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The objectives of this sabbatical were to examine the resilience of temperate corals (Astrangia poculata, Ellis & Solander 1787) and to address the following questions: 1. Do corals exhibit quiescence at warmer temperatures?; 2. Does Astrangia poculata exhibit quiescence across their geographic range?; 3. Does the microbial population on corals change during quiescence?; and 4. Does temperature cause a change in symbiotic state in A. poculata?

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.

Temperate marine ecosystems globally are undergoing regime shifts from dominance by habitat-forming kelps to dominance by opportunistic algal turfs. While the environmental drivers of shifts to turf are generally well-documented, the feedback mechanisms that stabilize novel turf-dominated ecosystems remain poorly resolved. Here, we document a decline of kelp Saccharina latissima between 1980 and 2018 at sites at the southernmost extent of kelp forests in the Northwest Atlantic and their replacement by algal turf. We examined the drivers of a shift to turf and feedback mechanisms that stabilize turf reefs. Kelp replacement by turf was linked to a significant multi-decadal increase in sea temperature above an upper thermal threshold for kelp survival. In the turf-dominated ecosystem, 45% of S. latissima were attached to algal turf rather than rocky substrate due to preemption of space. Turf-attached kelp required significantly (2 to 4 times) less force to detach from the substrate, with an attendant pattern of lower survival following 2 major wave events as compared to rock-attached kelp. Turf-attached kelp allocated a significantly greater percentage of their biomass to the anchoring structure (holdfast), with a consequent energetic trade-off of slower growth. The results indicate a shift in community dominance from kelp to turf driven by thermal stress and stabilized by ecological feedbacks of lower survival and slower growth of kelp recruited to turf.

Facultatively symbiotic corals provide important experimental models to explore the establishment, maintenance, and breakdown of the mutualism between corals and members of the algal family Symbiodiniaceae. The temperate coral Astrangia poculata is one such model as it is not only facultatively symbiotic, but also occurs across a broad temperature and latitudinal gradient. Here, we report the de novo chromosome-scale assembly and annotation of the A. poculata genome. Though widespread segmental/tandem duplications of genomic regions were detected, we did not find strong evidence of a whole genome duplication (WGD) event. Comparison of the gene arrangement between A. poculata and the tropical coral Acropora millepora revealed 56.38% of the orthologous genes were conserved in syntenic blocks despite ∼415 million years of divergence. Gene families related to sperm hyperactivation and innate immunity, including lectins, were found to contain more genes in A. millepora relative to A. poculata. Sperm hyperactivation in A. millepora is expected given the extreme requirements of gamete competition during mass spawning events in tropical corals, while lectins are important in the establishment of coral-algal symbiosis. By contrast, gene families involved in sleep promotion, feeding suppression, and circadian sleep/wake cycle processes were expanded in A. poculata. These expanded gene families may play a role in A. poculata’s ability to enter a dormancy-like state (“winter quiescence”) to survive freezing temperatures at the northern edges of the species’ range.

Cycling of organic carbon in the ocean has the potential to mitigate or exacerbate global climate change, but major questions remain about the environmental controls on organic carbon flux in the coastal zone. Here, we used a field experiment distributed across 28° of latitude, and the entire range of 2 dominant kelp species in the northern hemisphere, to measure decomposition rates of kelp detritus on the seafloor in relation to local environmental factors. Detritus decomposition in both species were strongly related to ocean temperature and initial carbon content, with higher rates of biomass loss at lower latitudes with warmer temperatures. Our experiment showed slow overall decomposition and turnover of kelp detritus and modeling of coastal residence times at our study sites revealed that a significant portion of this production can remain intact long enough to reach deep marine sinks. The results suggest that decomposition of these kelp species could accelerate with ocean warming and that low-latitude kelp forests could experience the greatest increase in remineralization with a 9% to 42% reduced potential for transport to long-term ocean sinks under short-term (RCP4.5) and long-term (RCP8.5) warming scenarios. However, slow decomposition at high latitudes, where kelp abundance is predicted to expand, indicates potential for increasing kelp-carbon sinks in cooler (northern) regions. Our findings reveal an important latitudinal gradient in coastal ecosystem function that provides an improved capacity to predict the implications of ocean warming on carbon cycling. Broad-scale patterns in organic carbon decomposition revealed here can be used to identify hotspots of carbon sequestration potential and resolve relationships between carbon cycling processes and ocean climate at a global scale.