There are no less than 100 producers of toxic and biologically active/biocidal substances among 1100 cyanobacterial and microalgal species in the Black Sea. Our investigations include two aspects: (1) evaluation of ecological and biochemical connections between cyanobacteria and microalgae and herbivorous organisms ("grazers"), and (2) formation of new approaches to the using of hydrobionts and their metabolites in human affairs. Experimental data confirm that biological and ecological constructiveness of mass and active cyanobacteria and microalgae species lays not only in toxic effect. It is complex, multiple and many-sided challenge and a part of co-evolutionary process, which resides very close to allelochemical interactions between plants and herbivorous organisms in terrestrial ecosystems. Selectivity of biocidal effect of cyanobacteria and microalgae and high level of their controlled cultivation make possible to create new preparations for pest control.