Mud volcanos
(MVs) form when low-density sedimentary rock rises to the surface, they often
occur in areas of tectonic activity where the Earth’s Crust in weakened. These
MVs emit pressurized fluids that consist of water, liquids, hydrocarbons, solid
particles and gases (methane being the prevalent one). However, methanotrophic
bacteria in the surface sediments of the MVs act as environmental filters and
provide energy sources for other organisms which are released in the seeping
fluids. Indeed, evidence suggest that in the Gulf of Cadiz almost all of the
methane emitted by MVs is consumed by benthic communities and not transported
further. While previous studies have described microbial communities in MV
sediments, our knowledge of their community structure and functions are still
limited. In their paper, Coelho et al.
(2016) set out to characterize bacterial and microeukaryotic communities in
deep-sea MVs in the Gulf of Cadiz and to determine possible interactions between
different groups.
Surface sediment
samples (0-1 cm) were taken from deep-sea MVs in the South West Iberian Margin
(SWIM) during a cruise in 2012. SWIM is located on the border between the
Eurasian and the African plates, leading to the formation of many MVs. The
sampling sites were sorted into three categories of seepage activity, based on
the thickness of hemipelagic sediment and presence of chemotrophic fauna. DNA
was extracted from the samples and 16S and 18S rRNA pyrosequencing was applied.
The sequences were classified into operational taxonomic units (OTUs) using the
QIIME and UPARSE softwares.
The authors
were able to show that the composition of the bacterial and microeukaryotic
communities changed significantly with seepage activity. Moreover, the number
of qualified sequences also varied between sample sites. 99.07% of the
bacterial sequences and 97.16% of the eukaryotic sequences were assigned to
known phyla. At the most active MVs, Methylococcales
and Lobosa dominated bacterial
and microeukaryotic communities respectively. In contrast, Rhodospirillales, Nitrospirales
and SAR202 were more abundant in bacterial communities at less active sites. Radiolaria abundance in the
microeukaryotic communities was also higher at less active MVs. The significant
match between bacterial and microeukaryotic datasets, might either be explained
by abiotic processes or interactions between different groups. While this study
cannot confirm either theory, the authors suggest that Lobosa might graze on Methylococcales,
making them first-level consumers of the methanotrophic bacteria. However,
further study is necessary investigate possible interactions between microbes
in MV sediments.
In conclusion, I found this paper to be quite
an interesting study on a topic I personally did not know much about.
However, this is a baseline study and the authors acknowledge the limitations
of their paper regarding the detection of possible interactions between
bacteria and eukaryotes. The study is also mainly focused on the surface
sediments of MVs, therefore excluding anaerobic methanotrophic archaea and
sulfate reducing bacteria in the deeper sediments. For further studies, I would
be interested in seeing how microbial communities might vary between geographic
locations, depth of the MV and location in sediment.
Reference:
Coelho, F.
J., Louvado, A., Domingues, P. M., Cleary, D. F., Ferreira, M., Almeida, A.,
... & Gomes, N. C. (2016). Integrated analysis of bacterial and
microeukaryotic communities from differentially active mud volcanoes in the
Gulf of Cadiz. Scientific Reports, 6. Link:
http://www.nature.com/articles/srep35272
Hi Johanna,
ReplyDeleteThanks for another great post - I was unaware of work done on the microbiology of MV sediments so thank you for bringing it to our attention. I was thinking about the Lobosa-Methylococcales correlation and was wondering if the relationship could be symbiotic/syntrophic as opposed to predatory? The authors don't seem to mention this as an alternative, but amoebae can host a diversity of endosymbionts and interact with environmental bacteria in complex ways (such as the case with Burkholderia) (DiSalvo et al, 2015). This relationship might be very interesting indeed and merits further investigation.
DiSalvo, S., Haselkorn, T. S., Bashir, U., Jimenez, D., Brock, D. A., Queller, D. C., & Strassmann, J. E. (2015). Burkholderia bacteria infectiously induce the proto-farming symbiosis of Dictyostelium amoebae and food bacteria. Proceedings of the National Academy of Sciences, 112(36), E5029-E5037. http://www.pnas.org/content/112/36/E5029.short
Thanks,
Davis
Hi Davis,
ReplyDeleteThanks for your comment. You’re right, the authors don't mention symbiosis as a possible alternative. They do however refer to two studies. Murase & Frenzel (2008) were able to show that Lobosa amoebae in rice field sediments grazed on methanotrophic bacteria. Additionally, a recent study (Pernice et al., 2015) showed the abundance of heterotrophic protists in deep-water sediments to be strongly correlated with prokaryote abundance, again suggesting active grazing. This led Coelho et al. to conclude that the interaction between Lobosa and Methylococcales was likely to be predation.
However, I agree with you that the possibility of a mutualistic interaction should not be discarded so quickly and the paper you referenced seems like an great addition to the debate.
Thanks again,
Johanna
References:
Murase, J., & Frenzel, P. (2008). Selective grazing of methanotrophs by protozoa in a rice field soil. FEMS microbiology ecology, 65(3), 408-414. Link: http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6941.2008.00511.x/full
Pernice, M. C., Forn, I., Gomes, A., Lara, E., Alonso-Sáez, L., Arrieta, J. M., ... & Sintes, E. (2015). Global abundance of planktonic heterotrophic protists in the deep ocean. The ISME journal, 9(3), 782-792. Link: http://www.nature.com/ismej/journal/v9/n3/full/ismej2014168a.html