COMPONENTS OF PHOTOSYNTHESIS IN THE INTERTIDAL SACCATE ALGA HALOSACCION-AMERICANUM (RHODOPHYTA, PALMARIALES)

Rates of photosynthesis for the intertidal saccate alga Halosaccion americanum Lee were determined under submersed and emersed conditions. By fitting the data to a hyperbolic tangent function, Pmax was 4.08 mmol CO6 .cntdot. m-2.cntdot.h-1 and Ik was 116.4 .mu.E.cntdot.m-2.cntdot.s-1 under submersed conditions. Under emersed conditions, Pmax was 1.89 mmol CO2.cntdot.m-2.cntdot.h-1 and Ik was 22.9 .mu.E.cntdot.m-2.cntdot.s-1. Dark fixation represented 3.7% of Pmax in submersed thalli, whereas it equalled 33.3% of Pmax in emersed thalli. Photosynthetic uptake from the thallus cavity represented a significant source of carbon, achieving 68.8% of that from the atmosphere and 29.4% of that from seawater. Retained seawater also greatly reduced drying under emersed conditions. Experimental thalli lost 70.4% of their water after 120 min under desiccating conditions, whereas control thalli lost only 6.3%. Emersed photosynthetic rates were enhanced by desiccation. At times, rates for desiccated thalli were two times those of fully-hydrated ones. Only after water loss exceeded 47% did photosynthetic rates fall below fully-hydrates rates. Utilizing data from this study a model was constructed to determine total photosynthetic production of H. americanum over a single daylight period. These calculations demonstrate that photosynthetic contributions from emersed photosynthesis and retained seawater are significant. Because production from all sources is almost equal, total photosynthetic over a single days does not change greatly regardless of the time spent in air or in water.