Insights from the 7th International Symposium of Deep Sea Corals (ISDSC7)
The deep sea is only starting to be revealed to science but if one thing it is known to be certain, is that deep-sea reefs are at least as diverse as their peers in shallower waters. As for shallow-water corals, deep-sea corals can be cataloged as individual coral polyps, forming colonies that contain many polyps of the same species which at the same time, constitute reefs of one or more species. Nonetheless, as expected, deep-sea corals don’t host photosynthetic symbionts as visible light doesn’t reach such depths and therefore, they would not be undergoing hazards like bleaching, in theory. Does this mean deep-sea corals are not vulnerable to the increasing temperatures in the ocean waters?
Anthropogenic input of carbon dioxide has resulted in warming oceans and a rapid development of acidified waters and shoaling of the aragonite saturation horizon [1, 2]. As explained by Dr. Leslie Wickes in her presentation at the 7th International Symposium of Deep Sea Corals (ISDSC7), coral suitable habitats are going through a big contraction process. This shrinkage goes from the top, conveyed by physiological stress due to the rising temperatures as is the case when bleaching events are observed, and from the bottom with decreased calcification rates affecting calcium carbonate-based organisms, like hard corals. With no doubt, the most harmful stressor related to human activities on the ocean ecosystems is the increased acidification. Ocean acidification is essentially the increase in free hydrogen ions concentration which results in reef dissolution , a decrease in fish reproduction rates , and harmful algal blooms (HABs) , among others. During the ISDSC7, the ocean acidification impacts on deep coral reefs were exposed by both, in-field experiments and under laboratory conditions. One of the most studied deep-sea corals is Lophelia pertusa, a stony coral widely distributed that has been proven to be an important reef actor in the climate resilience of protected areas and sanctuary resources. Several works presented at the ISDSC7 showed that L. pertusa growth and feeding rates, were undeniably being affected by acidification  while other studies, showed that at least 75% of the Northeast Atlantic reefs could not adapt to the changing conditions of the water related to acidification.
Therefore, the most logical question would be: how can we monitor and eventually how could we model ocean acidification to further predict its impacts on deep-sea reefs? Dr. S. Hennige was next to address this question during his talk Will we lose the reefs of the deep (and how will we know)? To answer this question, Dr. Hennige and collaborators studied the porosity of corals’ skeletons. Their results showed that the integrity of microscopical building blocks along the coral’s structure remained the same across differently acidified environments both for live and for dead corals but eventually, and after a prolonged exposure to acidified conditions, these building blocks started to get lose resulting in two different configurations: a protected skeleton configuration where all the blocks were upon, next and below other blocks and an exposed skeleton configuration where several blocks were missing along the structure. Interestingly, these results corroborate what has been previously observed at the macroscopic level about reefs tri-dimensionality: species that are more resilient to acidification or that lack calcium carbonate-based skeletons have an obvious advantage.
Lastly, I would like to finish by drawing attention to a message from Dr. Marco Taviani’s keynote presentation on the first day of the symposium: protect what is needed to be safeguarded first, sites weigh not equally.
 Gómez, C. E., Wickes, L., Deegan, D., Etnoyer, P. J., & Cordes, E. E. (2018). Growth and feeding of deep-sea coral Lophelia pertusa from the California margin under simulated ocean acidification conditions. PeerJ, 6, e5671. https://doi.org/10.7717/peerj.5671
 NCCOS – National Center for Ocean Costal Sciences. Vulnerability of Deep-Sea Coral Ecosystems to Ocean Acidification. NOAA: https://coastalscience.noaa.gov/project/vulnerability-deep-sea-coral-ecosystems-ocean-acidification/
 Cooley, S. R., & Doney, S. C. (2009). Anticipating ocean acidification’s economic consequences for commercial fisheries. Environmental Research Letters, 4(2), 024007. https://doi.org/10.1088/1748-9326/4/2/024007
 Paerl, H. W., Gardner, W. S., Havens, K. E., Joyner, A. R., McCarthy, M. J., Newell, S. E., … Scott, J. T. (2016). Mitigating cyanobacterial harmful algal blooms in aquatic ecosystems impacted by climate change and anthropogenic nutrients. Harmful Algae, 54, 213–222. https://doi.org/10.1016/j.hal.2015.09.009
 Etnoyer, P. J., Wickes, L. N., Silva, M., Dubick, J. D., Balthis, L., Salgado, E., & MacDonald, I. R. (2016). Decline in condition of gorgonian octocorals on mesophotic reefs in the northern Gulf of Mexico: Before and after the Deepwater Horizon oil spill. Coral Reefs, 35(1), 77–90. https://doi.org/10.1007/s00338-015-1363-2
 Hennige, S. J., Wicks, L. C., Kamenos, N. A., Perna, G., Findlay, H. S., & Roberts, J. M. (2015). Hidden impacts of ocean acidification to live and dead coral framework. Proceedings of the Royal Society B: Biological Sciences, 282(1813), 20150990. https://doi.org/10.1098/rspb.2015.0990