Glycerol outflow in Symbiodinium under osmotic and nitrogen stress

In marine nutrient-poor environments, the success of Symbiodinium-cnidarian symbioses depends on the translocation of photosynthetically fixed carbon to the host. However, the mechanisms involved in this translocation are not well understood. Glycerol has been identified in the literature as one of the main photosynthates translocated from symbiont to host. Considering that glycerol is a preferred regulatory osmolyte in many unicellular eukaryotes and exposure of isolated symbionts to host homogenate results in the liberation of glycerol, we investigated the potential role of glycerol and its release in Symbiodinium. We studied the response to high osmolarity in two cultured species of Symbiodinium, examining glycerol production, specific activity of the enzyme glycerol 3-phosphate dehydrogenase and expression of the coding gene. We also assessed the production of glycerol and glucose under osmotic stress and nitrogen depletion. Results showed that cells exposed to high osmolarity conditions induced the synthesis of glycerol, although it was not retained effectively inside cells. In addition, nitrogen depletion also induced the synthesis of glycerol, although values were an order of magnitude lower. Interestingly, a significant decrease in glucose levels was detected in osmotically stressed cultures and under low nitrogen, possibly associated with the storage of carbon. While glycerol is a metabolite induced by osmotic stress, our results do not support its role as osmolyte. We conclude that glycerol may have a role as a sink for reducing power to prevent feedback inhibition of photosynthesis, particularly under low nitrogen, conditions that may be prevalent in the symbiosis.