Kinetics of photosynthetic response to ultraviolet and photosynthetically active radiation in synechococcus WH8102 (CYANOBACTERIA)

Abstract

The picoplanktonic cyanobacteria, Synechococcus spp., (Nägeli) are important contributors to global ocean primary production that can be stressed by solar radiation, both in the photosynthetically active (PAR) and ultraviolet (UV) range. We studied the responses of PSII quantum yield (active fluorescence), carbon fixation (14C assimilation) and oxygen evolution (membrane inlet mass spectrometry) in Synechococcus WH8102 under moderate UV and PAR. PSII quantum yield decreased during exposure to moderate UV and UV+PAR, with response to the latter being faster (6.4 versus 2.8 min, respectively). Repair processes were also faster when UV+PAR exposure was followed by moderate PAR (1.68 min response time) than when UV was followed by very low PAR (10.5 min response time). For the UV+PAR treatment, the initial decrease in quantum yield was followed by a 50% increase ("rebound") after 7 min exposure, showing an apparent photoprotection induction. While oxygen uptake increased with PAR, it did not change under UV, suggesting that this oxygen-dependent mechanism of photoprotection, which may be acting as an electron sink, is not an important strategy against UV. We used propyl gallate, an antioxidant, to test for plastid terminal oxidase (ptox) or ptox-like enzymes activity, but it caused nonspecific and toxic effects on Synechococcus WH8102. We studied the responses of PSII quantum yield, carbon fixation and oxygen evolution in Synechococcus WH8102 under levels of PAR and UVR that are representative of a mixed layer from oligotrophic waters. Fluorescence kinetics measurements showed that these radiation levels inhibited PSII activity, however, an apparent induction of photoprotection was observed. Although oxygen uptake is a mechanism of photoprotection when cells are exposed to increase levels of PAR, our results show that this is not an important strategy against UVR. © 2014 The American Society of Photobiology.