PHOTOSYNTHETIC RESPONSE TO TEMPERATURE AND DESICCATION OF THE INTERTIDAL ALGA MASTOCARPUS-PAPILLATUS

When exposed to air during low tide, intertidal macroalgae experience a terrestrial environment and often encounter extreme levels of heatin g and desiccation. Two aspects of photosynthesis may be influenced by this increase in temperature and decrease in water content during expo sure to air: (1) the rate of aerial photosynthesis itself, and (2) the rate at which aquatic photosynthesis recovers upon immersion in water at high tide. This laboratory study examines the effect of air temper ature and desiccation on photosynthesis of the intertidal macroalga Ma stocarpus papillatus Kutzing. Plants were collected at Hopkins Marine Station, California, USA (36-degrees-37'N; 121-degrees-54'W) between J uly and December 1990. When apical tips were exposed to 15 to 25-degre es-C air for 2 h, photosynthesis was rapidly recovered upon reimmersio n in seawater. Recovery was delayed, but complete, when tissue was exp osed to 30-degrees-C air, but did not occur after exposure to 35-degre es-C air. Desiccation did not influence either the rate or the ultimat e level of recovery upon reimmersion. Photosynthesis in air generally decreased with increasing desiccation, with no net photosynthesis occu rring below 25% relative water content. Net photosynthesis of hydrated thalli increased with air temperature from 15 to 30-degrees-C, then d ecreased at 35-degrees-C. Dark respiration of hydrated thalli increase d over the entire temperature range. This study indicates that thallus heating and desiccation during periods of exposure to air can potenti ally influence the total carbon budget of M. papillatus.