The deep sea surface sediment sustains one of the largest continuous ecosystems on the planet, including diverse communities of microbes whose distribution and biogeography have become an increasingly popular area of research. The article quotes the cosmopolitan hypothesis ‘everything is everywhere, but the environment selects’, which in context to the article, suggests that the way bacteria are distributed across the sea floor is largely influenced by their ability to cope with surrounding environmental barriers and how these barriers affect their ability to disperse. Physical and physiological factors that can affect dispersal in marine habitats are pH, temperature, oceanic and near-bed currents which affect the sediment-water interface exchange, resulting in the dispersal of particles such as the microbes.
This article focuses on the eastern South Atlantic Ocean and the Walvis Ridge which forms a barrier separating three deep sea basins as well as the northward and southward flow of water in the deep region of the sea (below 3000m). It has been shown that the Walvis Ridge acts as a physical and physiological barrier, preventing the dispersal of various life in the deep sea, such as some crustacean species. However this article attempts to find out whether it also affects microbe dispersal by determining the bacterial distribution in the three deep sea basins of the south Atlantic ocean through the analysis of 16s ribosomal RNA gene sequences and using a community fingerprinting method, terminal restriction fragment length polymorphism (T-RFLP), a technique used to profile the abundance of unknown microbial communities using PCR amplification of a target gene.
The results showed a third of the fragments from the T-RFLP were detected in all three basins suggesting many common phylotypes between them. However, major groups such as Chloroflexi were restricted to individual basins and there was a significant difference in communities between Angola and Cape basins, which was consistent with the different sediment particle size in Cape basin, thus suggesting that environmental factors do influence communities. Although there were high dispersal rates suggesting limited effects of barriers, when looking at the 16s rRNA and T-RFLP analysis, the distribution of many microbes is limited showing that there is an influence of barriers on dispersal of microbes in the South Atlantic Ocean.
The study proved that there was an effect of environmental barriers on microbe dispersal, but the authors also strengthened their work by looking at numerous other variables that could affect the results, such as the effects of spatial structure, correlation between genetic diversity and geographic distance and they also compared all their results to previous studies on different areas of the ocean. These added evaluations allow the effect of possible confounding variables to be resolved and prevents the results from being limited. However, more regions or basins, differing in geographical distance from the ridge could have been analysed to give a more representative evaluation. Also, perhaps another potential barrier could be the different currents acting upon the individual basins as this was not mentioned or more focus could have been put on the variations in physiological differences between the basins, as if there was little variation then it could explain the high similarity in diversity.
A review of Schauer, R. Bienhold, C. Ramette, A. and Harder, J. (2010) Bacterial diversity and biogeography in deep-sea surface sediments of the South Atlantic Ocean, The ISME Journal. 4, 159-170
2 comments:
Jelena - an interesting paper. It's been a long-held belief that microbes are not affected by biogeographical influences (like plants and animals), but this paper seems to add to evidence that casts doubt on this. There is a nice article in Science (2005) that supports this http://www.sciencemag.org/content/310/5750/960.full.pdf
Hi Colin, I was unable to get access to the article you suggested, however I did find this article (http://onlinelibrary.wiley.com/doi/10.1016/j.femsec.2004.03.013/full) which I thought was very interesting and took the research a step further by supporting the theory that bacteria are greatly affected by physical and physiological barriers but also going further into the effects this has on the bacterial populations and their evolution.
It begins by explaining that it is generally thought that microbes are not affected by environmental barriers and that dispersal is an on-going process for microbes because of some of their attributes. Firstly, it states that microbial populations are so large that this increases the probability of dispersal and makes it very likely. Also, it goes further to explain that microbes are able to form resistant inactive stages which allow them to survive a diverse range of hostile environments and also adapt to them, suggesting that adaptation is the cause of evolutionary change in microbes, instead of the isolation of populations which would be caused by the environmental barriers.
This article argues that instead of resorting to adaptation as an explanation for evolutionary change in microbes, it explains the possibility of evolution through genetic drift, which up until recently was considered extremely rare in microbes. The article continues its theory by providing support and contributing results from hot springs and from free-living soil bacteria from different continents, concluding that through various rRNA analysis, completely unique 16s rRNA sequences were found in the isolated environments, suggesting the presence of geographic clades. Thus concluding that isolation of populations is possible due to restricting dispersal of the microbes (through environmental barriers) and that this is an important factor in the evolution of microbial populations and therefore, genetic drift, founder effects or neutral evolution should be considered, instead of just adaptation.
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