Who wants salty fish???

The importance of microflora is well documented both in the literature and in this blog. Briefly, symbiotic bacteria present in the GI tract of organisms confer a number of benefits from vitamin production to increased immunity. Within the field of microbiomes, examination of the symbionts and how they respond to biotic and abiotic influence is not numerous. This lack of understanding may be particularly important in light of climate change, where changes in the microbial community could affect host fitness. Some studies have purported that a microbiome has contributed vastly to the evolutionary success of man. However, how these assemblages assemble is poorly understood, as such much of the literature is filled with debate. A multitude of hypotheses aim to explain this, two of the main ones are niche-appropriation, a competition driven process and neutral theory, where stochastic processes determine colonisation. With this in mind, Schmidt et al. (2015) investigated the role of salinity acclimation on the microbial consortia present in/on the euryhaline fish, Poecilla sphenops (Mexican Molly).

After acclimation to eliminate the effects of previous environmental history, fish were acclimated between 0 and 2ppt. Following acclimation fish were exposed to four salinities for 12 days; 0, 5, 18 and 30ppt. Fish were then homogenised, tank water was filtered and DNA analysis undertaken for both sample types.

Results showed that the Gammaproteobacteria were the main constituents of the microbiome in all fish, across all salinity treatments. Generally, the same 2-5 OTUs were present in all individuals; however the frequency of these changed in accordance with the prevailing salinity. Vibrio and Enterobacteriaceae spp. replaced Aeromonas and Cetobacterium spp. at higher salinities. Furthermore, significant overlap was observed between the microbiomes of fish at 0 and 5ppt, and 18 and 30ppt. There were however, some outliers to this general rule, where a particular OTU was far more abundant in an individual despite being subject to the same conditions. Significant differences were observed between the microbial communities present in tissues and the surrounding water. Further, microbial communities in the water did not affect those in the tissues of fish.

Researchers concluded that microbial community assembly is a competition driven process and not a stochastic one owing to the lack of effect external microbial communities had on host colonisation. Additionally, changes in salinity caused changes in the microfloral composition to be observed; some species would not have the capacity to function optimally at varying salinities, and would thus be outcompeted. As such the neutral theory of microbiome assembly seems to be apparent. Despite this, there are some potential flaws in this investigation, many of which are detailed by authors themselves. The potential role of disease (some fish died before experimentation), replication cycles and diet were not considered; likewise analysis of the initial microbiome was not undertaken. Whilst this investigation does provide some interesting insights in to microbiome dynamics in response to the changing environment, it is evident that a more informed experimental design would be beneficial and thus, make the results more convincing. To expand upon this investigation, a multi-stressor approach may be useful. In addition, with the use of axenic fish, could the role of the microbiome in fitness be examined? Again as with most studies, the Fungi, Archaea and eukaryotes were ignored, despite evidence suggesting their significance in microbiome dynamics.  

Jack 

References

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