Bacterial community composition of size-fractioned aggregates within the phycosphere of cyanobacterial blooms in a eutrophic freshwater lake

Abstract

Bacterial community composition of different sized aggregates within the Microcystis cyanobacterial phycosphere were determined during summer and fall in Lake Taihu, a eutrophic lake in eastern China. Bloom samples taken in August and September represent healthy bloom biomass, whereas samples from October represent decomposing bloom biomass. To improve our understanding of the complex interior structure in the phycosphere, bloom samples were separated into large (>100 µm), medium (10-100 µm) and small (0.2-10 µm) size aggregates. Species richness and library coverage indicated that pyrosequencing recovered a large bacterial diversity. The community of each size aggregate was highly organized, indicating highly specific conditions within the Microcystis phycosphere. While the communities of medium and small-size aggregates clustered together in August and September samples, large- and medium-size aggregate communities in the October sample were grouped together and distinct from small-size aggregate community. Pronounced changes in the absolute and relative percentages of the dominant genus from the two most important phyla Proteobacteria and Bacteroidetes were observed among the various size aggregates. Bacterial species on large and small-size aggregates likely have the ability to degrade high and low molecular weight compounds, respectively. Thus, there exists a spatial differentiation of bacterial taxa within the phycosphere, possibly operating in sequence and synergy to catalyze the turnover of complex organic matters.

Figure 1. Rarefaction analysis of the 16S rRNA gene sequences among phycosphere samples using an evolutionary distance threshold of 3% (i.e., 97% similarity).

Figure 2. Pareto-Lorenz curves derived from phycosphere samples in Lake Taihu.

The 16S rRNA gene sequences were divided in OTUs based on a sequence similarity threshold of 97% and the OTUs were ranked from high to low, based on their abundance. The Pareto-Lorenz evenness curve is the plot of the cumulative proportion of OTU abundance (y-axis) against the cumulative proportion of OTUs (x-axis). The Fo index, i.e. the combined relative abundance of 20% of the OTUs, is shown. The 45° diagonal is the Pareto-Lorenz curve of a community with perfect evenness.

Figure 3. Principle component analysis of the phycosphere samples including cyanobacterial reads (A) and excluding cyanobacterial reads (B).

The Yue and Clayton measure of dissimilarity between the structures of the communities was estimated and visualized using the dist.shared and pcoa commands of Mother.

Figure 4. Relative abundance and bacterial composition obtained by pyrosequencing from phycosphere samples in September and October, by phylum.

Phylogenetic classification for the pyrosequencing analysis obtained from Ribosomal Database Project Classifier analyses.

Figure 5. Organic carbon-utilization conceptual model illustrating the role of bacteria on different-sized aggregates in the cyanobacterial phycosphere.