Evidence for phosphate-dependent control of symbiont cell division in the model anemone Exaiptasia diaphana

Abstract

Reef-building corals depend on symbiosis with photosynthetic algae that reside within their cells. As important as this relationship is for maintaining healthy reefs, it is strikingly delicate. When ocean temperatures briefly exceed the average summer maximum, corals can bleach, losing their endosymbionts. Although the mechanisms governing bleaching are unknown, studies implicate uncoupling of coral and algal cell divisions at high temperatures. Still, little is known regarding the coordination of host and algal cell divisions. Control of nutrient exchange is one likely mechanism. Both nitrogen and phosphate are necessary for dividing cells, and although nitrogen enrichment is known to increase symbiont density in the host, the consequences of phosphate enrichment are poorly understood. Here, we examined the effects of phosphate depletion on symbiont growth in culture and compared the physiology of phosphate-starved symbionts in culture to symbionts that were freshly isolated from a host. We found that available phosphate is as low in freshly isolated symbionts as it is in phosphate-starved cultures. Furthermore, RNAseq revealed that phosphate-limited and freshly isolated symbionts have similar patterns of gene expression for phosphate-dependent genes, most notably upregulation of phosphatases, which is consistent with phosphate recycling. Similarly, lipid profiling revealed a substantial decrease in phospholipid abundance in both phosphate-starved cultures and freshly isolated symbionts. These findings are important because they suggest that limited access to phosphate controls algal cell divisions within a host.