The present disclosure describes a DNA fragment mixture for the prevention or for the treatment of at least one pathogenic, parasitic, or infesting species of plants or of the environment, wherein the DNA mixture consists of random fragments of total DNA of at least one pathogenic, parasitic, or infesting species, and/or at least one phylogenetically similar species, against which the prevention and treatment are directed. Further, the disclosure describes a process and related system for improvement of the production/growth of microorganisms at high yield in bioreactors or photobioreactors, or of plants in different culture systems, where the nucleic acids of the organisms produced/grown by such a process are removed from the culture medium and the culture medium, deprived of these nucleic acid, is used again in the process.