Photophysiology of the marine cyanobacterium Synechococcus sp. WH8102, a new model organism

Abstract

Synechococcus spp. constitute a major and ubiquitous component of marine ecosystems. The genome of one strain of this genus, WH8102, has recently been completely sequenced. Since it can also be genetically manipulated, this clone has the potential to become a new model organism; however, to date, it remains poorly characterised in terms of pigment composition, optical properties and photophysiology. It has a very high phycourobilin to phycoerythrobilin (PUB:PEB) ratio (ca. 1.95 at low light), and is therefore representative of Synechococcus populations found in oligotrophic areas of the ocean. We show here that this strain has a very wide growth irradiance range from 650 μmol photons m -2 s -1 continuous white light, with a maximum growth rate (μ max = 1.13 ± 0.02 d -1 ) at 207 pmol quanta m -2 s -1 (I max ). As cells acclimated to high light, drastic variations in the chlorophyll a (chl a), β-carotene and phycoerythrin (PE) contents were observed, reaching a quasi steady state around I max . In contrast, the zeaxanthin content remained approximately constant whatever the light level. Similarly, the carbon and nitrogen contents did not significantly vary with irradiance. Red and orange fluorescences, as measured by flow cytometry, were found to correlate well with chl a and PE contents, respectively. Spectrometric analyses of phycobilisome (PBS)-containing fractions from cells grown under different photon fluxes suggest a specific reduction of the PEII content relative to other phycobiliproteins (PBPs) during acclimation of the PBSs to high light.