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

Abstract

The cold tolerance capacity of samples of the marine green alga Caulerpa taxifolia, obtained from Moreton Bay, Brisbane, Australia, was investigated by exposing samples to seawater temperatures of 9 to 15C, for periods of 4 to 10 wk, after maintenance at 22C. Residual effects of cold water exposure were evaluated by re-acclimating samples to 22C. Phenotypic expression and survivorship were monitored throughout both cold treatment and re-acclimation phases. Measurements of photosynthesis and respiration were made toward the end of the cold treatments and after re-acclimation. Samples exposed to 9 and 11C water exhibited retraction or loss of chloroplasts (or chlorophyll) from the mid-rib regions of the pseudo-fronds. After 4 wk of exposure to 9C the only green coloured regions of the fronds were the extremities of the pinnules; 1 to 2 wk later these samples began to decompose. Samples kept at 11C retained the bulk of their photosynthetic pigments and survived throughout experiments. The stolons of samples tended to grow upward toward the seawater surface rather than parallel to the substratum. Samples at all treatment temperatures tended to become progressively detached from the substratum through stolon 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, buoyed by metabolic gases trapped in the mucilage. During cold treatment, the maximal rates of gross and net photosynthesis (P m g and P m n ), normalised to dry weight or tissue protein content, and the ratio of P m g to dark respiration (-R) varied directly with temperature. The irradiance required for compensation (I c ) varied inversely with temperature. The rate of dark respiration increased with cold exposure. Calculation of whole day net production indicated that rates of photosynthesis among samples incubated at 13C or below were insufficient to maintain existing tissue biomass. There were no residual effects of cold exposure after re-acclimation, except that 11C samples failed to increase in relative protein content as a function of ambient irradiance, perhaps due to nitrogen limitation. Results indicate a cold-tolerance threshold of 9 to 11C, thus genetic modification does not need to be invoked to explain the occurrence of C. taxifolia in the northwest Mediterranean.