Seasonality and environmental control of freshwater SAR11 (LD12) in a temperate lake (Lake Erken, Sweden)

Abstract

THE SAR11 clade is ubiquitous and abundant in planktonic environments. In freshwater lakes, the clade is represented by tribe LD12 which is phylogenetically distinct from the marine SAR11. We studied the ecology of LD12 in a temperate dimictic lake (Lake Erken, Sweden), by analyzing its seasonal dynamics with quantitative PCR, CARD-FISH and 454 pyrosequencing of the 16S rRNA gene. Results showed that LD12 can be as numerous in freshwater bacterioplankton as their marine SAR11 siblings. They exhibited strong seasonality and made up from 1.8 to 40% of the total bacterial 16S rRNA pool (mean 14%) with pronounced peaks in summer and late fall. Except in spring, LD12 was the dominant Alphaproteobacteria, contributing on average 72% of the 16S rRNA within this class. The LD12 population was dominated by a single persistent ribotype, suggesting low local divergence, at least at the phylogenetic resolution accessed with rRNA genes. The relative abundance of LD12 was positively correlated to nutrient concentrations (phosphate, ammonia, nitrate, and silica) and water transparency whereas the relative abundance was lower during periods characterized by high phytoplankton biomass. Based on these observations we propose that LD12 are poor competitors during periods of high phytoplankton productivity and associated release of labile organic compounds, but thrive when availability of inorganic nutrients is high. Similar to the marine SAR11 sibling group, local LD12 populations appear to respond in contrasting ways to nutrient availability in different lakes, pointing to either ecological divergence within the tribe or variations in the interplay between environmental driver variables. © Inter-Research 2013.