By Anna Huang
Coral reefs are amongst the most biologically diverse ecosystems on earth and crucial in sustaining human life on Earth. Unfortunately, the once colourful coral reefs, teeming with marine life, have turned into a heart-breaking sight over the past decade. Large regions of coral reefs all across the world are now grey, brittle and dying from bleaching. Coral bleaching is a global crisis because it negatively impacts biodiversity, food security and safety (healthy coral reefs absorb the force of waves and storm surges). Bleached corals also aggravate the already present fishing crisis, as fish and crustacean species are deprived of sites to spawn and develop.
Coral bleaching occurs when zooxanthella, symbiotic algae, die or leave the coral tissue due to stress factors, which include bacterial and viral diseases, as well as pollution. Zooxanthella live in healthy reef-building coral polyps and provide them with nutrients (through photosynthesis) as well as their vibrant colours. Whilst global warming is another widely known cause of coral bleaching, certain ingredients in skincare, especially sunscreen can also cause abrupt bleaching of hard corals, even at extremely low concentrations (Tibbetts, 2008). Sunscreen is applied to the skin to protect it from photoaging and photocarcinogenesis and is a crucial part of everyday skincare. However, some preservatives and sunscreen filters, especially organic/chemical ultraviolet (UV) filters (which protect the skin from UV light by absorbing UV rays into less harmful ways), can have dire underlying consequences for marine life (Latha et al., 2013; Lee, Corvalan & Saraum, 2018).
In situ experiments have shown that parabens (such as butylparaben, which is used as preservative in cosmetics), and chemical UV filters, such as cinnamate, benzophenone derivatives (e.g. oxybenzone) and camphor derivatives can stimulate dormant viral infections in zooxanthellae. These chemicals induce the lytic cycle in viruses within zooxanthellae and cause them to replicate until the host cell lyses, spilling viral particles and spreading the infection to nearby coral communities (Danovaro et al., 2008; Tibbetts, 2008; Wijgerde et al., 2020).
Benzophenone derivatives are arguably the most harmful ingredient, since its toxicological behaviours range from a molecular level to multi-organ system pathologies. Benzophenones (including oxybenzone) increase the susceptibility of corals to bleaching and are mutagens that increase the rate of damage to DNA in coral cells when exposed to sunlight (Downs et al., 2015). Benzophenones cause genetic damage by inducing the formation of cyclobutene pyrimidinic dimers (molecular lesions formed from thymine and cytosine bases), oxidative DNA damage, single-strand DNA breaks etc. (Downs et al., 2015). In addition, oxybenzone (or benzophenone-3) exposure can cause deformation and a loss of motility in coral planula (larval forms of coral), which at higher oxybenzone concentration progresses to cell death and catastrophic tissue lysis (Downs et al., 2015). Oxybenzone is a photo-toxicant, which means that under greater light intensity (especially in the UV or near-UV spectrum) its adverse effects are exacerbated, making it highly toxic to aquatic life (Danovaro et al., 2008; Downs et al., 2015).
Whilst the harmful effects of oxybenzone on marine environments has incentivised some countries, like Hawaii, to put a ban on the sale of oxybenzone-containing sunscreen, oxybenzone is still found as an active ingredient in many other personal care products. These include shampoos, anti-aging creams and lip balms, continuing to be a ubiquitous contaminant in marine environments (Wijgerde et al., 2020). Furthermore, the environmental contamination caused by skincare chemicals is only expected to increase as a result of rising production and consumption of cosmetic (sun) products.
Coral reefs experience significant threat from harmful skincare ingredients that wash off swimmers in reef waters or reach the ocean through wastewater. Therefore, it is important to note that sunscreens with inorganic/physical UV filters are less harmful to corals (e.g. titanium or zinc oxide). These, unlike organic/chemical UV filters, protect the skin from photoaging and photocarcinogenesis by reflecting UV rays instead of absorbing them (Gabros, Nessel & Zito, 2020). However, even these sunscreens are not all created equal. Some contain nanoparticulate titanium oxide and zinc oxide, which generate reactive oxygen species (ROS) when exposed to UV radiation and have toxic effects on marine phytoplankton (Miller et al., 2012; Sánchez-Quiles & Tovar-Sánchez, 2014). Sunscreens that are ‘reef-safe’ and non-toxic to other marine life are hard to find but try to look for sunscreens labelled as ‘mineral sunscreens, which are non-nanotized and will not harm marine life.
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