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Biodiversity of bacteria and protists along a salinity gradient in the Fraser River estuary Nawar, Ferdous


A fundamental goal of microbial ecology is to understand how microbial diversity is distributed over space and time. At broad scales, abiotic factors drive microbial community composition, and salinity plays a particularly important role. Do all microbes respond in the same way? Here I investigate the diversity of two domains encompassed within the blanket term ‘microbes’ the bacteria and protists (unicellular eukaryotes). I examined their diversity and composition across a river to ocean transect and compare the shifts in diversity that occur in these two domains. I collected 122 samples from the Fraser River, its plume and the adjacent coastal water at three time points covering the spring freshet. I used a combined approach of high-throughput sequencing and inverted microscopy to investigate the changes in diversity and community composition. As expected, there was a strong turnover in the community composition of bacteria and protists across the salinity gradient. I found that diversity of both the bacteria and protists was highest in the freshwater and brackish water with low salinity, and decrease with increasing salinity, and observed a high degree of separation between marine and freshwater community composition. However, the shift from communities with predominately fresh to marine water taxa occurs at the higher taxonomic level for bacteria (phylum/class) and lower taxonomic level (family/genus) for protists. Moreover, the inflection point for taxonomic distribution tends to occur around at 20 salinity for bacteria and 10 salinity for protists. Overall, bacteria are distributed more broadly across the salinity gradient than protists and more likely to be present across time points. In contrast, protists tend to be found in a narrower salinity range and are more often restricted to either marine or freshwater, and found at a single time point.

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