Marine natural flavonoids: chemistry and biological activities. 
As more than 70% of the world's surface is covered by oceans, marine organisms offer a rich and unlimited resource of structurally diverse bioactive compounds.  These organisms have developed unique properties and bioactive compounds that are, in majority of them, unparalleled by their terrestrial counterparts due to the different surrounding ecological systems.  Marine flavonoids have been extensively studied in the last decades due to a growing interest concerning their promising biological/pharmacological activities.  The most common classes of marine flavonoids are flavones and flavonols, which are mostly isolated from marine plants.  Although most of flavonoids are hydroxylated and methoxylated, some marine flavonoids possess an unusual substitution pattern, not commonly found in terrestrial organisms, namely the presence of sulphate, chlorine, and amino groups.  This review presents, for the first time in a systematic way, the structure, natural occurrence, and biological activities of marine flavonoids.