Physiological responses of Gracilariopsis longissima (S.G. Gmelin) Steentoft, L.M. Irvine and Farnham (Rhodophyceae) to sub-lethal copper concentrations

Abstract

Through a series of comparative experiments the relative effects of copper (Cu) exposure on the growth and physiology (chlorophyll fluorescence, ion leakage, O(2) evolution and pigmentation) of the red seaweed Gracilariopsis longissima was investigated. Of the various physiological end-points measured, growth proved to be the most sensitive with reductions in relative growth rate (RGR) observed at a concentration of 12.5 microg l(-1) Cu, with zero growth above about 300 microg l(-1). A significant increase in ion leakage and reduction in phycobiliprotein concentrations were evident, but only at the highest concentration tested (500 microg l(-1)), at which point shrinkage of apical tips also occurred. Photosynthetic rates, as measured by chlorophyll fluorescence and oxygen evolution, were first impaired at 250 microg l(-1), with a 30% reduction in photosynthetic efficiency (F(v)/F(m)) and a 60% reduction in oxygen evolution. There were no discernible effects on respiration rates or chlorophyll a and beta-carotene content over this concentration range. It was hypothesised that the observed uncoupling of growth and photosynthesis at low Cu concentrations might be explained by the release of dissolved organic matter (DOC), resulting in less available energy for growth. No such increase in DOC was apparent. Alternative explanations to account for the uncoupling, including the diversion of energy for maintenance of cell integrity and induction of protective mechanisms, are discussed.