Certain diatom diets cause adverse effects on the reproductive success of copepods. Diatom-derived polyunsaturated aldehydes, formed upon cell damage, were made responsible for these effects. A joint biochemical and analytical approach was conducted to get a deeper insight into the formation and abundance of diatom-derived aldehydes. An enzymatic cascade of lipase/lipoxygenase and lyase activities, which can even be initialized in the copepod gut, transforms C20/C16-polyunsaturated fatty acids into diverse structurally related aldehydes.

Nevertheless most exciting studies focused only on laboratory experiments and only few diatom-species were examined for their deleterious potential. It is thus difficult to get a reliable picture of the potential ecological impact of unsaturated aldehydes in the field. To overcome these limitations I elaborated an in situ derivatisation protocol of unsaturated aldehydes for monitoring the chemical defense potential of phytoplankton during field studies and of cultivated strains. Taking into account the biosynthesis, diatom containing samples are damaged in the presence of the trapping-reagent O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (= PFBHA) under near physiological conditions. This leads to an in situ derivatisation without inhibition of aldehyde-biosynthesis. I found that only 18 out of 51 investigated species release unsaturated aldehydes. In parallel, I investigated the effect of phytoplankton on the reproductive success of Calanus helgolandicus (Western English Channel, Roscoff, France). The recruitment rates of copepods derived from diatom enriched diets were temporal less than predicted based on the reproductive responses on non-diatom control food. Several hypotheses have been put forward to explain the suppression of copepod recruitment. Food quality is a major factor influencing growth, reproduction and survival of zooplankton. However, results show that the diatom-derived production of deleterious unsaturated aldehydes cannot account for the reduction of egg production rate and of hatching success or malformed larvae of Calanus helgolandicus in coastal waters off Roscoff. Moreover, the reproductive response was not affected by e.g. polyunsaturated fatty acids indicating food quality.

This work also presents evidences that the lipoxygenase mediated pathways in diatoms are very diverse. Besides the aldehyde-producing diatoms and species, which do not release any aldehyde, others transform C20-polyunsaturated fatty acids into acidic unsaturated aldehydes and a further second fragment. For instance, I found that the marine diatom Stephanopyxis turris transforms eicosapentanoic acid into halogenated compounds, such as 3-chloro-1,5Z-octadiene and 1-chloro-2Z,5Z-octadiene, and the 12-oxododeca-5Z,8Z,10E-trienoic acid (= 12-ODTE) catalyzed by a newly identified hydroperoxide-halolyase. For further systematic investigations of the wound activated oxylipin formation, I have explored the release of volatiles of the model organism Physcomitrella patens. The moss contains high levels of arachidonic acid as well as the typical C18-polyunsaturated fatty acids of higher plants. The oxylipin profile has revealed the preferential transformation of arachidonic acid. An involved multifunctional lipoxygenase (PpLOX) and a hydroperoxid-lyase (PpHPL) were cloned (Prof. Ivo Feussner, University Göttingen, Germany) and characterized. While PpLOX, a novel lipoxygenase with a fatty acid chain cleaving lyase activity, provides (R)-1-octen-3-ol (ee > 94 %) and (2Z)-octen-1-ol and 12-ODTE, PpHPL transforms 12-hydroperoxy- arachidonic acid exclusively into (3Z)-nonenal and 11-oxoundeca-5Z,8Z-dienoic acid demonstrated by knockout lines.

Email: twichard@princeton.edu