Energy balance during active carbon uptake and at excess irradiance in three marine macrophytes

Abstract: The marine environment is an important habitat where many processes occur that affect life conditions on earth. Macrophytes and planktonic oxygen evolvers are an essential component for almost all marine life forms and have developed in an environment that differs largely from the terrestrial habitats. For instance in regards to available ionic forms of inorganic carbon and moving water masses which affects incoming light. It is therefore relevant to examine the physiology of algae and marine plants to identify their unique features and differences to terrestrial plants that once orginated from algae. By using chlorophyll fluorescence measurements alone or combined with measurements of oxygen evolution and protein analysis photosynthetic strategies to withstand excess energy have been evaluated under a variety of experimental conditions. Furthermore metabolic pathways involved in energy transfer from photosynthesis to the site of active carbon uptake have been examined. The following was found:' The ratio between photosynthetic gross oxygen evolution and estimated electron transport rate varies in Ulva spp depending on previous history of light and dark exposures. To obtain P/I curves with ratios close to the theoretical 1:4 value, measurements should be performed on separate pieces of tissue at each irradiance level. ' Under carbon deficient conditions, the estimated ETR is larger than the gross oxygen evolution, which may be due to the so called “water-water” cycle and absorption changes in PSII which are not corrected for in the calculation of ETR. ' Upon exposure to high irradiances (1500 µmol photons m-2s-1) the PSII core protein D1 is broken down with a concomittant reduction in ETR in Ulva spp. With the decrease in electron transport between PSII and PSI the acidification of the lumen decreases and the ability to dissipate excess energy as heat. At prolonged irradiance, an acclimation occurs with a lesser or no breakdown of D1 indicating an additional photo-protective strategy other than heat dissipation.' Laminaria saccharina is dependent on mitochondrial respiration for active utilization of bicarbonate. By extruding protons outside the plasmalemma an acidification takes place that favors the conversion of bicarbonate into carbon dioxide that then can diffuse in to the cell. These proton pumps are driven by ATP supplied to a large degree from mitochondria, likely through the reductant NADPH produced photochemically. ' The marine angiosperm Zostera marina is dependent on mitochondrial respiration for utilization of bicarbonate in a manner similar to that in Laminaria saccharina . However, the water-water cycle may supply additional ATP to the proton pumps in Zostera marina. Both species exhibit a lag-phase at the onset of illumination after a dark incubation period and at least part of this lag-phase is due to a lag in an activation of mitochondrial supported bicarbonate utilization. It is clear that the marine environment holds complex plant and algae species and much is still to discover about the oxygen evolvers that grow beneath the water surface.