Light-harvesting of mesophotic corals

Netanel Kramer, a Ph­D candidate at Tel-Aviv University, discusses with us his recently accepted paper, “Efficient light-harvesting of mesophotic corals is facilitated by coral optical traits.

About the Paper

Admiring the beauty of a coral. © Netanel Kramer
Admiring the beauty of a coral. © Netanel Kramer

Corals are light-dependent organisms that take the concept of “solar cells” very literally. Corals rely on a symbiotic relationship with tiny algae residing within their tissues to convert sunlight into energy. Since corals are powered by solar energy, they can be in the well-lit, shallow tropical waters down to depths of between 30 to 150 meters, also known as Mesophotic Coral Ecosystems. At such depths, light is extremely limited, as only 1-5% of solar irradiance penetrates these deep waters. And yet, we still find corals flourishing and growing under such low-light conditions! How then can corals at mesophotic depths survive and grow under extremely limited light conditions?

Previous studies have found that shallow-water corals are highly efficient at collecting and redirecting light compared to land-based plants. The corals’ highly optimized photosynthesizing capabilities are largely attributed to the light-scattering mechanisms within the coral tissue and skeleton. However, studies characterizing the bio-optical properties of mesophotic corals are still lacking. The fact that corals from the same species can occupy a wide depth bathymetry (e.g., 5-50m) suggests that they developed strategies and specialized optical modifications to cope with and adjust to extreme light environments. 

In this paper, my co-authors and I performed a comprehensive investigation on the photobiology of four widely depth-distributed corals and hypothesized that the optical properties of deep-water corals are better modified to utilize low-light than shallow corals. Our findings show that mesophotic corals were highly efficient at redistributing light to their symbiotic algae as compared to their shallow-water counterparts. The enhanced, light-harvesting capacity of mesophotic corals was achieved by a set of eco-physiological and optical traits, including enhanced light scattering by their skeletons, higher algae cell densities, and greater chlorophyll-a content. Combined, these characteristics facilitated up to three-fold higher light absorption compared to their shallow counterparts!

About the Research

The comparison of the light-harvesting capacities of mesophotic corals to that of their shallow-water conspecifics provides an unprecedented understanding of the coral-algae symbiosis in low-light environments. This work has allowed us to expand our current knowledge on how mesophotic corals function under dim-light conditions, highlighting the optical mechanisms that allow corals to grow at the lower photic zone.

These results and conclusions of this study should encourage future investigations into the role of bio-optics and irradiance in structuring coral communities along the depth gradient. Also, our findings suggest that the enhanced light-harvesting capabilities of mesophotic corals may render them specifically susceptible to environmental disturbances and coral bleaching. As such, my next step will be to investigate the impact of various environmental changes on mesophotic corals. 

About the Author

Living next to the sea and ever since my first dive as a teen, the wonders below the ocean surface have always fascinated me, especially the beauty and design of coral reefs! During my studies for my undergraduate degree, I worked at the Underwater Observatory and Marine Park in Eilat, Israel, and performed a research project at the coral reefs of Eilat. The exposure to working with marine life has encouraged me to pursue a graduate degree.

Netanel Kramer collecting corals from 45m depth. © Ronen Liberman
Netanel Kramer collecting corals from 45m depth. © Ronen Liberman

Currently, I am a Ph.D. candidate based in Yossi Loya’s lab at Tel-Aviv University, Israel, and the location for my fieldwork is at the Interuniversity Institute for Marine Sciences (IUI) in Eilat, which provides convenient access to both the shallow and mesophotic reefs. I continue to explore the “unseen” coral reefs of deeper waters, and employ an interdisciplinary approach to explore the interaction of light with coral architecture in the fields of ecology, bio-optics, and ecophysiology. Following this paper, I am currently exploring how the macro- and micro-skeletal structures of corals influence light distribution in different coral morphologies and light regimes.

One of the best things about being an ecologist is that this field of science is so broad, there is always something new to learn and to study! My advice to others in this field is to endeavour to maintain a healthy balance between academic life and personal life. Find a hobby (whether it’s running on the beach, cooking, photography, and eating dark chocolate or something else) that can help you to unwind. Also, try to maintain a good attitude even when there are tough days (and I have had my fair share of those!), take a deep breath, look up and remember what caused you to become excited in the first place and why you do what you do.

Read the article in full here

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