Better Out than In – or why superoxide in coral may be misunderstood

Reactive oxygen species (ROS) such as superoxide are created through the reduction of molecular oxygen to water and play a critical role in coral bleaching. The internal accumulation of Superoxide have previously been linked to physiological stress. The build-up of these toxic chemicals within the coral are thought to damage the photosynthetic capability of Symbiodinium cells and impair mitochondrial electron transport in the coral host. This eventually leads to the expulsion of the algae companions, resulting in the loss of coral colour, energy production and organic carbon.

Previously, internal superoxide has been considered a detrimental molecule, however newly discovered pathways and sources of superoxide have revealed that a diverse group of heterotrophic bacteria enzymatically produce extracellular superoxide at the corals surface. Unlike the traditional detrimental effects of having intercellular superoxide, external production may aid the corals resilience to disease and increase thermotolerance.

Despite the bad reputation of superoxide, very little is known about the origins, distributions and ecological underpinning of superoxide production in natural coral communities. Previous studies utilized indirect evidence of oxidative stress based on observations, gene expression and proteomics resulting in often inaccurate and biased results. However Diaz et al. utilized recent developments in non-invasive chemiluminescent techniques allowing the team to capture the first in-situ measurements of external superoxide production by several species of thermally stressed and bleaching corals.

Observations during a bleaching event saw corals that were susceptible to bleaching lacked the production of external superoxide, while resistant corals had high concentrations of external superoxide. Additional observations were also carried out on corals that weren’t undergoing physiological stress, these corals still carried out the production of superoxide. These findings were then reinforced by a parallel study which examined corals in a lab environment. The combination of these results has resulted in scientists realizing that superoxide may be vitally misunderstood and instead of a being a detrimental molecule, it may be an essential molecule for the well-being of coral. Future lab experiments are essential for investigating the relationship between superoxide and physiological stressors without the natural variability of the coral reef. Focus should also be directed to the handful of other stressors corals experience such as pathogens.

The measurement of superoxide isn’t without its hardships, due to the volatile 30 second lifespan of superoxide, even these successful measurements shown will be seeded with unavoidable bias. However the research team have carving their way through previous bias methodologies and unreliable data using novel techniques to reveal important discoveries that go against the grain of studies on the area. This initial discovery may lead to future research that is essential to achieve a better understanding of the physiology of corals, leading us towards future developments in coral management and help develop bleaching mitigation strategies.

Diaz, J., Hansel, C., Apprill, A., Brighi, C., Zhang, T., Weber, L., McNally, S. and Xun, L. (2016). Species-specific control of external superoxide levels by the coral holobiont during a natural bleaching event. Nature Communications, 7, p.13801.