Foraminifera – single-celled organisms that live predominantly in the sea – play an important role in the ocean ecosystem. However, tests have shown that they and their algae symbionts are damaged by ingredients in sunscreen. This also applied to sunscreens sold as “environmentally friendly”. Their residues are even more harmful to the health of the protozoa, as environmental biologists have now discovered.
Due to their long-lived shells, foraminifera are important index fossils for past geological eras – but they are still an important part of the marine ecosystem today. Some of these single-celled sea creatures can take in algae or cyanobacteria as symbionts. They thus make a decisive contribution to the marine carbon and nitrogen cycle in both deep and shallow waters.
Effects tested on symbiotes
However, this important process could be affected, particularly in coastal areas and river deltas, by pollution from sunscreen residues. Michael Lintner from the University of Vienna and his colleagues have now taken a closer look at the consequences of this contamination for the foraminifera. The research team focused on the foraminiferal species Heterostegina depressa, which belongs to the larger foraminifera and hosts diatoms as a symbiont. These provide their host with important substances such as sugar or glycerol and are therefore essential for their metabolic activity. The photosynthetic performance of these diatoms is significantly influenced by physical and chemical parameters and thus also by human pollution.
“In detail, we analyzed possible effects of commercially available sunscreens on the activity of the photosymbionts on H. depressa using fluorescence microscopy,” explains Lintner. The changes in the photosynthetic activity of the diatoms can be tracked microscopically. The research team selected four different sunscreens, two of which are sold as “conventional” and another two as “eco-friendly”. They also tested the effect of pure ensulizol, which is often used as a UV blocker in sunscreens.
Damage to diatoms also caused by “eco-friendly” sunscreen
The results showed that pure ensulizol caused a significant decrease in the photosynthetic activity of the diatoms. The researchers observed the same for conventional sunscreens, albeit to a lesser extent. According to the researchers, the reason why conventional sunscreens have a negative effect on the photosymbionts of Heterostegina depressa is that they also contain ensulizol. It has long been known that this substance is stored in plant cells and thus has a negative effect on cell mechanisms. “Even the smallest concentrations strongly inhibit the activity of the algae. The concentration of ensulizol in the sunscreen is lower than that of the lowest pure ensulizol tested,” report Lintner and his colleagues.
However, the research team observed a sharp decrease in photosynthetic activity and a negative impact on the health of the foraminifera even with sunscreens sold as environmentally friendly. Compared to conventional sunscreens, the impairments caused by these agents were even greater, the team reports. Interestingly, the environmentally friendly sunscreens contain no ensulizol at all. So what then ensures that the diatoms are inhibited in their photosynthetic activity?
“We assume that in particular metal nanoparticles such as titanium dioxide or zinc oxide from ‘environmentally friendly’ sunscreens cause this effect,” explain Lintner and his colleagues. “In the last decade, the use of titanium dioxide nanoparticles has increased significantly. The nanoparticles are able to aggregate and pose a possible risk to the organism”. Since titanium dioxide is the only metal found in both of the sunscreens tested, the research team suspects that it is likely responsible for the eco-friendly sunscreen’s toxic effects. However, further studies are needed to prove this
Source: University of Vienna; Specialist articles: Scientific Reports, doi: 10.1038/s41598-022-06735-1