When people think of coral reefs, they usually imagine warm, shallow tropical waters; not cold, dark deep ocean waters. But there are coral reefs living in the dark at the bottom of the ocean as well. They are hard to access and not as vibrant as their warm-water relatives, still they harbor almost as many species and serve as nursery grounds for many commercially important fish . These deep-sea coral reefs are primarily formed by the reef-building coral Desmophyllum pertusum (formerly Lophelia pertusa) .
These reefs are abundant in every ocean except the pacific, and provide critical habitat for thousands of marine species, which, in turn, provide food security to people living in coastal regions . Yet, these hidden, deep reefs face severe pressures from both climate change and other anthropogenic factors such as bottom trawling, oil and gas exploitation, deep sea mining, pollution, and waste disposal . Therefore, active restoration measures must be taken to support and enhance their recovery.
One of the factors that could improve restoration success is the establishment of healthy interactions between the coral host and its microbiome. Host associated microbes include bacteria, archaea, protists, fungi, and viruses. They can contribute to energy and nutrient acquisition, reproduction, mitigation of toxic compounds, pathogen control, and induction of coral larvae settlement . Despite their known beneficial effects, the role of the microbiome of cold-water corals during coral larvae settlement and recruitment remains unclear .
In November 2022, I went to the Tjärnö Marine Laboratory in Sweden to investigate these corals’ microbiome. Thanks to the projects LIFE Lophelia  and RESTORESEAS, which has amongst its goals to restore lost reefs in the marine protected area of Kosterfjorden-Väderöfjordens Natura 2000, I was able to sample corals from the species D. pertusum at a depth of 100 m. The samples were collected using a remote operated vehicle (ROV) equipped with a robotic arm and collection basket. I was able to see the beauty of the deep-sea reefs for the first time over the video feed of the ROV. There I saw countless deep-sea sponges, fish, squat lobsters and many more animals residing in the reef structures.
Seeing such an abundance of life at 100 m below the ocean surface, motivated me even more to protect the reefs that live and grow there. The collected samples will provide me with insights into the microbial composition associated with D. pertusum and hints on which microbes induce settlement, leading to higher restoration success rates. In the future, I will conduct settling trials in the aquarium of the Marine Laboratory.
My next tasks are to find the best DNA-extraction methods to acquire the genetic material of the samples and uncover the genetic code of the microbiome by Nanopore sequencing.
Written by Christian Pruckner, Natural History Museum Vienna
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