Written By Tim Bateman
It is widely accepted that light is necessary for coral reefs to survive and grow, but we also know that pollution can have detrimental impacts on corals and the organisms that call reefs home. Ongoing coastal development in tropical regions increases the presence of artificial light at night (ALAN) on reefs around the world. The main threats from human light pollution are sky glow, light scattered by particles in the atmosphere, and direct light from sources such as street and building lights that shine on the water. These lights can cause oxidative stress in corals and disrupt the natural circadian rhythm of reef residents. Reef organisms rely on their circadian rhythm for spawning, hunting, feeding, and diurnal migrations, and disrupting these behaviors negatively impacts the ecological function of reefs.
ALAN is increasing worldwide, but coastal development in tropical regions has altered the intensity and spectra of light on reefs at night. A recent study by Tamir and colleagues quantified light pollution in the Red Sea and the Gulf of Aqaba and found some surprising results (Figure 1). Light at the water’s surface at night was more than two orders of magnitude higher along the shoreline compared to the middle of the gulf, just 9.5 kilometers away. This same study also found an artificial light signal all the way down to 30 meters, highlighting how far and deep-reaching the effects of ALAN can be. Not only are coastal reefs seeing more light at night, but artificial lights are also changing the spectra or color of light on reefs. Light spectra also profoundly impacts reef organisms as behavior patterns have been linked to the overlap between the spectra of artificial light and the light absorbed by light sensing proteins called cryptochromes. For example, these cryptochromes are linked to temporal synchronization processes in corals. Artificial light is significantly altering the intensity and spectra of light on reefs at night, and this will alter the physiology and behavior of many reef residents, adding to the stress of climate change reefs are currently experiencing.
Corals are particularly vulnerable to ALAN’s adverse effects because they rely so heavily on light cues for critical behavioral and physiological functions. The most important and well-studied of these behaviors is coral spawning, which relies on moonlight cues to synchronize the release of eggs and sperm from corals into the water column. If spawning is not synchronized across the reef, the chances of egg fertilization drop dramatically and can reduce coral reproduction and recruitment. For example, when the pillar coral Dendrogyra cylindrus was raised in aquaria under artificial light, spawning lasted seven days. However, when spawning naturally on reefs, D. cylindrus spawning lasted only three days, concentrating the release of eggs and sperm and increasing the chance for fertilization success. Similarly, coral larvae exposed to artificial light treatments displayed a 30% reduction in recruitment success, further highlighting the detrimental effects of light pollution on coral reproduction (Figure 3). Successful spawning and recruitment are critical to reefs’ continued survival, but the adverse effects of light pollution add to the stress coral reproduction is already experiencing from climate change.
ALAN negatively impacts photosynthetic processes in coral symbionts, impacting coral energy reserves as corals can receive up to 100% of their required food from their symbionts. Oxidative stress caused by ALAN induced photoinhibition and damage to the photosynthetic machinery in corals. Specifically, a study of two coral species found that ALAN increased reactive oxygen species production and increased oxidative damage to lipids in both species. Corals are already experiencing oxidative stress caused by increasing climate temperatures, and adding oxidative stress from ALAN will only accelerate the decline of reefs worldwide.
As the human population continues to grow, coastal development will increase, but we must consider the impacts of urbanization on the surrounding environment. Globally, reefs are under attack from local and global stressors, and ALAN further exacerbates many of the adverse effects.
References and Further Reading
Levy, O. et al. Light-responsive cryptochromes from a simple multicellular animal, the coral Acropora millepora. Science 318, 467–470, doi: 10.1126/science.1145432 (2007).
Levy, O., Marangoni, L. F. D., Benichou, J. I. C., Rottier, C., Beraud, E., Grover, R. and Ferrier-Pages, C. Artificial light at night (ALAN) alters the physiology and biochemistry of symbiotic reef building corals. Environmental Pollution 266, 11 (2020).
Tamir, R. et al. The spectral and spatial distribution of light pollution in the waters of the northern Gulf of Aqaba (Eilat). Sci. Rep. 7, 42329; doi: 10.1038/srep42329 (2017).
Neely KL, Lewis CL and Macaulay KA. Disparities in Spawning Times Between in situ and ex situ Pillar Corals. Front. Mar. Sci. 7:643. doi: 10.3389/fmars.2020.00643 (2020).