Surprising growth rates discovered in world's deepest photosynthetic
corals

Date:
June 15, 2020
Source:
University of Hawaii at Manoa
Summary:
New research has revealed unexpectedly high growth rates for deep
water photosynthetic corals. The study alters the assumption that
deep corals living on the brink of darkness grow extremely slowly.



FULL STORY ==========================================================================
New research published in the journal Coral Reefs revealed unexpectedly
high growth rates for deep water photosynthetic corals. The study,
led by Samuel Kahng, affiliate graduate faculty in the University
of Hawai'i at M?noa School of Ocean and Earth Science and Technology
(SOEST), alters the assumption that deep corals living on the brink of
darkness grow extremely slowly.


========================================================================== Leptoseris is a group of zooxanthellate coral species which dominate the
coral community near the deepest reaches of the sun's light throughout the Indo- Pacific. Symbiotic microalgae (called zooxanthellae) live within
the transparent tissues some coral -- giving corals their primary color
and providing the machinery for photosynthesis, and in turn, energy.

Deeper in the ocean, less light is available. At the lower end of their
depth range, the sunlight available to the Leptoseris species examined in
the recent study is less than 0.2% of surface light levels. Less light
dictates a general trend of slower growth among species that rely on
light for photosynthesis.

Previous studies suggested that photosynthetic corals at the bottom of
the ocean's sunlit layer grow extremely slowly -- about 0.04 inch per
year for one species of Leptoseris. Until recently, there were very few
data on growth rates of corals at depths greater than about 225 feet
given the logistical challenges of performing traditional time series
growth measurements at these depths.

Kahng, who is also an associate professor at Hawai'i Pacific University, collaborated with SOEST's Hawai'i Undersea Research Laboratory (HURL),
the Waikiki Aquarium, National Taiwan University and Hokkaido University
to collected colonies of Leptoseris at depths between 225 and 360 feet
in the Au'au Channel, Hawai'i using HURL's Pisces IV/V submersibles. The research team used uranium-thorium radiometric dating to accurately
determine the age of the coral skeletons at multiple points along its
radial growth axis -- much like one might determine the age of tree
rings within a tree trunk.

"Considering the low light environment, the previous assumption was
that large corals at these extreme depths should be very old due to
extremely slow growth rates," said Kahng. "Surprisingly, the corals were
found to be relatively young with growth rates comparable to that of
many non-branching shallow water corals. Growth rates were measured to
be between nearly 1 inch per year at 225 feet depth and 0.3 inches per
year at 360 feet depth." The research team found that these low light,
deep water specialists employ an interesting strategy to dominate their preferred habitat. Their thin skeletons and plate-like shape allow for
an efficient use of calcium carbonate to maximize surface area for light absorption while using minimal resources to form their skeleton. These
thin corals only grow radially outward, not upward, and do not thicken
over time like encrusting or massive corals.

"Additionally, the optical geometry of their thin, flat, white skeletons
form fine parallel ridges that grow outward from a central origin,"
said Kahng. "In some cases, these ridges form convex spaces between them
which effectively trap light in reflective chambers and cause light to
pass repeatedly through the coral tissue until it is absorbed by the photosynthetic machinery." The strategic efficiency of Leptoseris
enabling its robust growth rates in such low light has important
implications for its ability to compete for space and over-shade slower
growing organisms.

"It also illustrates the flexibility of reef building corals and suggests
that these communities may be able to develop and recover from mortality
events much faster than previously thought," said Kahng.


========================================================================== Story Source: Materials provided by University_of_Hawaii_at_Manoa. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Samuel E. Kahng, Takaaki K. Watanabe, Hsun-Ming Hu, Tsuyoshi
Watanabe,
Chuan-Chou Shen. Moderate zooxanthellate coral growth
rates in the lower photic zone. Coral Reefs, 2020; DOI:
10.1007/s00338-020-01960-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200615140901.htm

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