Excess phosphate loading shifts bacterioplankton community composition in oligotrophic coastal water microcosms over time

Phosphorus (P), primarily as phosphate, is considered as a key trophic factor of eutrophication in coastal environments. Information on the response of bacterial community to excess phosphate loading in oligotrophic coastal water could provide insights into the micro-ecological effects on the expanding trend of nutrient-enrichment in coastal water. In this study, temporal dynamic of bacterioplankton community composition was investigated using 16S Illumina MiSeq sequencing technique in oligotrophic seawater microcosms manipulated with no perturbation (control) and a certain level of phosphate by two loading modes: 1) one-off loading at the beginning of incubation; and 2) periodic loadings every two days over a 16-day duration. The results showed that overall bacterial alpha-diversity temporally changed and achieved the lowest value in phosphate loading (P-loading) microcosms at day 8; phosphate-addition increased the Shannon index and Pielou's evenness compared with those of the control at day 16. Bacterioplankton community turnover across all the treatments showed a similar temporal pattern. P-loading significantly affected bacterioplankton community composition with negative correlations of Methylophilaceae and Flavobacteriaceae and positive correlation of Saprospiraceae. Periodic loading treatments demonstrated a greater effect on community variation than one-off loading treatments did. Moreover, excess P-loading accelerated the temporal succession of the bacterioplankton community composition. These results provide insights into the pattern of bacterioplankton community phylogenetically responding to excess P-loading in the manipulated oligotrophic coastal system.