Diatoms are highly successful unicellular algae occurring in ocean and fresh water phytoplankton, as well as in biofilms on solid substrates. They are exceedingly abundant and are among the most important primary sources sustaining the marine food chain. Despite this, little is known about the chemical defense of these micro algae. Two of the few reported examples are the aldehydes decadienal 5 and decatrienal 6 (see Scheme 1) from the diatom Thalassiosira rotula, which reduce the hatching success from eggs of copepods (zooplankton grazers). [1] This observed activity explains the paradox that herbivorous copepods are less successful feeding on diatoms, although these algae are considered as high-quality food.
Here I provide biosynthetic and kinetic data on the formation of fatty acid derived metabolites in planktonic diatoms, demonstrating that the release of a,b,g,d-unsaturated dienals is widespread among this class of algae. The enzymatic mechanism to produce these metabolites is efficiently activated seconds after cell disruption and leads to high local concentrations of the defensive metabolites 5 and 6 or of structurally related potentially active aldehydes like 9.
The simultaneous production of C 11 hydrocarbons and 9-oxonona-5Z,7E-dienoic acid from C 20 fatty acids was demonstrated with the benthic diatom Gomphonema parvulum. [2,3] The polar dienoic acid contains the same aldehydic that were hardly soluble in the cleavage cocktail were washed off the solid support with a suitable solvent. Before concentration cleavage solutions were neutralized with pyridine. Analytical HPLC was carried out on C18 reversed-phase columns (250 ร 4 mm) with linear gradients of acetonitrile in water/0.1 % TFA and a flow of 1 mL min ร1 . Product peaks were characterized by ESI-MS. Experimental details for the synthesis of 8, 9, 11, and 12 are found in the Supporting Information.