16 / 100
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
16
ucture
acclimatization strategies (both ecological and biological).
These include the following (reviewed in Kahng et al. 2010,
2014):
• Minimizing self-shading and maximizing surface area at
a colony morphology level (e.g. horizontally flattened or
encrusting colony morphologies), at a cellular level (e.g.
monolayered zooxanthellate), and possibly at a subcellular
level.
• Reducing the amount of tissue biomass, surface area and
respiratory demand to increase growth efficiency.
• Reducing skeletal mass per unit colony area to reduce
energy requirements.
• Optimizing skeletal light-scattering properties (Figure
2.10).
The reflective properties of calcium carbonate play an
important role in increasing the light-harvesting efficiency of
mesophotic corals (Enríquez et al. 2005, Kahng et al. 2012a,
Kahng 2014) and may also occur in other organisms, such
as calcareous green algae and coralline red algae. For a plant
leaf (or non-calcareous macroalgae), light passes through the
tissue only once and, unless absorbed by pigments, is lost. In
contrast, the skeleton of a coral can reflect light back through
the tissue, thereby increasing the probability of absorption.
Light-harvesting efficiency is not only influenced by skeletal
composition, but can also be affected by the light-scattering
properties of skeletal micromorphology. Internal scattering
can increase the probability of light absorption, independent of
pigment concentration, by increasing the photon path length
within the coral tissue (Figure 2.10).
Location can also affect the amount of ambient light available
for mesophotic corals and algae. On flat or gently sloping
areas, sessile organisms can be exposed to diffuse low light
throughout the day, but on a steep slope, light is limited
because the slope obstructs the light for a portion of the day
(Brakel 1979). Thus, an MCE in clear water may have ample
light at a given depth in areas with flat open seafloor, but may
Figure 2.11.
A near-vertical mesophotic reef slope on thewestern
side of Tobi (Hatohobei) Island, Palau at 55 m in depth. This area
is heavily shaded during morning periods when the sun is in the
east, casting a shadow across the area (photo Patrick L. Colin).
become light-limited on a slope that is shaded for much of the
day (Figure 2.11).
Mesophotic corals exhibit several adaptations relative to
dependence on low light at depth, one of which is the switch
from autotrophic (i.e., energy from light) to heterotrophic
(i.e., energy from consumed foods) nutrition. This has been
demonstrated using stable isotope techniques in scleractinian
corals,
Montastraea cavernosa
(Lesser et al. 2010) and in
a facultative zooxanthellate gorgonian from a temperate
ecosystem (Gori et al. 2012). Specifically, planktonic
resources, which are often higher on mesophotic reefs (e.g.
Lesser and Slattery 2013) due to upwelled nutrients (Leichter
and Genovese 2006, Leichter et al. 2007), are captured by the
coral’s tentacles, thereby offsetting the lmss of energy from
phototrophic sources.
Figure 2.10.
The absorption of light is influenced by the micromorphology of coral and algal skeletons.
Source: Enríquez et al. 2005, Kahng et al. 2012a, Kahng 2014
Porites
structure
Leptoseris
structure
Flat skeleton
Leaf
E ect of morphology on light harvesting
Water column
Tissue
Coral skeleton
Sunlight