Thursday, 7 November 2013

MGA Diversity and Function in the North Pacific Ocean

Marine Group A (MGA) bacteria were first discovered in 16S rRNA gene clone libraries produced from surface waters of the Atlantic and Pacific Oceans. However, almost nothing is known about MGA, which is currently a candidate phylum. The report expands on a previous study on the diversity of MGA in a range of O2-deficient waters of the Northeast subarctic Pacific Ocean (NESAP), including an anoxic fjord, Saanich Inlet (SI). While MGA are ubiquitous in the dark ocean, they tend to be most dominant and diverse in interior regions of the ocean with distinct oxyclines, such as in oxygen minimum zones (OMZs) and permanent or seasonally stratified anoxic basins. Climate change is causing OMZs to expand and intensify, which could have large negative implications on ocean ecology and biogeochemistry, so this study aimed to contribute to a greater understanding of the metabolic diversity and ecosystem function of these bacteria to predict any systematic effects.

Phylogenetic analysis of MGA in the NESAP and SI

To compensate for the lack of reference genomes for MGA, 18 large-insert DNA fragments affiliated with certain MGA subgroups were identified to study MGA function and metabolism. 17 of these fragments containing MGA 16S rRNA genes were linked to 5 previously defined and 3 new MGA subgroups using metagenomic libraries.

The authors tested how the 16S rRNA-based patterns of MGA distribution contributed to the establishment of ecotypes along dissolved oxygen gradients, as a response to O2  deficiency. 46 end-sequenced fosmid libraries from both NESAP and SI waters were screened for clones containing 16S rRNA sequences, and sequences for 14 fosmid inserts affiliated with MGA were exposed. Comparative analyses were also made against four publicly available MGA-associated large-insert fragments. This demonstrated a widespread difference in genomes, although patterns did not consistently reflect previous observations in clone libraries. There was a greater proportion of MGA distribution in the NESAP (open ocean) than in SI (coastal basin), where the highest number of end sequences were in dysoxic and suboxic samples, so it’s suggested that dominant MGA subgroups are adapted to O2 deficiency in this area.  

Role of the metabolism of MGA in the marine sulphur cycle

Predicted protein-coding genes associated with oxygen deficiency adaptations and sulphur-based energy metabolism, such as polysulphide reductase (psrABC, or PSR), were discovered on multiple fosmids. The authors suggest that the sequences encoded on two particular fosmids from the NESAP and SI are associated with O2-deficient environments. Two other fosmids were identified possessing proteins homologous to PSR, suggesting that specific MGA subgroups may have the capacity to generate energy via dissimilatory polysulphide reduction to hydrogen sulphide (H2S) or dissimilatory H2S oxidation.

So far, Wolinella succinogenes is the only microorganism in MGA to show evidence for the role of PSR in sulphur-based energy metabolism. The presence of PSR homologs on MGA-affiliated genome fragments suggests MGA may have a dominant role in the cryptic sulphur cycle of O2-deficient marine systems.

Active sulphur cycles linked to sulphur-oxidising gamma and epsilonproteobacteria activities are found in oxygen-deficient marine systems, including OMZs and permanent or seasonally stratified anoxic basins. This paper gives initial indications of the scale of the metabolic processes of the MGA and their significance in the marine environment, based on the group’s diversity, but suggests a wider scope for further work. It would also be interesting to investigate how the role of MGA in the marine sulphur cycle affects other marine microbial communities, especially other anaerobic bacteria.

Wright, J.J., Mewis, K., Hanson, N.W., Konwar, K.M., Maas, K.R., and Hallam, S.J. (2013) Genomic properties of Marine Group A Bacteria indicate a role in the marine sulphur cycle. The ISME Journal, doi:10.1038/ismej.2013.152

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