Deep-Sea Corals & Cold Seeps: Unraveling a Mysterious Relationship

Corals in the deep sea are some of the oldest living animals on Earth. They create a framework that provides a home and food for many other creatures. These corals can also survive in unusual places like cold seeps, where gases seep from the seafloor. We knew little about the relationship between corals and cold seeps or if corals benefited from or were negatively affected by them.

A recent discovery led by scientists from the Pennsylvania State University has shed light on this mystery. They found that corals can feed on organic material that originated from chemosynthesis fueled by the gases from the cold seeps, not just the photosynthetically derived food trickling down from the sunlight ocean surface water, as previously thought. This new research, published in the journal Global Change Biology, also showed that corals' ability to consume this chemosynthetic food was linked to specific bacteria known to process sulfur compounds to conduct chemosynthesis.

Cold seeps are unique areas on the seafloor where methane and other substances leak out from the Earth's crust. These seeps harbor special ecosystems, and the animals that live there have developed unique adaptations to survive in these conditions. Some of these adaptations include the ability to use food produced by bacteria. These bacteria transform chemicals like methane and sulfur into organic matter via a process called chemosynthesis. Because sunlight does not reach the deep sea, primary production through photosynthesis is limited to organic material that sinks down from the surface waters to the deep ocean. Therefore, chemosynthetic food is vital for the survival of organisms in cold seep areas, creating small "oases" of life in the middle of vast mud flats. Additionally, many cold-seep animals have specialized enzymes in their cells that help them break down and detoxify harmful substances found in the seep environment. These adaptations, among others, have enabled cold-seep animals to thrive in these fascinating habitats.


Small organisms climb corals to live and reach higher ground to obtain food. (Image credit: ECOGIG/Erik Cordes)

Deep-sea corals have long been observed to live near cold seeps in different parts of the world's oceans, but the nature of their relationship with these seeps remains a mystery. While it was known that many other organisms obtained nourishment through chemosynthesis, there was no evidence of corals deriving their food from chemosynthetic sources, not even in fossil samples. As a result, it was widely believed that corals primarily used cold seeps as a sturdy substrate once the release of seep chemicals ceased. The research team focused on studying two coral species, Callogorgia delta and Paramuricea sp. type B3, found near and far from active seepage sites in the deep Gulf of Mexico to evaluate this claim.

The researchers discovered that deep-sea corals also have the ability to actively feed on chemosynthetic food sources. Additionally, they noticed that the types of bacteria (microbiome) associated with corals near seeps were different from those found further away. Both coral species showed an increase in a specific bacterial symbiont called SUP05, which is also found in other seep animals. This bacterial symbiont plays a crucial role in enabling seep organisms to consume sulfur compounds and convert them into chemosynthetic food. Harboring chemosynthetic bacteria is thought to be beneficial for coral survival because it allows corals to obtain food indirectly from cold seeps.

When researchers compared corals living near cold seeps to those in non-seep areas, they found no noticeable differences in their overall health, growth rate, amount of living tissue, or ability to recover from injuries. Even the smaller organisms living on the corals, called epifauna, were similar between the seep and non-seep colonies, showing no significant changes. Interestingly, the study also revealed a positive correlation between the presence of chemosynthetic food and healthier tissue in coral colonies. In other words, corals near cold seeps are just as healthy as those elsewhere, and they enjoy extra benefits from the nearby gas emissions, which provide them with an additional source of nutrients.

The discovery sheds new light on the complex relationship between corals and cold seeps, expanding our understanding of ecological dynamics in the deep-sea environment. Deep-sea corals are long-lived and grow slowly, and by gaining a deeper understanding of these majestic species, we can take more effective steps to ensure their continued contributions to the overall health and balance of the marine environment. This highlights the intricate and complex connections that exist within our oceans, underscoring the significance of safeguarding these delicate habitats. As we continue to explore the wonders of the deep sea, it becomes increasingly clear that preserving these ecosystems is crucial for the long-term health of our planet.

To dive into the full article, visit: https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.16447

This feature appeared in Environment, Coastal & Offshore (ECO) Magazine's 2023 Deep Dive II special edition Marine Environmental Research, to read more access the magazine here.

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