**Preface to "Bioactive Molecules from Marine Microorganisms"**

Microorganisms live in all types of habitats and under variety of environmental conditions, including extreme ones. They are the most abundant organisms on Earth, highly diverse and classified into different domains. The diversity of marine microorganisms and their products represents high, but still unexplored, pharmaceutical potential. The most frequently observed effects of microbial products include anticancer, anticoagulant, antibacterial, antiviral, neurotoxic and immune-modulating activities. Metabolites of marine microorganisms have also been used as templates for the development of new pharmaceuticals with unique mechanisms of action. Despite the enormous potential offered by marine microorganisms, so far only a few of their metabolites have been successfully developed into FDA-approved drugs or have entered clinical trials. The fact that only a small fraction of marine microorganisms and their bioactive molecules has been discovered gives a great chance to further explore them and develop them into high added value products.

For this Special Issue book, ten papers focusing on novel bioactive molecules from different marine microorganisms, including fungi, cyanobacteria, actinobacteria and diatoms, were selected. The isolated biomolecules represent different structures and showed anticancer, antiviral, antifungal, antibacterial, anti-inflammatory and enzyme-inhibiting activities. One of the papers is a review article on microviridins, a class of bioactive cyanobacterial peptides.

> **Hanna Mazur-Marzec, Anna Toru ´nska-Sitarz** *Editors*
