A buffer sensitive inorganic carbon utilisation system in Zostera marina

The effect of a buffer, tris(hydroxymethyl)aminomethane (TRIS), was determi ned on the rate of net photosynthesis in Zostera marina. Addition of 50 mM TRIS (pH 8.1) to natural seawater reduced net photosynthetic rates of Z. ma rina by some 70% as compared with rates in non-buffered seawater of the sam e pH. This suggests that proton extrusion, and the maintenance of acidic zo nes in the diffusion boundary layer, is of importance for supplying inorgan ic carbon (C-i) to the photosynthesising cells of this plant. Acetazolamide (AZ), an inhibitor of periplasmic carbonic anhydrase (CA) activity, also r educed photosynthetic rates by some 20%, while an anion exchange inhibitor had no effect. The pattern of inhibition by AZ and TRIS is in agreement wit h the operation of two separate C-i utilising mechanisms, one sensitive to TRIS (but not to AZ) and one sensitive to AZ (but not to TRIS). The means b y which protons mediate the influx of C-i may be via (non-catalysed) extrac ellular conversion of HCO3- to CO2 within the acid zones or, alternatively, some type of direct uptake of the anion coupled with inwardly moving proto ns following their active extrusion. At pH 8.1, the additive effect of AZ a nd TRIS suggested that both mechanisms were operating in parallel. However, at a higher pH value (8.6), the AZ effect was negligible, suggesting that only the TRIS sensitive mechanism was operating. One of the implications of the present findings is that photosynthesis versus C-i concentration depen dencies show lower rates at normal C-i conditions, and higher saturation co ncentrations, in the presence of buffers. This puts into question some of t he high C-i saturation points reported earlier for seagrasses, including Z. marina. (C) 2001 Elsevier Science B.V. All rights reserved.