Corals adapted to extreme and fluctuating seawater pH increase calcification rates and have unique symbiont communities

Ocean acidification (OA) is a severe threat to coral reefs mainly by
reducing their calcification rate. Identifying the resilience factors of
corals to decreasing seawater pH is of paramount importance to predict the
survivability of coral reefs in the future. This study compared corals
adapted to variable pHT (i.e., 7.23–8.06) from the semi-enclosed
lagoon of Bouraké, New Caledonia, to corals adapted to more stable
seawater pHT (i.e., 7.90-8.18). In a 100-day aquarium experiment, we
examined the physiological response and genetic diversity of
Symbiodiniaceae from three coral species (Acropora tenuis, Montipora
digitata and Porites sp.) from both sites under three
stable pHNBS conditions (8.11, 7.76, 7.54) and one fluctuating
pHNBS regime (between 7.56 and 8.07). Bouraké corals consistently
exhibited higher growth rates than corals from the stable pH environment.
Interestingly, A. tenuis from Bouraké showed the highest growth rate under
the 7.76 pHNBS condition, whereas for M. digitata and Porites sp.
from Bouraké, growth was highest under the fluctuating regime and the
8.11 pHNBS conditions, respectively.  While OA generally
decreased coral calcification by ca. 16%, Bouraké corals showed higher
growth rates than corals from the stable pH environment (21% increase for
A. tenuis to 93% for M. digitata, with all pH conditions pooled). This
superior performance coincided with divergent symbiont communities that
were more homogenous for Bouraké corals. Corals adapted to variable pH
conditions appear to have a better capacity to calcify under reduced pH
compared to corals native to more stable pH conditions.  This
response was not gained by corals from the more stable environment exposed
to variable pH during the 100-day experiment, suggesting that long-term
exposure to pH fluctuations and/or differences in symbiont communities
benefit calcification under OA.