The ocean absorbs about one-third of anthropogenic carbon dioxide (Sabine et al., 2004), resulting in a decrease in oceanic pH. As pH drops, the concentration of carbonate ions decreases. Together, carbonate ion availability and pH exert a strong influence over the rates of calcification in organisms that use calcite or aragonite for skeletal, test, and shell formation (Georgian, DeLeo, et al., 2016;Melatunan, Calosi, Rundle, Widdicombe, & Moody, 2013;Waldbusser et al., 2015). The aragonite saturation horizon (ASH) is the boundary between waters saturated and undersaturated with respect to aragonite and is a potential boundary between net calcification and net dissolution of coral skeleton. The ASH currently lies between approximately 200 and 2,000 m in the world's ocean . This is deeper than the depth range that corals with photosynthetic symbionts can live, but nearly all cold-water AbstractCold-water corals (CWCs) are important foundation species in the world's largest ecosystem, the deep sea. They support a rich faunal diversity but are threatened by climate change and increased ocean acidification. As part of this study, fragments from three genetically distinct Lophelia pertusa colonies were subjected to ambient pH (pH = 7.9) and low pH (pH = 7.6) for six months. RNA was sampled at two, 4.5, and 8.5 weeks and sequenced. The colony from which the fragments were sampled explained most of the variance in expression patterns, but a general pattern emerged where upregulation of ion transport, required to maintain normal function and calcification, was coincident with lowered expression of genes involved in metabolic processes; RNA regulation and processing in particular. Furthermore, there was no differential expression of carbonic anhydrase detected in any analyses, which agrees with a previously described lack of response in enzyme activity in the same corals. However, one colony was able to maintain calcification longer than the other colonies when exposed to low pH and showed increased expression of ion transport genes including proton transport and expression of genes associated with formation of microtubules and the organic matrix, suggesting that certain genotypes may be better equipped to cope with ocean acidification in the future. While these genotypes exist in the contemporary gene pool, further stresses would reduce the genetic variability of the species, which would have repercussions for the maintenance of existing populations and the ecosystem as a whole. K E Y W O R D S coral, deep sea, ocean acidification, RNAseq, stress response, transcriptomics S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section.

Microbial communities associated with deep-sea animals are critical to the establishment of novel biological communities in unusual environments. Over the past few decades, rapid exploration of the deep sea has enabled the discovery of novel microbial communities, some of which form symbiotic relationships with animal hosts. Symbiosis in the deep sea changes host physiology, behavior, ecology, and evolution over time and space. Symbiont diversity within a host is often aligned with diverse metabolic pathways that broaden the environmental niche for the animal host. In this review, we focus on microbiomes and obligate symbionts found in different deep-sea habitats and how they facilitate survival of the organisms that live in these environments. In addition, we discuss factors that govern microbiome diversity, host specificity, and biogeography in the deep sea. Finally, we highlight the current limitations of microbiome research and draw a road map for future directions to advance our knowledge of microbiomes in the deep sea. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

There are numerous studies highlighting the impacts of direct and indirect stressors on marine organisms, and multi-stressor studies of their combined effects are an increasing focus of experimental work. Lophelia pertusa is a framework-forming cold-water coral that supports numerous ecosystem services in the deep ocean. These corals are threatened by increasing anthropogenic impacts to the deep-sea, such as global ocean change and hydrocarbon extraction. This study implemented two sets of experiments to assess the effects of future conditions (temperature: 8 °C and 12 °C, pH: 7.9 and 7.6) and hydrocarbon exposure (oil, dispersant, oil + dispersant combined) on coral health. Phenotypic response was assessed through three independent observations of diagnostic characteristics that were combined into an average health rating at four points during exposure and recovery. In both experiments, regardless of environmental condition, average health significantly declined during 24-hour exposure to dispersant alone but was not significantly altered in the other treatments. In the early recovery stage (24 hours), polyp health returned to the pre-exposure health state under ambient temperature in all treatments. However, increased temperature resulted in a delay in recovery (72 hours) from dispersant exposure. These experiments provide evidence that global ocean change can affect the resilience of corals to environmental stressors and that exposure to chemical dispersants may pose a greater threat than oil itself. The combination of human population growth and global industrial development is driving potentially irreversible anthropogenic impacts on the natural world. The current rate and scale of human development is unparalleled, and evidence has shown that human actions are altering the global climate and environment at startling rates 1-4. Increased atmospheric CO 2 concentrations contribute to both a decrease in global ocean pH (i.e. ocean acidification) as well as an increase in ocean temperatures 5-7. Sea surface temperatures have increased on average by 0.6 °C over the past 100 years and models suggest that the deep-sea is warming in some areas at a rate of 0.01-0.1 °C per decade 4,8,9. The oceans' surface and bathyal depths (200-4,000 m) are likely to experience increases in temperature of up to 4 °C by 2100 4,10,11. Furthermore, the ocean's continuous absorption of excess atmospheric CO 2 results in a decrease in pH; the surface ocean pH is already 0.1 units lower than preindustrial times and may decline by another 0.3-0.4 units by 2100 12-14. In addition to a changing global climate, human activities continue to expose oceanic environments to a wide range of pollutants 15. Hydrocarbon pollution is one of the most common and widespread forms of marine pollution and can have long-standing effects on marine environments despite standing regulations and monitoring efforts 16. Upwards of 1.3 million tons of oil is released annually into marine environments; however, roughly 47% of the oil released is from natural seep...

No abstract