SEASONAL ADAPTATION AND SHORT-TERM METABOLIC RESPONSES OF GELIDIUM-SESQUIPEDALE TO VARYING LIGHT AND TEMPERATURE

The responses of Gelidium sesquipedale productivity, photosynthetic efficiency and respiration to changes in light and temperature were measured on plants acclimated to different light/temperature combinations simulating average winter and summer conditions at 2 different depths. Photosynthetic efficiency was reported relative to frond surface area computed by a method described in this study. Algal pigment contents were monitored during acclimation. Results suggest that seasonal acclimation to light and temperature may have a significant effect on photosynthetic efficiency and on the dependence of productivity on light and temperature. This effect may be the opposite of what is generally expected from acclimation to photon flux density alone but is in good agreement with previously reported results indicating an increase in photosynthetic efficiency in plants grown at higher temperatures. There is some evidence that temperature adaptation may compensate light adaptation through changes at physiological levels other than the pigmentary system. Significant seasonal effects on respiration, as well as a strong temperature dependence, are apparent. Winter acclimated plants had higher respiratory rates than summer acclimated ones. Among the former, plants acclimated to the greater depth exhibited lower respiratory rates than those acclimated to the shallower depth, which is an important adaptation for surviving under low light conditions by reducing energetic losses. The compensation photon flux density values obtained in this work for G, sesquipedale suggest that during winter the average net photosynthesis may be negative for a significant part of its subtidal populations.