Abstract:
Increased ocean temperatures are thought to be triggering
mass coral bleaching events around the world. The intracellular
symbiotic zooxanthellae (genus Symbiodinium) are
expelled from the coral host, which is believed to be a response
to photosynthetic damage within these symbionts. Several
sites of impact have been proposed, and here we probe the
functional heterogeneity of Photosystem I1 (PSII) in three
coral species exposed to bleaching conditions. As length of
exposure to bleaching conditions (32°C and 350 pmol photons
s ) increased, the QA- reoxidation kinetics showed a rise
in the proportion of inactive PSII centers (PSIIx), where Qe
was unable to accept electrons. PSIIx contributed up to 20 %
of the total PSII centers in Pocillopora damicornis, 35% in
Acropora nobilis and 14% in Cyphastrea serailia. Changes in
FJF, and amplitude of the J step along fast induction curves
were found to be highly dependent upon the proportion of
PSIIx centers within the total pool of PSII reaction centers.
Determination of PSII antenna size revealed that under
control conditions in the three coral species up to 60% of
PSII centers were lacking peripheral light-harvesting complexes
(PSIIfi). In P. darnicornis, the proportion of PSIIfi
increased under bleaching conditions and this could be
a photoprotective mechanism in response to excess light.
The rapid increases in PSIIx and PSIIP observed in these
corals under bleaching conditions indicates these physiological
processes are involved in the initial photochemical damage
to zooxanthellae.
