Acquisition of inorganic carbon by Endarachne binghamiae (Scytosiphonales, Phaeophyceae)

Abstract

Photosynthetic acquisition of inorganic carbon was studied in the brown seaweed Endarachne binghamiae J. Agardh. Photosynthesis was saturated at 245 µmol photons m−2 s−1 and photoinhibition did not occur at an irradiance as high as 750 µmol photons m−2 s−1. The dependence of O2 evolution on inorganic carbon (Ci) concentration demonstrated that the normal Ci composition in natural seawater was not saturating for irradiance-saturated photosynthesis. Three lines of evidence demonstrated that E. binghamiae was able to acquire as a source of Ci for photosynthesis: (i) the high value of photosynthetic conductance for CO2 (220.6 µm s−1); (ii) the high pH compensation point of 9.7; and (iii) the measured photosynthetic rates being in excess of the theoretical maximum rates supported solely by the CO2 supply from the spontaneous dehydration of in the bulk seawater. In order to establish the mechanism of Ci acquisition, specific inhibitors and a proton buffer were applied to examine their inhibitory effects on photosynthesis. No inhibitory effects were found for the proton buffer tris(hydroxymethyl)aminomethane and the anion exchanger inhibitor, 4,4′-diisothiocyano-stilbene-2,2′-disulphonate. By contrast, photosynthetic O2 evolution in natural seawater was significantly depressed by the extracellular carbonic anhydrase (CA) activity inhibitor, acetazolamide, and the plasma membrane P-type H+-ATPase inhibitor, vanadate. These results suggested that carbon acquisition from the natural seawater was mostly through the external CA-mediated dehydration mechanism, and that P-type H+-ATPase (proton pump) in the plasma membrane simultaneously functioned in photosynthesis of E. binghamiae. Additional experiments on the O2 exchange versus pH value relationship indicated that, in contrast to photosynthesis, dark respiration of E. binghamiae was insensitive to the change of pH in the seawater, which resulted in a decreasing instantaneous balance between net carbon gain and respiratory carbon loss at high pH values in seawater.