Deep-Sea Corals in Hawaii Thrive in Dim Light

Jim Crocker
2nd March, 2024

Deep-Sea Corals in Hawaii Thrive in Dim Light

Image Source: Natural Science News, 2024

Key Findings

  • In Hawai'i's deep 'Au'au Channel, corals and algae thrive with minimal sunlight
  • These corals rely on both photosynthesis and consuming organic material for energy
  • Deeper corals share nitrogen more efficiently with their algae, aiding survival
In the depths of the ocean, beyond where most sunlight can reach, lie the mesophotic coral ecosystems, mysterious and less explored than their shallow-water counterparts. These communities thrive between 30 and 150 meters below the ocean's surface, in a world where sunlight is scarce. A recent study by researchers at Villanova University[1] has shed light on the survival strategies of reef-building corals and their symbiotic algae in these deep waters, particularly in the 'Au'au Channel off Maui, Hawai'i. Corals are fascinating organisms, partly because they have developed a symbiotic relationship with photosynthetic algae known as zooxanthellae. These algae live within the coral's tissues, providing it with energy through photosynthesis, while the coral provides the algae with a protected environment and the compounds necessary for photosynthesis. This partnership is crucial for the survival and growth of coral reefs. However, the question of how these corals survive in the dimly lit mesophotic zone has puzzled scientists. The study aimed to understand how depth-specialist corals balance the use of energy from light-dependent photosynthesis (autotrophy) and the consumption of organic material from their environment (heterotrophy). To investigate this, researchers analyzed the stable carbon (δ13C) and nitrogen (δ15N) isotope values of both the coral hosts and their algal symbionts across different species and depths ranging from 65 to 125 meters. Isotopes are forms of an element with different numbers of neutrons, and their ratios can tell us about the dietary habits and ecological roles of organisms. The findings revealed that the symbiont δ13C was consistently higher than that of the host coral by 0.5‰ at all depths. This indicates that despite the low light conditions, photosynthesis remains a critical energy source for these corals. Furthermore, the difference in δ15N between the hosts and their symbionts decreased with depth, suggesting a more efficient nitrogen sharing and reliance on autotrophy in deeper waters. These results are striking because they show that even in environments with less than 1% of surface irradiance, corals continue to rely significantly on their symbiotic algae for survival. The study also found that these isotopic trends did not vary significantly across different coral species or algal haplotypes, suggesting a common strategy among the Leptoseris coral community to cope with the challenges of deep-water life. The research builds on previous studies that have examined how coral morphology[2] and environmental factors[3] affect coral productivity and survival. For instance, it was known that coral morphology influences carbon fixation efficiency[2], which is directly related to how corals and their symbionts use light for energy. This new study complements these findings by showing that despite morphological differences, mesophotic corals maintain a consistent reliance on photosynthesis across species. The work also ties into the understanding of coral bleaching and resilience. Previous research[4] has suggested that corals with smaller polyp sizes and less overlap in their trophic niches with their symbionts are more resistant to bleaching. The current study does not directly address bleaching but offers insight into the fundamental nutritional strategies that may underpin resilience in low-light environments. Additionally, the study supports the idea that corals can adapt to varying light conditions without a change in their symbiotic algae[5], as evidenced by the consistent stable isotope values across different depths and species. In conclusion, the research from Villanova University advances our understanding of the delicate balance between autotrophy and heterotrophy that allows mesophotic corals to survive in the dim blue depths. It emphasizes the importance of photosynthesis and efficient nutrient sharing between corals and their symbionts, even in conditions where sunlight is a rare commodity. This knowledge is essential for conservation efforts, as it highlights the adaptability and resilience of coral ecosystems in the face of changing ocean conditions.

EcologyMarine Biology

References

Main Study

1) Mesophotic corals in Hawai'i maintain autotrophy to survive low-light conditions.

Published 28th February, 2024

https://doi.org/10.1098/rspb.2023.1534

Related Studies

2) Incident light and morphology determine coral productivity along a shallow to mesophotic depth gradient.

https://doi.org/10.1002/ece3.8066

3) Studying animal niches using bulk stable isotope ratios: an updated synthesis.

https://doi.org/10.1007/s00442-020-04654-4

4) Trophic strategy and bleaching resistance in reef-building corals.

https://doi.org/10.1126/sciadv.aaz5443

5) Photoacclimation and induction of light-enhanced calcification in the mesophotic coral Euphyllia paradivisa.

https://doi.org/10.1098/rsos.180527


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