Coastal eutrophication controls the bacterial community composition in different reef habitats of the Spermonde Archipelago, Indonesia.

Coastal eutrophication is a key driver of shifts in bacterial communities, but related knowledge is very scarce. With fringing and patch reefs at varying distances from the coast the Spermonde Archipelago in southern Sulawesi, Indonesia offers ideal conditions to study the effects of coastal eutrophication along a spatially defined gradient. The present study investigated bacterial community composition of three coral reef habitats: the water column, sediments and mucus of the hard coral genus Fungia, along that cross-shelf environmental and water quality gradient. The main research questions were: (1) How do bacterial communities respond to changes in water quality along a spatial gradient? (2) Which water quality parameters influence the bacterial community composition? (3) Are there bacteria community differences between the different investigated microbial habitats? For this purpose a range of key water parameters were measured at 8 stations in distances from 2 to 55 km from urban Makassar. This was supplemented by sampling of bacterial communities of important microbial habitats using 454 pyrosequencing. Findings revealed that the population center Makassar had a strong effect on the concentrations of Chlorophyll a, suspended particulate matter (SPM) and transparent exopolymer particles (TEP), which were all significantly elevated at the inshore compared the other 7 sites. Shifts in the bacterial communities were specific to each sampled habitat. In the water column, the relative abundance of Gammaproteobacteria increased with distance from Makassar, while that of Alphaproteobacteria decreased.In the sediments, there was a pronounced dominance of Gammaproteobacteria at the inshore site, which decreased along the gradient. There was no gradual shift in bacterial classes for samples obtained from the Fungia mucus. We observed a strong positive correlation between Bacteroidia, Chlamydiia and Acidobacteria_Gp6 and chlorophyll a, TEP and SPM. Deinocooci, Verrumicrobiae and Alphaproteobacteria were, on the other hand, positively correlated to DOC concentrations. In addition, we observed very distinct communities between the investigated habitats. Our data shows strong changes in the bacterial community composition at the inshore site for water column and sediment samples. Alarmingly, this led to a much higher prevalence of potentially pathogenic bacteria at the chronically impacted site closest to Makassar.