Unlocking the role of the symbiotic community in the calcification process of the temperate coral Astrangia poculata

Symbiotic and aposymbiotic colonies of the temperate coral Astrangia poculata were reared in 15ºC, 27ºC, or ambient conditions. Scanning electron microscopy (SEM) was used to describe how these physiological and environmental conditions impact skeletal structure as well as to examine the morphology of the calicoblastic ectoderm. Buoyant weight data over time showed that symbiont state and temperature both significantly affect growth rates. SEM of A. poculata skeletons revealed that aposymbiotic colonies appear to have a lower density of calcium carbonate at growing septal spines. Quantitative analysis of roughness of septal spines revealed that aposymbiotic colonies have a rougher surface texture than symbiotic colonies. This roughness trend is strongest in the colonies reared at 27ºC, which were also the fastest growing colonies. SEM of both the skeleton and tissue revealed a pervasive presence of bioeroders in the skeleton. Light sheet microscopy using the L-SPI Single Plane Illumination system was used to confirm and characterize this community of bioeroders. Finally, we studied skeletons of the tropical corals Porites astreoides and Acropora cervicornis to understand how A. poculata skeletal structure compares to its tropical counterparts. Few studies have examined the skeleton of A. poculata or corals in general using SEM. These results unlock new insights into the skeletons of temperate corals and the role of the associated community in the calcification process.