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

In this investigation, seawater (33 parts per thousand) was diluted with re spect to salinity, inorganic carbon and osmolality and the effects on photo synthesis in Zostera marina leaves were measured with oxygen electrodes. It was found that the decrease in photosynthetic activity, seen after dilutio n of the seawater medium with distilled water, mainly could be explained by the decrease in the inorganic carbon content. Decreased salinity, i.e. dil ution of osmolytes but retained ambient level of inorganic carbon (2 mM), d id not affect the photosynthetic activity significantly. Dark respiration i n the leaves remained unaffected even after severe dilution (seawater : dis tilled water, 1 : 5 v/v). Exposure to acetazolamide, an inhibitor of extrac ellular carbonic anhydrase, caused a 65% reduction of net photosynthesis co mpared to control in ambient seawater (pH 8.2 and 33 parts per thousand). I t is discussed whether inorganic carbon limitation, especially in waters wi th seasonally low light availability, could affect the whole plant carbon b alance negatively and thereby limit its distribution. (C) 1999 Elsevier Sci ence B.V. All rights reserved.