COMPONENTS OF PHOTOSYNTHESIS IN THE INTERTIDAL SACCATE ALGA HALOSACCION AMERICANUM (RHODOPHYTA, PALMARIALES)1

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Abstract
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, P max was 4.08 mmol CO2. m−2. h−1 and Ik was 116.4 μE. m−2. s−1. under submersed conditions. Under emersed conditions, P max was 1.89 mmol CO2. m−2. h−1 and Ik was 22.9 μE. m−2. 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‐hydrated rates. Utilizing data from this study a model was constructed to determine total photosynthetic production of H. americanum over a single daylight period. These caluclations demonstrate that photosynthetic contributions from emersed photosynthesis and retained seawater are significant. Because production from all sources is almost equal, total photosynthesis over a single day does not change greatly regardless of the time spent in air or in water.