The aim of our work was to study the occurrence of inorganic carbon pumps in the cell membrane and their importance in the supply of C for photosynthesis in different macrophyte species. This was performed by checking and comparing responses of several green, brown and, especially, red marine macroalgae species under CO2 disequilibrium conditions in the presence of buffer and/or inhibitors of carbon uptake. In addition, the effect of the different treatments was also checked in the marine phanerogam Zostera marina.
Zostera marina, Ulva sp., Palmaria palmata, Gracilaria sp., Polysiphonia sp., Dumontia contorta, Fucus serratus, Chondrus crispus, Laminaria saccharina, Phycodrus sp.
As a first approach, CO2 partial pressure and pH traces in natural seawater (control conditions) were measured using a combined system (IRGA and pH electrodes). The response of 9 marine macroalgae species (Ulva sp., Palmaria palmata, Gracilaria sp., Polysiphonia sp., Dumontia contorta, Fucus serratus, Chondrus crispus, Laminaria saccharina, Phycodrus sp.) and of the seagrass Zostera marina was checked applying light-dark transitions. In these experiments two different responses were obtained: (a) plants displaying CO2 traces following that expected for CO2 partial pressure in eqyilibrium with the medium, indicating a use of HCO3(-) via a highly efficient external carbonic anhydrase (Ulva sp. cultured under strong light in a day/night cycle, and Fucus serratus) (b) plants showing traces out of the equilibrium. The CO2 disequilibrium (indicating a release of CO2 into the medium) could be expressed either in darkness (type b1), or both in light and darkness (type b2). Plants performing as a type b2 showed a pronounced and transitory increase in CO2 partial pressure immediately after light was set off, interpreted as a transient CO2 outlet in darkness. The presence of a less pronounced external carbonic canhydrase activity was suggested in those plants only showing disequilibrium responses in darkness (type b1; Phycodrus sp. and Polysiphonia sp.), these algae probably relying on a diffusive CO2 enrty as the main Ci source for photosynthesis. The use of buffer (TRIS) and specific inhibitors for carbon uptake (acetazolamide and ethoxyzolamide) opened the possibility to discern between different responses and underlying mechanistic aspects. Disequilibrium responses both in light and darkness were considered as an evidence of a carbon-concentrating mechanism (CCM) in the cell membrane. In one group of plants this CCM was inhibited by both acetazolamide and buffer (mainly red algae: Gracilaria sp., Chondrus crispus, Dumontia contorta and Laminaria saccharina), while one unique algae was not sensitive neither to buffer nor to acetazolamide (Palmaria palmata). In the first case, the existence of external carbonic anhydrase activity linked to a proton excretion was proposed as a possible CCM, while a direct HCO3(-) uptake was postulated in Palmaria palmata.