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Assessing physiological responses that correspond to the normal range of seasonal variation can provide a better understanding of how environmental stressors may impact physiology. Most tropical corals exhibit seasonal variation in their host and symbiont physiology within a narrow range of environmental conditions. In temperate regions and at the northern end of its distribution, Astrangia poculata must adapt to wide ranges in seasonal variability. The species is facultatively symbiotic, and it is unclear if or how symbiotic state and, consequently, host physiology is affected by environmental seasonality. We collected colonies of A. poculata with a visible range of symbiotic states from Fort Wetherill State Park in Jamestown, RI in fall, winter, spring, and summer seasons of 2018–2019. We measured physiological parameters, including symbiotic state [chlorophyll (Chl) a and c2], total lipid content, and stable carbon (δ13C) and nitrogen (δ15N) isotopes of the host and symbiont. Seasonal variation occurred in all physiological parameters we studied. Specifically, Chl a, c2, and lipid content all reached low points in the spring, suggesting a lag, where the consequences of the coldest temperatures in the winter took up to three months to manifest in the tissue. There were seasonal fluctuations in host:symbiont ratios of δ13C, reflecting changing rates of autotrophy relative to heterotrophy during the year. While some autotrophy occurred during the year, isotopic evidence indicated that carbon acquisition in A. poculata was mostly heterotrophic in the winter. Based on δ15N, the symbiont was primarily responsible for nitrogen assimilation, although other sources likely contributed. Both carbon acquisition and nitrogen acquisition were more similar to that of other aposymbiotic coral species, regardless of the symbiotic state of A. poculata. Therefore, it may be more appropriate to view A. poculata as a unique aposymbiotic coral that is capable of symbiosis, rather than the reverse. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

5 citations (Crossref) [2023-10-31]