Light Pollution at Night Severely Disrupts the Reproductive Cycle of Corals Israeli study reveals

When humans are exposed to excessive light, they may suffer a variety of harmful effects on their health including stress, headaches, a decrease in sexual function and an increase in anxiety. Shift employees who work through the night instead of sleeping are at a higher rate of cancer. 

For those who need to be awake at night, light at night also has an acute effect on alertness and mood. 

It affects not only people, but also animals. Some species of spiders avoid areas that are lit up at night, and light pollution can pose a serious threat to nocturnal wildlife that are active looking for prey and other activities at night. 

Light pollution from building exterior and interior lighting, advertising, outdoor area lighting, offices, factories, streetlights and illuminated sporting facilities is a byproduct of industrial civilization. Awareness of its harmful effects began early in the 20th century, but efforts to cope with them did not begin until the 1950s.

Now, researchers at Bar-Ilan University in Ramat Gan (near Tel Aviv) have found that due to the high rate of urban development in marine coastal areas around the world, light pollution could further threaten populations of coral communities that are already under severe degradation. The new study, just published in the journal Current Biology under the title “Coral gametogenesis collapse under artificial light pollution,” shows how light pollution is negatively impacting the reproductive cycle of two coral species. 


The coral reproductive cycle (the development of sperm and eggs) is controlled by a biological rhythm a few months during the year in harmony with natural environmental conditions. At the conclusion, sperm and eggs are released into the water for external fertilization. The mechanism that leads to synchronized spawning is thought to be controlled by both an exogenous (environmental) and endogenous apparatus (biological clock). 


Successful production of gametes (eggs and sperm) and their fertilization, as well as the development of viable offspring and survival of new coral recruits are possibly the most important processes for replenishing degraded reefs. In addition, , sexual reproduction maintains coral populations and supports evolutionary processes which may enhance fitness. 


For this study, the researchers collected and tracked two coral species, Acropora millepora and Acropora digitifera, from the Indo-Pacific Ocean.  A total of 90 colonies were transferred to the Bolinao Marine Laboratory, which is located in an area in the Philippines with no light pollution. The colonies were placed in outdoor tanks, exposed to natural sunlight, moonlight, and seawater. Exposure to the sun was adjusted to equivalent levels experienced by corals at their collection depth of five meters. 


Coral colonies were divided into three groups: two experimental and one control, with each group composed of 15 colonies from each Acropora species divided randomly into three tanks. The experimental groups were treated with LED lamps possessing both cold (yellowish, with less blue light) and warm (white, with more blue light) spectra. 


For three months, the LED lamps were activated every day from sundown until sunrise. The control groups were exposed to the same conditions as the experimental colonies (natural solar light, moonlight phases) but without supplemental light at night. Chlorophyll fluorescence yield was assessed on a monthly basis as an indicator of colony health. 


The results clearly showed that light pollution caused delayed gametogenesis and unsynchronized gamete release, emphasizing the importance of natural periodic illumination, both solar and lunar, as a critical factor in cueing spawning synchronicity and the gametogenic process.


“Both key coral species were affected by ecological light pollution,” commented the study’s lead author, Prof. Oren Levy, of Bar-Ilan’s Faculty of Life Sciences. “They exhibited a lack of synchrony in the reproductive state, which was reflected in the number of oocytes per polyp, gametogenesis, and gamete maturation. This was further reflected at the population level where only corals exposed to natural light cycles succeeded in spawning synchronization.” 


To shed light on how relevant their findings are on a worldwide scale, the researchers created a first-of-its-kind global map that highlights areas most threatened by artificial light at night, including the Caribbean Sea and the Pacific and Indian Oceans.  One striking example is the Gulf of Aqaba/Eilat in the northern Red Sea, where considerable urban lighting is present close to shore. Here, the least affected area is 47% brighter than a natural night sky, and this rises to a maximum of 60 times brighter. Light treatment with both cold and warm LED’s had a similar impact on the gametogenesis cycle, added Levy. 


With the global transition towards LED lighting, which tends to have higher emissions in the blue spectrum, more near shore coral reefs could be affected by artificial light, as blue light penetrates deeper into the water. This spectral shift is expected to be amplified by the current rapid population growth in coastal regions. Levy and team’s results demonstrate that artificial light must be considered in conservation plans for coral reefs near areas of human activity and their light pollution impact assessment can help incorporate an important variable in coral reef conservation planning. 


In follow-up research, Levy hopes to find out whether there are corals more adapted to light pollution and, if so, what mechanism underlies their resilience. 


The Bar-Ilan team was assisted by the Interuniversity Institute for Marine Sciences in Eilat and Tel Aviv University team members Inbal Ayalon and Dr. Yaeli Rosenberg, with help from Patrick Cabaitan from the Marine Science Institute at the University of the Philippines and light pollution specialists Dr. Christopher Kyba and Dr. Helga Kuechly from the German Research Centre for Geosciences GFZ.





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