Mixed Microbiomes in one species of Euryhaline fish! Coincidence?

Microbiomes living within bodies and organs have been studied extensively in fish as they influence greatly the health, bodily functions and behaviours of that species depending on the community assemblage. A reduction in an individual’s immune system can be caused by a consistent turnover of microbes, a useful symbiont has a reduced abundance or becomes outcompeted due to the appearance of a stronger competitor, the host’s fitness and diet can then be affected depending on the function of the new microbe. This explains the ‘niche-appropriation’ hypothesis, possibly leading to mass mortalities. A contradictory hypothesis, ‘neutral theory’, states communities have the same competitive strengths leading to local microbiome communities representing the regional pool, overall. I agree there is less of an opinion for the latter as not all environmental bacteria can accommodate to every available niche hosts offer. Schmidt et al., (2015) explains these, consistently pulling on them in justifying their results. This is limiting due to unknown abilities of dispersal distances of microbes, they saturate across all oceans, making it hard to classify regional pools.

Microbiome-host interactions is a very recent topic of interest, including the appreciation of the importance of how they connect to the surrounding environment in identifying and understanding changes in assemblages and its effects, positive and negative. This type of study allows scientists to understand how microbiomes affect host physiology and how the community composition influences its function in relation to the host’s health. The ion-balancing abilities in Euryhaline fish meant its perfect for salinity studies as it can stabilize internal plasma and their gut salinity to ~7ppt, accommodating the microbial regional pools.

Schmidt et al., (2015) chose four salinities; 0.0ppt, 5.0ppt, 18.0ppt, 30.0ppt to acclimatise Poecilia sphenops to whilst controlling their diets of tropical fish flakes. Each tank contained two fish, every four tanks designated to a salinity, allowing for three microbiome community comparison studies. First, the two-individual fish were compared, second, paired fish were compared to pairs in other same salinity tanks producing local community pools. Thirdly, different salinities were compared for local community comparisons as different regional pools were present. Changes between fish microbiomes was examined and compared to microbiomes within the water by filtering out the bacteria to extract DNA. The V6 hypervariable region of the bacterial 16S rRNA gene was used when amplifying DNA and Illumina HiSeq used to give paired-end sequencing of DNA strands, clustering the data into operational taxonomic units. By assigning habitat type and microbial identities, using ecological distance metrics, the similarities and differences between the water, tissues and salinity of the microbiome communities produced the result.

Their results found a change in the microbiome assemblage within fish. As salinity gradient increased turnover of the few dominant, but abundant, bacteria taxa occurred, yet, with some similarities, for example; Vibrio and Enterobacteriaceae were found in the two high salinity treatments when Aeromonas and Cetobacterium with the two lowest salinities. Fish in the same salinity tanks resulted in similar microbiomes, thus confirming the ‘niche-appropriation’ hypotheses on colonization with niche availability, not randomly, as the most abundant OTU in the fish did not correspond to the most abundant in the water. Physiological processes within the fish could also influence the assemblages.

This author states it is still not clear how community assemblages form in relation to the connections from environmental pools yet their results aid the growing research conducted of microbe-host interactions with seasons. To me this paper provides a good insight into how microbial communities within fish are pooled together in relation to habitat change, but is difficult to understand how salinity effects fits into current research about seasonality. It would be interesting to read the consequences of microbes found. I think this would be a good base for future research into diseases caused by these microbes with specific reference to aquaculture or humans.

Reviewed Article:

Schmidt, V. T., Smith, K. F., Melvin, D. W. & Amaral-Zettler, L. A. (2015), Community assembly of a euryhaline fish microbiome during salinity acclimation. Molecular Ecology, 24(10), 2537–2550.

http://onlinelibrary.wiley.com.plymouth.idm.oclc.org/doi/10.1111/mec.13177/full