Distinct bacterial communities associated with photosynthetic picoeukaryotes in Lake Dongting throughout the seasonal cycle as revealed by flow cytometry sorting

Abstract

Associations and interactions between bacteria and phytoplankton are prevalent in aquatic systems. However, the coexistence of bacteria with the smallest phytoplankton groups is mostly unknown. Here, the associated bacteria (AB) of photosynthetic picoeukaryotes (PPEs) in the Yangtze-connected Lake Dongting were studied using flow cytometry sorting combined with high-throughput sequencing of the 16S rRNA gene throughout the seasonal cycle. Bacteria in the corresponding whole water (WWB) were also investigated for comparison. Our results revealed a distinct bacterial community of AB compared to WWB. The taxonomic and phylogenetic diversity indices, as well as the richness, of AB were significantly lower than those of WWB; however, AB showed higher Pielou’s evenness. Proteobacteria (53.8%) was the most abundant phylum in AB, whereas Actinobacteria was predominant in WWB (36.0%). Several abundant clades, including Flavobacterium, Pseudomonas, Rheinheimera, Novosphingobium, and Sphingomonas, were significantly enriched in AB. Functional prediction analysis identified 116 KEGG pathways differing significantly between AB and WWB. Particularly, functional properties involved in cellular processes (mainly bacterial chemotaxis), environmental information processing, organismal systems and xenobiotics biodegradation and metabolism were greatly enriched in AB. Total nitrogen, temperature and chlorophyll a correlated significantly with the community dissimilarity between AB and WWB. Our results shed new light on the bacterial diversity and function associated with the smallest phytoplankton.