ESTIMATION OF OXYGEN EVOLUTION BY MARINE-PHYTOPLANKTON FROM MEASUREMENT OF THE EFFICIENCY OF PHOTOSYSTEM-II ELECTRON FLOW

The relation between photosynthetic oxygen evolution and Photosystem I I electron transport was investigated for the marine algae Phaeodactyl um tricornutum, Dunaliella tertiolecta, Tetraselmis sp., Isochrysis sp , and Rhodomonas sp.. The rate of Photosystem II electron transport wa s estimated from the incident photon flux density and the quantum effi ciency of Photosystem II electron transport as determined by chlorophy ll fluorescence, The relation between the estimated rate of Photosyste m II electron transport and the rate of oxygen evolution was investiga ted by varying the ambient light intensity. At limiting light intensit ies a linear relation was found in all species. At intensities approac hing light saturation, the relation was found to deviate from linearit y, The slope of the line in the light-limited range is species depende nt and related to differences in absorption cross-section of Photosyst em II. The observed non-linearity at high irradiances is not caused by photorespiration but probably by a Mehler-type of oxygen reduction. T he relationship could be modelled by including a redox-state dependent oxygen uptake, In the diatom Phaeodactylum tricornutum, the photochem ical efficiency of dark adapted open Photosystem II centers was found to be temperature-dependent with an optimum near 10 degrees C.