Biological soil crusts (BSCs) are key components of ecosystem productivity in arid lands and they cover a substantial fraction of the terrestrial surface. In particular, BSC N 2 -fixation contributes significantly to the nitrogen (N) budget of arid land ecosystems. In mature crusts, N 2 -fixation is largely attributed to heterocystous cyanobacteria; however, early successional crusts possess few N 2 -fixing cyanobacteria and this suggests that microorganisms other than cyanobacteria mediate N 2 -fixation during the critical early stages of BSC development. DNA stable isotope probing with 15 N 2 revealed that Clostridiaceae and Proteobacteria are the most common microorganisms that assimilate 15 N 2 in early successional crusts. The Clostridiaceae identified are divergent from previously characterized isolates, though N 2 -fixation has previously been observed in this family. The Proteobacteria identified share 498.5% small subunit rRNA gene sequence identity with isolates from genera known to possess diazotrophs (for example, Pseudomonas, Klebsiella, Shigella and Ideonella). The low abundance of these heterotrophic diazotrophs in BSCs may explain why they have not been characterized previously. Diazotrophs have a critical role in BSC formation and characterization of these organisms represents a crucial step towards understanding how anthropogenic change will affect the formation and ecological function of BSCs in arid ecosystems.
We used metatranscriptomics to study the gene transcription patterns of microbial plankton (0.2 to 64 m) at a mesohaline station in the Chesapeake Bay under transitions from oxic to anoxic waters in spring and from anoxic to oxic waters in autumn. Samples were collected from surface (i.e., above pycnocline) waters ( The similarity between metatranscriptomes changed at a lower rate during the transition from oxic to anoxic waters than after the return to oxic conditions. Transcripts related to photosynthesis and low-affinity cytochrome oxidases were significantly higher in shallow than in deep waters, while in deep water genes involved in anaerobic metabolism, particularly sulfate reduction, succinyl coenzyme A (succinyl-CoA)-to-propionyl-CoA conversion, and menaquinone synthesis, were enriched relative to in shallow waters. Expected transitions in metabolism between oxic and anoxic deep waters were reflected in elevated levels of anaerobic respiratory reductases and utilization of propenediol and acetoin. The percentage of archaeal transcripts increased in both years in late summer (from 0.1 to 4.4% of all transcripts in 2010 and from 0.1 to 6.2% in 2011). Denitrification-related genes were expressed in a predicted pattern during the oxic-anoxic transition. Overall, our data suggest that Chesapeake Bay microbial assemblages express gene suites differently in shallow and deep waters and that differences in deep waters reflect variable redox states.
Biological soil crusts (BSCs) are key components of ecosystem productivity in arid lands and they cover a substantial fraction of the terrestrial surface. In particular, BSC N 2 -fixation contributes significantly to the nitrogen (N) budget of arid land ecosystems. In mature crusts, N 2 -fixation is largely attributed to heterocystous cyanobacteria; however, early successional crusts possess few N 2 -fixing cyanobacteria and this suggests that microorganisms other than cyanobacteria mediate N 2 -fixation during the critical early stages of BSC development. DNA stable isotope probing with 15 N 2 revealed that Clostridiaceae and Proteobacteria are the most common microorganisms that assimilate 15 N 2 in early successional crusts. The Clostridiaceae identified are divergent from previously characterized isolates, though N 2 -fixation has previously been observed in this family. The Proteobacteria identified share 498.5% small subunit rRNA gene sequence identity with isolates from genera known to possess diazotrophs (for example, Pseudomonas, Klebsiella, Shigella and Ideonella). The low abundance of these heterotrophic diazotrophs in BSCs may explain why they have not been characterized previously. Diazotrophs have a critical role in BSC formation and characterization of these organisms represents a crucial step towards understanding how anthropogenic change will affect the formation and ecological function of BSCs in arid ecosystems.
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