Abstract.A model that predicts UV effects on marine primary productivity using a biological weighting function (BWF) coupled to the photosynthesis-irradiance response (BWF/P-E model) has been implemented for two strains of the picoplanktonic cyanobacteria Synechococcus, WH7803 and WH8102, which were grown at two irradiances (77 and 174 µmol m −2 s −1 photosynthetically available radiation (PAR)) and two temperatures (20 and 26 • C). The model was fit using photosynthesis measured in a polychromatic incubator with 12 long-pass filter configurations with 50 % wavelength cutoffs ranging from 291 to 408 nm, giving an effective wavelength range of 280-400 nm. Examination of photosynthetic response vs. weighted exposure revealed that repair rate progressively increases at low exposure but reaches a maximum rate above a threshold exposure ("E max "). Adding E max as a parameter to the BWF/P-E model provided a significantly better fit to Synechococcus data than the existing "E" or "T " models. Sensitivity to UV inhibition varied with growth conditions for both strains, but this was mediated mainly by variations in E max for WH8102 while both the BWF and E max changed for WH7803. Higher growth temperature was associated with a considerable reduction in sensitivity, consistent with an important role of repair in regulating sensitivity to UV. Based on nominal water column conditions (noon, solstice, 23 • latitude, "blue" water), the BWF E max /P-E model estimates that UV + PAR exposure inhibits Synechococcus photosynthesis from 78 to 91 % at 1 m, and integrated productivity to 150 m 17-29 % relative to predicted rates in the absence of inhibition.

Abstract. A model that predicts UV effects on marine primary productivity using a biological weighting function (BWF) coupled to the photosynthesis-irradiance response (BWF/P-E model) has been implemented for two strains of the picoplanktonic cyanobacteria, Synechococcus, WH7803 and WH8102, which were grown at two irradiances (77 and 174 μmol m−2 s−1 PAR) and two temperatures (20 °C and 26 °C). The model was fit using photosynthesis measured in a polychromatic incubator with 12 long-pass filter configurations with 50% wavelength cutoffs ranging from 291 to 408 nm, giving an effective wavelength range of 280–400 nm. Examination of photosynthetic response vs weighted exposure revealed that repair rate progressively increases at low exposure but reaches a maximum rate above a threshold exposure ("Emax"). Adding Emax as a parameter to the BWF/P-E model provided a significantly better fit to Synechococcus data than the existing "E" or "T" models. Sensitivity to UV inhibition varied with growth conditions for both strains, but this was mediated mainly by variations in Emax for WH8102 while both the BWF and Emax changed for WH7803. Higher growth temperature was associated with a considerable reduction in sensitivity, consistent with an important role of repair in regulating sensitivity to UV. Based on nominal water column conditions (noon, solstice, 23° latitude, "blue" water), the BWFEmax/P-E model estimates that UV + PAR exposure inhibits Synechococcus photosynthesis from 77–91% at 1 m, and integrated productivity to 150 m 15–27% relative to predicted rates in the absence of inhibition.

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 ((14)C 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.

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