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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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2.2.1.
Living in the shade
Corals existing in the low-light environment of themesophotic
zone, like the plants in the understory of a rainforest, can have
specialized morphology and physiological traits (Kuhlmann
1983, Kahng et al. 2014) that enable capture and efficient use
of as much light as possible. For example, in shallow water,
the Caribbean coral
Montastraea cavernosa
normally has a
boulder-like shape (Figure 2.9a), while at mesophotic depths,
it exhibits a flattened phenotype, which enhances light capture
(Figure 2.9b; Lesser et al. 2010). Moreover, deep (> 50 m)
mesophotic corals can have unique zooxanthellae clades that
are adapted to low light and not found in shallower depths
(Lesser et al. 2010, Bongaerts et al. 2011a, 2013b, Nir et al.
2011, Pochon et al. 2015).
In shallow water, adaptation to high light irradiance
dominates coral photophysiology (e.g. photo-protective
proteins, antioxidant enzyme capacity and self-shading
morphologies; Falkowski and Raven 2007). However,
because light attenuates exponentially with increasing depth,
photosynthetic organisms eventually become light-limited
(Kirk 1994). Corals (and algae) transplanted to lower light
regimes often increase photosynthetic pigment concentrations
per unit area to maximize utilization of ambient light. While
potentially advantageous at intermediate depths, this form
of shade adaptation becomes self-limiting with increasing
depth, as the incremental gain in photosynthetic production
per unit pigment diminishes (Falkowski et al. 1990, Stambler
and Dubinsky 2007). Therefore at lower mesophotic depths,
zooxanthellate corals employ multiple adaptation and
Figure 2.9. (a)
In shallow waters, the Caribbean coral
Montastraea cavernosa
exhibits a boulder-like morphology, shown at 5 m (photo
John Reed), and
(b)
in mesophotic waters, a flattened morphology, shown at 75 m (photo Mike Echevarria).
(a)
(b)