4. Discussion
Producing secondary metabolites like mycosporine like amino
acids or pigments in response to UV radiation is a well-known protection
mechanism in algae and cyanobacteria [11–16]. Possible
effects on primary metabolites such as carbohydrates, amino acids
and nucleosides have not been reported to our knowledge, even
though they are essential substrates for secondary metabolic pathways
[28]. Results of our study support this statement, because at
least some of the compounds which were found to be upregulated
are precursors of known photoprotectants. For example, tyrosine
and phenylalanine are required for melanin and bacterial pyomelanin
biosynthesis [29], as well as for the formation of mycosporine
like amino acids. Therefore, our data at least partially explains
already known strategies and contributes to a holistic understanding
of defense mechanisms of algae and cyanobacteria against UV
stress.
A few studies could show an upregulation of primary metabolites
in higher plants when they are exposed to UV radiation
[28,30]. However, as no such effects have been described for algae
and cyanobacteria, our study is the overall first evidence for a similar
strategy in these organisms. This is not only of relevance for
developmental biology but also might pave way for new applications.
Several studies have reported on antioxidative, DNA protective
and anti-inflammatory effects of nucleosides; especially
adenosine and guanosine seem to be beneficial in this respect
[31–33]. Guanosine is able to prevent deamination of cytosine in