Extracellular self-DNA inhibits microalgae growth: insights on self-inhibition in freshwater and marine environment

The extracellular DNA (exDNA) is defined as the DNA outside the cell that can be actively or passively released1. It can be located in different natural habitats (e.g. soil, sediments, oceans and freshwater) as free molecule, complexed with inorganic and organic particles2,3 or organized in extracellular structures4-6. The role of exDNA has been mainly investigated in soil, water column and in aquatic sediments, where nucleotides function as a nutrient 7, phosphate 8 and carbon 9 sources 8,10or as building blocks for the synthesis of new molecules11. Recently, the exDNA has been shown to have an inhibitory effect on the growth of conspecific (self-exDNA), while the same was not evident for heterologous (nonself-) exDNA12. The inhibitory effect of self-exDNA was proposed as a universal phenomenon, although evidence is mainly reported for terrestrial species13. In this research, we show the inhibitory effect of self-exDNA on the growth of the microalgae Chlamydomonas reinhardtii and Nannochloropsis gaditana, a freshwater and a marine species, respectively. This study revealed the formation of peculiar heteromorphic structures embedded in an extracellular matrix after the treatment with self-exDNA. The differential effect of self and nonself-exDNA on both microalgae, accompanied by the inhibitory growth effect of self-exDNA are the first pieces of evidence provided about this phenomenon for species leaving in aquatic environments14.