Effect of low water temperature on metabolism and growth of a subtropical strain of Caulerpa taxifolia (Chlorophyta)

The cold tolerance capacity of samples of the marine green alga Caulerpa ta xifolia, obtained from Moreton Bay, Brisbane, Australia, was investigated b y exposing samples to seawater temperatures of 9 to 15 degrees C, for perio ds of 4 to 10 wk, after maintenance at 22 degrees C. Residual effects of co ld water exposure were evaluated by re-acclimating samples to 22 degrees C. Phenotypic expression and survivorship were monitored throughout both cold treatment and re-acclimation phases. Measurements of photosynthesis and re spiration were made toward the end of the cold treatments and after re-accl imation. Samples exposed to 9 and 11 degrees C water exhibited retraction o r loss of chloroplasts (or chlorophyll) from the mid-rib regions of the pse udo-fronds. After 4 wk of exposure to 9 degrees C the only green coloured r egions of the fronds were the extremities of the pinnules; 1 to 2 wk later these samples began to decompose. Samples kept at 11 degrees C retained the bulk of their photosynthetic pigments and survived throughout experiments. The stolons of samples tended to grow upward toward the seawater surface r ather than parallel to the substratum. Samples at all treatment temperature s tended to become progressively detached from the substratum through stole n extension without intermittent rhizoid attachment. Several samples became fragmented and covered with mucilaginous epiphytes during re-acclimation, such that those lacking rhizoid anchorages, rose to the seawater surface, b uoyed by metabolic gases trapped in the mucilage. During cold treatment, th e maximal rates of gross and net photosynthesis (P-m(g) and P-m(n)), normal ised to dry weight or tissue protein content, and the ratio of P-m(g) to da rk respiration (-R) varied directly with temperature. The irradiance requir ed for compensation (I-c) varied inversely with temperature. The rate of da rk respiration increased with cold exposure. Calculation of whole day net p roduction indicated that rates of photosynthesis among samples incubated at 13 degrees C or below were insufficient to maintain existing tissue biomas s. There were no residual effects of cold exposure after re-acclimation, ex cept that 11 degrees C samples failed to increase in relative protein conte nt as a function of ambient irradiance, perhaps due to nitrogen Limitation. Results indicate a cold-tolerance threshold of 9 to 11 degrees C, thus gen etic modification does not need to be invoked to explain the occurrence of C. taxifolia in the northwest Mediterranean.