Direct Evidence of Microbial Sunscreen Production by Scum-Forming Cyanobacteria in the Baltic Sea.

Mycosporine-like amino acids are water-soluble secondary metabolites that protect photosynthetic microorganisms from ultraviolet radiation. Here, we present direct evidence for the production of these compounds in surface scums of cyanobacteria along the Baltic Sea coast. We collected 59 environmental samples from the southern coast of Finland during the summers of 2021 and 2022 and analysed them using high-resolution liquid chromatography-mass spectrometry. Our results revealed the presence of microbial sunscreens in nearly all surface scum samples. Mycosporine-like amino acids are synthesised through the coordinated action of four biosynthetic enzymes encoded in a compact biosynthetic pathway. Bioinformatics analysis of the mysB biosynthetic gene from a surface scum indicated that the cyanobacteria responsible for production belonged to the Anabaena/Dolichospermum/Aphanizomenon species complex. We mapped the distribution of biosynthetic enzymes onto a phylogenomic tree, utilising 120 bacterial single-copy conserved genes from 101 draft or complete genomes within the species complex. This analysis showed that 48% of identified species possess the ability to produce these compounds, with biosynthetic pathways being most common in Dolichospermum and Aphanizomenon strains. We detected the production of porphyra-334 and shinorine, two widely reported family members, in Dolichospermum strains isolated from the Gulf of Finland's surface layer. The estimated content of porphyra-334 in Dolichospermum sp. UHCC 0684 was 7.4 mg per gram dry weight. Our results suggest that bloom-forming cyanobacteria could be a potential source of these compounds for cosmetic and biotechnological applications and may play a significant role in cyanobacterial bloom formation.