Effects of Extracellular Self- and Nonself-DNA on the Freshwater Microalga Chlamydomonas reinhardtii and on the Marine Microalga Nannochloropsis gaditana

The role of extracellular DNA (exDNA) in soil and aquatic environments was mainlydiscussed in terms of source of mineral nutrients and of genetic material for horizontal gene transfer.Recently, the self-exDNA (conspecific) has been shown to have an inhibitory effect on the growthof that organism, while the same was not evident for nonself-exDNA (non conspecific). Theinhibitory effect of self-exDNA was proposed as a universal phenomenon, although evidence ismainly reported for terrestrial species. The current study showed the inhibitory effect of self-exDNAalso on photosynthetic aquatic microorganisms. We showed that self-exDNA inhibits the growth ofthe microalgae Chlamydomonas reinhardtii and Nannochloropsis gaditana, a freshwater and a marinespecies, respectively. In addition, the study also revealed the phenotypic effects post self-exDNAtreatments. Indeed, Chlamydomonas showed the formation of peculiar heteromorphic aggregatesof palmelloid cells embedded in an extracellular matrix, favored by the presence of DNA in theenvironment, that is not revealed after exposure to nonself-exDNA. The differential effect of selfand nonself-exDNA on both microalgae, accompanied by the inhibitory growth effect of selfexDNAare the first pieces of evidence provided for species from aquatic environments.