Salt-Stimulated Bicarbonate-Dependent Photosynthesis in the Marine Angiosperm<i>Zostera muelleri</i>

Abstract

Millhouse, J. and Strother, S. 1986. Salt-stimulated bicarbonate-dependent photosynthesis in the marine angiosperm Zostera muelleri.—J. exp. Bot. 37: 965–976. Photosynthetic oxygen evolution in the seagrass Zostera muelleri Irmisch ex Aschers. was inhibited in iso-osmotic sucrose. The apparent affinity of the leaves for CO2 in seawater increases from pH 8·2 to 8·9 indicating that HCO3- as well as CO2 may act as a substrate for photosynthesis. The affinity for CO2 was lower in iso-osmotic sucrose and was not affected by pH. Under these conditions HCO3- was not a substrate for photosynthesis. The difference between the photosynthetic rate in seawater and iso-osmotic sucrose at the same concentration of CO2 was used to estimate HCO3- assimilation. The Briggs-Maskell equation, which allows for an unstirred layer around the tissue was more appropriate than the Michaelis-Menten theory for calculating the apparent affinity of the leaf slices for CO2. The apparent Km CO2 was calculated as 116 mmol m−3 at pH 8·2 by Michaelis-Menten kinetics but only 8·10 mmol m−3 by the Briggs-Maskell equation. The stimulation by various ions in Seawater of HCO3- use was investigated. The cations, in decreasing order of effectiveness were Ca2+, Mg2+, K+ and Na+ Anions were ineffective. No single cation at its concentration in seawater was capable of supporting HCO3- use at the rate observed in seawater. Acetazolamide, an inhibitor of carbonic anhydrase, inhibited the use of HCO3- for photosynthesis but had little effect on CO2 photosynthesis. Thus, carbonic anhydrase activity is required for HCO3--dependent photosynthesis.