Small-scale variability of protistan planktonic communities relative to environmental pressures and biotic interactions at two adjacent coastal stations

The aim of this study was to analyze planktonic protistan assemblages under different environmental pressures at 2 adjacent coastal stations (inshore and offshore; similar to 6.4 km apart) in the eastern English Channel (EEC). Samples were collected between March 2012 and June 2013, and analyzed using bioinformatic analysis and tag pyrosequencing of the V2-V3 hypervariable region of the 18S rRNA gene. In addition to the taxonomic composition of the protistan communities, the detected operational taxonomic units (OTUs) were sorted into 6 major functional groups based on their trophic roles in marine systems. Comparisons of 13 environmental factors, including physical and chemical variables, indicated significant differences between the 2 stations. However, the only significant relationship between environmental pressures and protistan community structure was detected when the trophic role of the OTUs was considered at the inshore station. In order to interpret differences in community structure, the effect of biotic inter actions was investigated through the examination of the co-occurrence networks of the protistan communities at both stations. In terms of the number of edges and connectivity of nodes, the analysis showed that the inshore station had more complex associations between OTUs than the offshore station. This strongly suggests that due to the higher and more variable environmental pressures the inshore station comparatively receives, its protistan community has developed a greater complexity of biotic connections. This in turn reflects the rapid responses of trophic interactions within the entire microbial community.