Is the evolution of the coral-algal symbiosis linked to fluctuations in seawater magnesium concentrations?

Abstract:

While Scleractinia first appear in shallow tropical seas during the Mid-Triassic, it is unclear when and why these corals established their symbiosis with a dinoflagellate alga (Symbiodinium microadriaticum). The development of this symbiosis was a major evolutionary innovation for corals, which was not previously observed in other coral taxa (Rugosa and Tabulata) and likely contributed to the rise of Scleractinia as the dominant reef builders. Inarguably, this symbiotic relationship is linked to increased calcification rates but dinoflagellate symbioses are also very common in non-calcifying marine invertebrates making it unclear whether the coral host or algal symbiont drives the establishment of this symbiosis. Recently, it has been suggested that the establishment of the coral-algal symbiosis is symbiont driven by the fluctuation of the Mg/Ca ratio of seawater at the beginning of the Mesozoic. Scleractinia precipitate aragonitic skeletons further suggesting they evolved in seawater with a high Mg/Ca ratio and that their mineralogy is linked to their environment. In order to determine how seawater chemistry influences host-symbiont interactions, we grew Pocillopora damicornis in seawater with elevated calcium and magnesium concentrations. Growth rates are higher than the control treatment when the Mg²⁺ concentration is increased by 200 ppm but are not significantly different than the control treatment when the Ca²⁺ concentration is increased by 200 ppm, suggesting that calcification is linked to the Mg²⁺ concentration of seawater. Growth rates are not, however, related to in-hospite symbiont density, which is similar in the control, +200 ppm Ca²⁺ and +200 ppm Mg²⁺ treatments. This similarity in symbiont density between treatments suggests that even when the chemistry of the surrounding seawater fluctuates, with respect to Ca²⁺ and Mg²⁺ ions, the coral host provides a stable environment in which the symbionts can reside. This preliminary work has implications for understanding the evolution of scleractinian corals and future work will investigate the role of elevated Mg²⁺ concentrations on the physiology of symbionts grown ex-hospite.