Coral bleaching, a widespread phenomenon documented as
increasing in frequency over the last two-three decades, is
defined as the loss of the endosymbiotic photoautotrophic
dinoflagellates (=zooxanthellae) and/or their photosynthetic
pigments when reef-building corals are subjected to
environmental stresses. The 1998 world-wide bleaching event
and the global warming phenomenon have not spared the
waters of the Ryukyus, Japan. Though elevated sea surface
temperature (SST) and solar irradiance have been mostly
implicated in mass bleaching events, the cellular and molecular
mechanisms of the bleaching process and susceptibility among
reef-building corals yielding in mortality have yet to be
thoroughly addressed. This thesis explores the bleaching
mechanisms, stress susceptibility and mortality patterns
experimentally among the reef-building corals exposed to
thermally and/or light induced stresses coupled with an attempt
to predict the recurrences of mass mortality of corals based on
historical and forecast SSTs in the region of the Ryukyus in
particular Okinawa and Ishigaki Islands, Japan. The predictive
models of mass mortality of corals or the “extinction dates” for
the examined reefs of Okinawa and Ishigaki Islands are yrs
2000-2010 and 2070-2080, respectively. However, assuming
that corals would acclimate to temperature increases of 2oC, the
predicted extinction dates could be prolonged by almost 100
yrs. The major findings of the experimental studies are: 1) both
partners in the symbiosis of coral/zooxanthellae are affected
during thermal stress, 2) the dark reaction (Calvin cycle
enzymes) is more vulnerable to thermal stress than the light
reaction (photosystem II, PSII) of photosynthesis of the
endosymbionts and damage to PSII is secondary, 3) the
differential responses of reef-building corals seem to be
symbiont genotype dependent as the Calvin cycle is inhibited
differently in distinct genotypes harbored by corals with
different vulnerabilities to thermal stress, 4) high temperature
reduces the threshold light intensity for photoinhibition of PSII
differently in corals with different bleaching susceptibilities, 5)
the glycine mycosporine-like amino acid functions as a
biological antioxidant in corals and along with the antioxidant
enzyme defense systems it plays a potential role in determining
bleaching susceptibility among corals, and 6) the host appears
to be the first partner to be affected by thermal stress. Taken
together, these results suggest that thermal stress affects the
host first and damage to the photosynthetic machinery of the
endosymbionts further adds to the oxidative stress burden and
results in severe bleaching and ultimately mortality of reef-
building corals. This sequence of events might occur to a
different extent or differently in coral species exposed to similar
levels of thermal stress exposure and thus account for
differential bleaching and/or mortality patterns observed among
reef-building corals. The prediction of mass coral mortality and
the experimentally explored thermally-induced bleaching
vulnerability and mortality patterns among reef-building corals
existing in the Islands of the Ryukyus should be highly helpful to
concerned governmental and non-governmental organizations
in prioritizing their efforts in restoration and preservation of
badly affected (Okinawa) and least affected (Ishigaki) areas,
respectively. It is also highly recommended that this predictive
model be run not only for the so-far nine coral reefs based
marine protected areas (MPAs) of Japan as a tool in determining
the vulnerabilities of the existing ones to global climate change
but also in designating new MPAs. In an effort to preserve
biodiversity of reef-building corals in the Japanese waters
coupling the SST-based predictive models with ground-truthing
surveys of biodiversity, bleaching and mortality of corals related
to global warming would be needed to identify the least global
warming prone potential habitat refuges, where both thermally
susceptible and resistant corals could continue to co-exist in the
natural marine environment.
Email: rbhagooli@gmail.com