Light threat is not so light for some static marine organisms

The issue of artificial light at night (ALAN) is rapidly growing in prominence. However, there are still significant research gaps before there is a good understanding of how a range of marine organisms are impacted by ALAN, especially when combined with other stressors.

It is important to understand these impacts not only in light of the multiple pressures faced by marine organisms, such as warming, acidification and pollution, but also as ALAN sources are expanding globally. In fact, ALAN is becoming pervasive with biologically-relevant light pollution affecting nearly 76% of the seafloor near well-lit cities, which is predicted to increase further in the future.

The study ‘The disruption of a symbiotic sea anemone by light pollution: Non-linear effects on zooxanthellae and molecular indicators’ was published in Science of the Total Environment and is a collaborative effort between the University of Prince Edward Island, Plymouth Marine Laboratory and the University of Exeter.

Study findings

The research demonstrated a significant negative effect of high ALAN levels on the symbiotic algae (zooxanthellae) that live on the anemones, and on the enzyme (superoxide dismutase) that helps both the anemone and algae cope with oxidative stress.

Both of these processes are crucial for the anemone’s survival and therefore important to understand:

• Zooxanthellae live symbiotically with some sea anemones, as well as other marine organisms such as coral, providing them with essential nutrients through photosynthesis in exchange for a safe and stable environment. Negative stress on either organism can have substantial detrimental effects on their survival.
• The superoxide dismutase (SOD) enzyme plays a crucial role in protecting symbiotic anemones and their algae from oxidative stress. The algae’s photosynthetic process can cause fluctuating oxygen levels within the anemone’s tissues and a form of reactive oxygen can be produced, which causes cellular damage to the anemone. SOD’s help neutralise this reactive oxygen, protecting the anemone from cells damage.

By integrating PML’s novel Marine Artificial Light at Night Research (MARLAN) Facility with its innovative tidal experiment system, natural light and tidal conditions for the urchins were mimicked. This allowed the anemones to acclimatise before experimental ALAN levels were introduced.

In comparison to natural conditions, anemones exposed to ALAN showed significantly higher zooxanthellae under mild ALAN (10 lx), whereas, counts were significantly lower under strong ALAN (50 lx) conditions.

10 lx is roughly the equivalent to the light levels at sunset, whereas, 50 lx is equivalent to an overcast day. In comparison, a well-lit office space requires approximately 300-500 lx.

Previous research has shown an increase in zooxanthellae following exposure to various light spectra, which reversed at a higher ALAN intensity (50 lx), resulting in a drastically lower number of symbionts, even below control levels. Another study showed that zooxanthellae growth rate increased with light intensity up to a threshold and then declined.

These studies suggest that after a threshold is reached, photoinhibition and damage to the photosynthetic apparatus were causally associated with the loss of symbionts. Photosynthesis rates were not measured in this latest study but the evident loss of zooxanthellae at the stronger ALAN levels suggests a similar mechanism is at play.

Anemones exposed to a low ALAN levels showed a decline in SOD concentration compared to controls, but the more severe ALAN levels caused a sharp 350% increase in SOD concentrations. Since the anemones were showing signs of bleaching, the study team interpret this as a physiological response involving a decline in the number of zooxanthellae well below control levels, and in parallel, the rise in SODs to cope with stress.

Dr K Devon Lynn, lead author, Post Doctoral Fellow at the University of Prince Edward Island and Research Fellow at Plymouth Marine Laboratory when conducting this research, said:

“These results support our working hypothesis and suggest a harmful influence of ALAN on the physiology and ecology of this species. Such influence can be associated with bleaching events which may contribute to, or possibly act synergically with, the effects of other bleaching events caused by global warming. It is therefore very likely that sea anemones, and other species with symbiont-associated algae, will experience at least partial bleaching when exposed to the more severe ALAN intensities for them, such as 50 lx”.

Prof. Pedro Quijon, co-author and Coastal Ecologist at the University of Prince Edward Island, continued:

“This study contributes to the growing body of research focused on immobile organisms by documenting ALAN sharp effects on two important life history features of the snakelocks anemone: its relationship with zooxanthellae and a molecular indicator of stress (SOD)”.

Dr Liz Talbot, co-author and Marine Ecologist at Plymouth Marine Laboratory, added:

“This study shows that ALAN exposure at intensities that we ourselves would perceive as relatively dim light, can have potentially profound impacts on the marine environment. Given the global distribution, this problem is already likely to be widespread. However, strategies for abating these impacts are already being explored. Modern lighting technologies offer the potential to reduce the ecological impacts of ALAN, although identifying how this is best achieved is challenging.”

“Unlike the solutions to many other environmental pressures, it is possible that those for ALAN may also have rapid benefits. While removing artificial light would likely not remove all ecological consequences of ALAN exposure immediately, where there are lags in recovery, these could be much shorter than those associated with the reduction of many other anthropogenic environmental pressures, such as climate change”.