Seasonal Patterns in Proteorhodopsin Gene Dynamics in the Arctic Ocean

Abstract

Photoheterotrophic microbes have the capacity to use both light and organic matter as an energy source and challenge our classical understanding of the marine C cycle. The proteorhodopsin gene (PR), encoding a membrane-bound retinal, is one of the functional genes that enable some bacteria to harvest light's energy. However, while PR gene has been reported from many oceanic regions, seasonal patterns of PR gene presence and expression remain poorly described, especially in polar oceans. During the Circumpolar Flaw Lead (IPY-CFL) study, we had the opportunity to investigate the annual seasonal pattern of PR gene presence and expression in the Amundsen Gulf of the Arctic Ocean. Surface seawater samples for microbial DNA and RNA were collected weekly from the Amundsen Gulf (November 2007 to July 2008). A subset of these samples was then chosen for targeted PR gene amplification using degenerate primers. Given the contrasting light-dark period in the Arctic, we expected PR to not be expressed during periods of ice cover and present throughout the open water season. PR was present in the genomic DNA of all of the samples tested. However the expression, inferred from the successful amplification of the PR gene from the RNA sample, coincided with the melt season and the return of sunlight only. Furthermore, there was an abrupt seasonal change in PR dynamics in the Arctic, which was mirrored by changes in bacterial diversity estimated using 16S rRNA gene amplicon visualized via denaturing gradient gel electrophoresis (DGGE). The DGGE indicated a rapid change in bacterial communities at ice melt. Light availability was the best indicator of PR dynamics and microbial community structure in the Arctic. Our data adds considerably to the known distribution and expression of the PR gene in marine bacterial communities, and the increased understanding of the timing and occurrence of the PR is a first step towards the integration of photoheterotrophs into Carbon cycle models in the Arctic Ocean