Utilization of different dissolved organic phosphorus sources by Symbiodinium voratum in vitro

This study examines the physiological responses of the Symbiodiniumvoratum (clade E) to two types of phosphates having different chemical bonds-phosphoesters (C-O-P bonds) and phosphonates (C-P bonds) to explore Symbiodinium cell growth and the molecular perspective of the P utilization process. Alkaline phosphatase (AP), PhnX, PhoA and PhoX expression was profiled for different P conditions using the RT-qPCR method. In a sterile system, Symbiodinium could decompose phosphoesters, such as ATP and glucose 6-phosphate (G-6-P), into dissolved inorganic P (DIP) to supplement inorganic phosphorus but could not directly use phosphoesters for growth. The growth rate and photosynthetic efficiency of zooxanthellae in phosphoester-containing media did not significantly differ from those in the DIP group but were significantly inhibited in medium containing phosphonates such as N-(phosphonomethyl)glycine (glyphosate) and 2-aminoethylphosphonic acid (2-AEP), as well as in DIP-poor medium. The phosphonate group DIP concentration did not change remarkably, indicating that phosphonates can neither be directly used by zooxanthellae nor decomposed into DIP. Our RT-qPCR results support our views that the phosphoesters (C-O-P) had been hydrolyzed outside the cell before being absorbed into the Symbiodinium cell, and implies that PhnX, PhoA and PhoX are perhaps responsible for transporting DIP from medium into cells and for storage of DIP.