Responses of respiration and photosynthesis of Scenedesmus protuberans Fritsch to gradual and steep salinity increases

Abstract

The effect of an increase in salinity on the physiology of the halotolerant chlorophyte Scenedesmus protuberans was studied in light-limited continuous cultures. It was observed that a gradual, as well as a steep increase in salinity resulted in lower biomass. However, the mechanisms by which this was achieved were different. In the culture that was exposed to a gradual salinity increase, respiration and the cellular protein content of the culture were initially unaffected. However, this culture was not able to maintain its cellular chlorophyll content and, consequently, gross and net photosynthesis decreased. The culture that was exposed to a steep salinity increase rapidly reacted by increasing its respiration and cellular protein content, which is ascribed to an induction of osmoregulation. This culture was able to maintain its gross photosynthesis rate. It is speculated that, in this species, a steep salinity increase induces a nearly immediate osmoregulatory response, allowing growth to continue. If the cells are exposed to a gradual salinity increase, induction of osmoregulation lags behind and, consequently, photosynthesis and growth rate will be affected. [KEYWORDS: DIATOM SKELETONEMA-COSTATUM, NITROGEN-METABOLISM, LIGHT, PHYTOPLANKTON, CYANOBACTERIA, STRESS, GROWTH, CHLOROPHYCEAE, PERFORMANCE, IRRADIANCE]