Photobiology in a scenario of global change

WALTER HELBLING, E; VIRGINIA ESTELA VILLAFAÑE

Tucuman

Congreso; DAAD; 2006

Even though the release of ozone-depleting substances has decreased dramatically in thelast decades, the ozone layer will take longer (i.e., ca 20 years) to recover than previouslythought due to the increase of CO2 in the atmosphere. In addition to the delay in therecovery of the ozone layer, other potential outcomes of global warming include theincreased stratification of the water column, with changes in both, ocean circulationpatterns and mixing rates. The combination of these variables might induce a higherstress in phytoplankton cells, as they would be exposed to higher UVR levels due to theircirculation within a shallower upper mixed layer (UML). Our data, from aphytoplankton time series from tropical China, show that the impact of UVR onphotosynthesis, when combined with different CO2 concentrations (from “normal” up to1100 ppm) and mixing rates, varied between the sampled assemblages. When thephytoplankton population was dominated by small cells, there were no differences in theimpact of UVR on the photosynthetic apparatus under various concentrations of CO2.However, when the phytoplankton population had a relatively large proportion of largedinoflagellates there were significant decreases in the UVR impact, with cells grownunder “normal” (i.e. 360 ppm CO2) being more sensitive than those grown under 1100ppm of CO2. The difference in UVR-sensitivity between cells grown under low and highCO2 concentrations was higher when they were kept static in the water column, anddecreased with increasing mixing speed. Our results suggest that large cells can takeadvantage from high CO2 concentrations and this, together with fast mixing, might helpcells to repair any UVR-induced damage. Our data also show that the combined impactof UVR, mixing and CO2 concentration is highly dependent on the species compositionand / or size structure of the assemblages.