Neue Lipoxygenase-/Hydroperoxid-Lyase-Biosynthesewege im Moos Physcomitrella patens

Formation of Halogenated Medium Chain Hydrocarbons by a Lipoxygenase/Hydroperoxide Halolyase-Mediated Transformation in Planktonic Microalgae

A rapid transformation of eicosapentaenoic acid is initiated upon disintegration of cells of the marine diatom Stephanopyxis turris. In contrast to any other fatty acid cleaving activity known, this diatom releases a blend of chlorinated C8 hydrocarbons in addition to an unsaturated 12-oxo acid. Using labeled precursors, trapping experiments, and selective inhibition of enzyme activities, we were able to show that a lipoxygenase activates the C20 fatty acid to a hydroperoxide, which is subsequently cleaved by a new hydroperoxide halolyase to the C8 and C12 products. This adds a fundamentally new pathway to the limited set of halogenating enzymatic activities known from nature.

Diatom/copepod interactions in plankton: the indirect chemical defense of unicellular algae

Numerous coexisting species can be observed in the open oceans. This includes the complex community of the plankton, which comprises all free floating organisms in the sea. Traditionally, nutrient limitation, competition, predation, and abiotic factors have been assumed to shape the community structure in this environment. Only in recent years has the idea arisen that chemical signals and chemical defense can influence species interactions in the plankton as well. Key players at the base of the marine food web are diatoms (unicellular algae with silicified cell walls) and their main predators, the herbivorous copepods. It was assumed that diatoms represent a generally good food source for the grazers but recent work indicates that some species use chemical defenses. Secondary metabolites, released by these algae immediately after wounding, are targeted not against the predators themselves but rather at interfering with their reproductive success. This strategy allows diatoms to reduce the grazer population, thereby influencing the marine food web. This review addresses the chemical ecology of the defensive oxylipins formed by diatoms and the question of how these metabolites can act in such a dilute environment. Aspects of biosynthesis, bioassays, and the possible implications of such a chemical defense for the plankton community structure are also discussed.