High nitrate concentrations alone did not result in altered
antioxidant enzyme (SOD and CAT) activities in this study, and
therefore had no effect on ROS production. The fact that antioxidant
enzyme activities increased under high temperature and high
light intensity conditions indicates that ROS levels also increased
as shown by Lesser et al. (1990). Our results suggest that high
temperatures and a light intensity of 600 μmol m−2 s−1 can increase
the production of ROS in host tissues and zooxanthellae, as
shown by the increased activity of antioxidant enzymes (Figs. 2
and 3). However, given that photosynthetic yield Fv/Fm did not
decrease, the photosystem was likely not damaged under a light
intensity of 600 μmol m−2 s−1 (Fig. 1(b)). A photon intensity of
600 μmol m−2 s−1 is not strong for coral–symbiotic algae, since presence of the superoxide anion, NO can be converted into highly
reactive peroxynitrite (ONOO−), which is much more toxic and
has the capacity to irreversibly damage mitochondria, antioxidant
enzymes, DNA, and lipid membranes (Pacher et al., 2007). Weis
(2008) suggested that high ROS or NO production by the symbiont
signals the host to initiate a bleaching response. High nitrate levels
also affect the resilience of the coral–symbiont relationship at high
temperatures (Chumun et al., 2013). It is likely that the ONOO−
could damage to host corals under the combined stresses in this
study, and it is believed that host corals discard their symbiotic algae
to reduce such chemicals. Therefore, it is concluded that nitrate
enrichment could be enhance oxidative stress and cause a faster
decrease in zooxanthellae density under high temperature and
high light stress conditions compared with a single stress alone.