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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