RESPONSES OF RESPIRATION AND PHOTOSYNTHESIS OF SCENEDESMUS PROTUBERANS FRITSCH TO GRADUAL AND STEEP SALINITY INCREASES

The effect of an increase in salinity on the physiology of the halotol erant chlorophyte Scenedesmus protuberans was studied in light-limited continuous cultures. It was observed that a gradual, as well as a ste ep increase in salinity resulted in lower biomass. However, the mechan isms by which this was achieved were different. In the culture that wa s exposed to a gradual salinity increase, respiration and the cellular protein content of the culture were initially unaffected. However, th is culture was not able to maintain its cellular chlorophyll content a nd, consequently, gross and net photosynthesis decreased. The culture that was exposed to a steep salinity increase rapidly reacted by incre asing its respiration and cellular protein content, which is ascribed to an induction of osmoregulation. This culture was able to maintain i ts gross photosynthesis rate. It is speculated that, in this species, a steep salinity increase induces a nearly immediate osmoregulatory re sponse, allowing growth to continue. If the cells are exposed to a gra dual salinity increase, induction of osmoregulation lags behind and, c onsequently, photosynthesis and growth rate will be affected.