New insights on the role of nitrogen in the resistance to environmental stress in an endosymbiotic dinoflagellate.

Endosymbiotic dinoflagellates provide the nutritional basis for marine invertebrates, especially reef-building corals. These dinoflagellates are sensitive to environmental changes, and understanding the factors that can increase the resistance of the symbionts is crucial for the elucidation of the mechanisms involved with coral bleaching. Here, we demonstrate how the endosymbiotic dinoflagellate Durusdinium glynnii is affected by concentration (1760 vs 440 µM) and source (sodium nitrate vs urea) of nitrogen after light and thermal stress exposure. The effectiveness in the use of the two nitrogen forms was proven by the nitrogen isotopic signature. Overall, high nitrogen concentrations, regardless of source, increased D. glynnii growth, chlorophyll-a, and peridinin levels. During the pre-stress period, the use of urea accelerated the growth of D. glynnii compared to cells grown using sodium nitrate. During the luminous stress, high nitrate conditions increased cell growth, but no changes in pigments composition was observed. On the other hand, during thermal stress, a steep and steady decline in cell densities over time was observed, except for high urea condition, where there is cellular division and peridinin accumulation 72 h after the thermal shock. Our findings suggest peridinin has a protective role during the thermal stress, and the uptake of urea by D. glynnii can alleviate thermal stress responses, eventually mitigating coral bleaching events.