strongly absorbs UVC radiation. Exposure to high-irradiance conditions caused cells to synthesize scytonemin and resulted in decreased UVC inhibition of photosynthetic carbon uptake. It was further demonstrated that scytonemin alone was sufficient for substantial protection against UVC damage. This represents the first experimental demonstration of biological protection against UVC radiation in cyanobacteria. These results suggest that scytonemin may have evolved during the Precambrian and allowed colonization of exposed, shallow-water and terrestrial habitats by cyanobacteria or their oxygenic ancestors.

The importance of sulfate respiration in the microbial mat found in the low-sulfate thermal outflow of Mushroom Spring in Yellowstone National Park was evaluated using a combination of molecular, microelectrode, and radiotracer studies. Despite very low sulfate concentrations, this mat community was shown to sustain a highly active sulfur cycle. The highest rates of sulfate respiration were measured close to the surface of the mat late in the day when photosynthetic oxygen production ceased and were associated with a Thermodesulfovibrio-like population. Reduced activity at greater depths was correlated with novel populations of sulfate-reducing microorganisms, unrelated to characterized species, and most likely due to both sulfate and carbon limitation.Cyanobacterial microbial mats are layered communities in which the combination of microbial activity and environmental gradients contributes to a fine vertical stratification of microbial populations and activities. They are self-sustaining communities that support all major biogeochemical cycles and are thought to be analogous to some of the earliest communities on Earth (9, 11). These attributes have made microbial mats attractive models for better understanding both the evolution and the ecology of microbial systems supporting the processes that now sustain our biosphere.Modern mat communities occur primarily in environments in which temperature or salinity suppresses grazing by higher eukaryotes (22,38,45). The mats that develop in low-sulfate, moderately thermophilic (50 to 72°C), moderately alkaline (pH ϳ8), siliceous hot spring effluents in Yellowstone National Park (YNP) are among the best characterized, and analyses of these systems have provided an increasingly complete view of their diversity and ecology (41-43). Most past studies of these mats have focused on the contribution of cyanobacteria (42), key biogeochemical players through both primary production and the generation of oxygen used by highly active populations of aerobes near the surface.Although all major anaerobic processes are also recognized to be present in the deeper, anoxic regions of these mats, much less attention has been directed toward resolving their contribution to mat biogeochemistry. In particular, since these mats develop in low-sulfate springs, the role of sulfate-respiring prokaryotes (SRP) has received relatively little attention (13, 33, 48). However, a low concentration of sulfate can support a highly active population of SRP in communities that sustain populations mediating both the oxidizing and the reducing pathways of the sulfur cycle. This has been observed in certain freshwater lakes (21) and in hot springs supporting the growth of photosynthetic biofilms (15, 16). The importance of sulfate respiration in low-sulfate environments is also of considerable significance for understanding the biogeochemistry of early Earth, in which oxidized sulfur species were much less abundant than today (6,20). The present study combined radiometric rate measurements, chemical profili...