ABSTRACT
BACKGROUND:
Corals, which form the
foundation of biodiverse reef
ecosystems, are under threat from warming
oceans. Reefs provide essential ecological services, including
food,
income from
tourism,
nutrient cycling, waste removal, and the
absorption of
wave energy to mitigate
erosion. Here, we studied the
coral thermal stress response using network
methods to analyze transcriptomic and polar
metabolomic data generated from the Hawaiian
rice coral Montipora capitata.
Coral nubbins were exposed to ambient or thermal stress conditions over a 5-week period, coinciding with a mass spawning event of this species. The major
goal of our study was to expand the
inventory of thermal stress-related
genes and metabolites present in M. capitata and to study
gene-metabolite interactions. These interactions provide the
foundation for functional or genetic
analysis of key
coral genes as well as provide potentially diagnostic markers of pre-bleaching stress. A
secondary goal of our study was to analyze the accumulation of
sex hormones prior to and during mass spawning to understand how thermal stress may impact reproductive success in M. capitata.
METHODS:
M. capitata was exposed to thermal stress during its spawning cycle over the
course of 5 weeks, during which
time transcriptomic and polar
metabolomic data were collected. We analyzed these data
streams individually, and then integrated both data sets using MAGI (Metabolite Annotation and
Gene Integration) to investigate molecular
transitions and
biochemical reactions.
RESULTS:
Our results reveal the complexity of the thermal stress phenome in M. capitata, which includes many
genes involved in
redox regulation,
biomineralization, and
reproduction. The size and number of modules in the
gene co-expression networks expanded from the initial stress response to the onset of bleaching. The later stages involved the
suppression of metabolite transport by the
coral host, including a variety of
sodium-coupled transporters and a putative
ammonium transporter, possibly as a response to reduction in algal
productivity. The
gene-metabolite integration data suggest that thermal
treatment results in the activation of
animal redox stress pathways involved in quenching molecular
oxygen to prevent an overabundance of
reactive oxygen species. Lastly, evidence that thermal stress
affects reproductive activity was provided by the
downregulation of CYP-like
genes and the irregular
production of
sex hormones during the mass spawning cycle. Overall,
redox regulation and metabolite transport are key components of the
coral animal thermal stress phenome. Mass spawning was highly attenuated under thermal stress, suggesting that global
climate change may negatively impact
reproductive behavior in this species.
