Net photosynthesis and respiration of the red algaHypnea spiciferain response to changes in temperature and desiccation

With the aid of a Gilson respirometer, net photosynthesis and respiration were measured in Hypnea spicifera (Suhr) Harv. apud J. Ag. at temperatures of 15, 20, 25 and 30°C under both immersed and emersed conditions. The effects of increasing desiccation on net photosynthesis and respiration after re-immersion were also determined. Up to 25°C, both under immersed and emersed conditions, net photosynthesis increased with an increase in temperature, with generally higher rates under immersed conditions. However, at 30°C under immersed conditions, the rate decreased to the values at 25 and 30°C under emersion. Respiration did not follow the same trends. With increase in temperature above 20°C there was a significant increase in respiration and, at all temperatures, respiration rates were significantly higher under emersed than under immersed conditions. Increased levels of desiccation caused significant reductions in net photosynthesis and respiration. After re-immersion, rates of both photosynthesis and respiration were proportional to those at the previous level of desiccation. The low rates of net photosynthesis with emersion and concomitant desiccation may explain the restriction of this species to the lower shore all along the coast.