Coral reefs are undoubtedly the treasures of our oceans. Every month during the new moon, cauliflower corals found thriving in the shallow waters of the Indian and Pacific oceans, release larvae that migrate through currents and swim to faraway locations for settlement. Swimming requires large amounts of energy reflected in high metabolic rates among larvae. But few survive this journey fraught with danger; they are at risk of predation and vulnerable to dynamic ocean environments. Adding to their woes, climate change may render our oceans warmer and more acidic by the of the century. How will cauliflower coral larvae respond to these environmental changes? A study suggests that only a fraction may possess the ability to adapt and persist.
Rapid climate change spurred by our reliance on fossil fuels is transforming our oceans; by the end of the century, global ocean conditions will be vastly different from today: Scientists at the Intergovernmental Panel on Climate Change (IPCC) have predicted that our oceans will be 1-3°C warmer and more than double the current acidity as a consequence of increased uptake of atmospheric carbon dioxide.
Little is known about how these coral larvae may respond physiologically to a disturbing combination of rapidly escalating sea temperatures and ocean carbon dioxide levels (characterized by a decrease in pH known as ocean acidification). These environmental stressors are expected to increase their demands for energy to maintain internal physiological processes. Rising temperatures may therefore lead to an increased metabolic rate, but it is thought that the interaction of both warming and rising acidity may reduce their window for thermal tolerance, thereby lowering metabolic rates overall.
To understand the physiological responses of coral larvae to future ocean conditions, a new study, by Rivest and Hoffman, investigated the metabolic responses of the cauliflower coral Pocillopora damicornis to these environmental stressors.
The team collected larvae from a fringing reef at Moorea, French Polynesia. Due to low numbers of larvae released, they had to use all for the study. They randomly assigned larvae to four experimental aquaria consisting of combinations of high and low levels of water temperature and carbon dioxide. A low temperature of 27.8°C represented the 5-year average temperatures at the reef site, and a low dissolved CO2 level (450 µatm) reflected approximate present sea conditions; in contrast, a high temperature of 30.6°C and a high CO2 level or acidity (950 µatm) simulated ocean conditions as forecast by the IPCC for 2100. The larvae were exposed to these experimental conditions for six hours.
To determine the present-day variability of temperature and acidity at the collection site, they continuously monitored the temperature and pH in the reef for a month preceding larval release. Both the temperature and pH oscillated in 24-hour cycles, and the ranges reflected the experimental levels they used for current conditions.
Under these conditions, they measured larval responses using two indirect metabolism indicators: rates of oxygen consumption and citrate synthase activity—an enzyme involved in releasing energy in the presence of oxygen. The latter reveals the capacity of larvae to satisfy their energy demands.
At the higher temperatures, larval oxygen consumption rates were elevated, while the rates declined at high CO2 levels. But citrate synthase activity was largely unaffected with increased temperatures or CO2. Inability to boost energy production by increasing citrate synthase activity might mean that P. damicornis larvae may be unable to cope with additional environmental stresses, according to the researchers. When exposed to such conditions, some larvae may be reach their biochemical limits to meet growing energy demands resulting in slower growth and reduced fitness for survival. On the contrary, prolonged exposure to the stressors, such as for days, could enhance citrate synthesis activity enabling acclimation, they explained.
In addition, they found that metabolism under the different environmental conditions varied considerably among the larvae depending on the day they were released during the spawning period. Larvae released later were more sensitive to the warm and acidic environment compared with the early-release larvae. This variation, the researchers believe, may allow corals to spread risk ensuring some larvae have a better ability to adapt and acclimatize to dynamic ocean conditions by altering their metabolism. But it is possible that if a larger number of larvae were sampled, the variation among the larvae will be lower than seen in this study.
The upshot of the study: only a portion of the coral larvae released each month may possess the ability and fitness to adapt to and tolerate future scenarios of increasingly warm and acidic oceans. Although P. damicornis is common and not currently classified as a threatened species, future climate conditions, along with other factors, may exacerbate coral reef decline, threatening the livelihood of millions of people who depend on coral reefs for their income.
Rivest EB, Hofmann GE (2014) Responses of the Metabolism of the Larvae of Pocillopora damicornis to Ocean Acidification and Warming. PLoS ONE 9(4): e96172. doi:10.1371/journal.pone.0096172