A buffer sensitive inorganic carbon utilisation system in Zostera marina

Abstract

The effect of a buffer, tris(hydroxymethyl)aminomethane (TRIS), was determined 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. marina by some 70% as compared with rates in non-buffered seawater of the same pH. This suggests that proton extrusion, and the maintenance of acidic zones in the diffusion boundary layer, is of importance for supplying inorganic carbon (C i) to the photosynthesising cells of this plant. Acetazolamide (AZ), an inhibitor of periplasmic carbonic anhydrase (CA) activity, also reduced photosynthetic rates by some 20%, while an anion exchange inhibitor had no effect. The pattern of inhibition by AZ and TRIS is in agreement with 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 by which protons mediate the influx of C i may be via (non-catalysed) extracellular conversion of HCO 3 − to CO 2 within the acid zones or, alternatively, some type of direct uptake of the anion coupled with inwardly moving protons following their active extrusion. At pH 8.1, the additive effect of AZ and 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 dependencies show lower rates at normal C i conditions, and higher saturation concentrations, in the presence of buffers. This puts into question some of the high C i saturation points reported earlier for seagrasses, including Z. marina.