Photosynthetic responses in Zostera marina to decreasing salinity, inorganic carbon content and osmolality

Abstract

In this investigation, seawater (33‰) was diluted with respect to salinity, inorganic carbon and osmolality and the effects on photosynthesis in Zostera marina leaves were measured with oxygen electrodes. It was found that the decrease in photosynthetic activity, seen after dilution of the seawater medium with distilled water, mainly could be explained by the decrease in the inorganic carbon content. Decreased salinity, i.e. dilution of osmolytes but retained ambient level of inorganic carbon (2 mM), did not affect the photosynthetic activity significantly. Dark respiration in the leaves remained unaffected even after severe dilution (seawater : distilled water, 1 : 5 v/v). Exposure to acetazolamide, an inhibitor of extracellular carbonic anhydrase, caused a 65% reduction of net photosynthesis compared to control in ambient seawater (pH 8.2 and 33‰). It is discussed whether inorganic carbon limitation, especially in waters with seasonally low light availability, could affect the whole plant carbon balance negatively and thereby limit its distribution.