Our group elucidates new chemical defence- and communication strategies of marine and freshwater organisms using the tools of modern bioorganic chemistry. The work aims to understand the role of secondary metabolites as mediators of ecological interactions. Isolation, structure elucidation and organic synthesis of natural products are important aspects of our work, but we believe that the full picture of the role of the compounds can only be obtained if metabolomics, biochemistry and ecology are brought in as well. Our interdisciplinary work gives new insights into the chemically mediated species interactions and the function of natural products. The picture by Charles Vidoudez and Matt Welling shows a microscopic view of the siphonous green alga Udotea spinulosa and a UPLC-MS chromatogram.
Diatoms are unicellular algae that count to the most important primary producers contributing more than 20 % to the global carbon fixation. They are among the most abundant algae of phytoplankton and are at the basis of the marine food chain. In recent years we have gained mainly indirect evidence that both, toxic as well as non-toxic phytoplankton species are capable of releasing metabolites that influence other algae. These compounds might act as infochemicals mediating intraspecific communication (A) or as allelochemicals (B) that influence other species in the plankton community. We pursue a metabolomics approach to profile metabolites released by the cells during growth in cultures or during phytoplankton blooms in the field. By correlating the data to ecological observations on interactions we aim to identify new roles for released metabolites.
The picture shows the grouping of metabolites released during the growth of a culture of the diatom Skeletonema costatum. It can be clearly seen that metabolites released by the cells differ during the development of a plankton culture. The same is true for metabolites observed during a phytoplankton bloom in the sea. Identified metabolites and release patterns can then be tested in bioassays to unravel the underlying ecological significance (see recent publications). We also employ this approach to monitor the development of the algal physiology and the response of algae to external factors by profiling intracellular metabolites.
Wound-activated defence in plankton
Our work on the fatty acid derived secondary metabolites of diatoms shows that some species are well defended by low molecular weight metabolites. This defence is activated upon cell disruption and provides high local concentrations of reactive aldehydes in close vicinity to the feeding herbivore. Diatoms are thus able to overcome the high cost of storing or releasing continuously defensive metabolites into their environment by activating their defence only on demand. Using fluorescent and stable isotope labelled fatty acids as metabolic probes we perform detailed mechanistic investigations on the lipoxygenase and hydroperoxide lyase dependant reactions that lead to defensive metabolites.
Schematic representation of the oxylipin-mediated diatom copepod interactions
(after G. Pohnert, ChemBioChem, 6: 946-959, 2005)
Algae showing a siphonous (coenocytic) organisation usually consist of hollow bags (siphons), in which cells are lacking. In the entire macroscopic organism there are no cross walls separating nuclei, thus the whole thallus consists of a continuous mass of cytoplasm contained only by the outer walls. Siphonous algae can thus be seen as giant unicellular organisms. In contrast to multicellular organisms these algae cannot perform tissue repair after woundig but have rather to rely on elaborate cell-repair mechanisms to seal wounds before the cytoplasm extrudes. We aim to understand the underlying mechanisms of rapid biopolymer formation that mediate wound repair mechanisms in these algae.
One model organism is the invasive macroalga Caulerpa taxifolia. After its accidental introduction into the Mediterranean Sea C. taxifolia now rapidly spreads along the coastal lines. This species causes massive damage to the local flora and fauna by supplanting the local vegetation. The alga adapted very well to its new habitat. Its success is based, among others, on a very efficient reproductive strategy and a highly active chemical defence. Till most recently it was assumed that this defence is nearly exclusively based on the sesquiterpene caulerpenyne.
We found that caulerpenyne is efficiently transformed after tissue disruption of the alga. An esterase releases highly unstable 1,4-bis-aldehydes that can be detected using trapping reactions.
Caulerpa taxifolia grown in our aquaria
Wound activated transformation of caulerpenyne (top) to reactive aldehydes.
Interestingly, this conversion leads to a highly reactive molecule featuring a 1,4-dialdehyde structure. This metabolite acts as an efficient protein cross linker after wounding and is involved in the fast wound closure mechanism of the alga. We could characterize a new biopolymerisation prozess, which takes place immediately after wounding, leading to a complex copolymer of the entity of algal proteins and the excess cross linker. The function of this mechanism in the formation of the wound plug, which seals the giant cells after disruption could be proven by biochemical and chemical processes.
Rapidly after wounding the formation of a polymer wound plug is observed (left). This process can be inhibited by esterase-inhibitors, which prevent the deacetylation of caulerpenyne.
We are also beginning to understand the new role of caulerpenyne as precursor of reactive defensive compounds thus providing a bi-functional character of this molecule for the alga. Field studies on the detailed mechanism of chemical defence are underway in close collaboration with the group of Alexandre Meinesz at the University of Nice. The way how certain marine snails (Sacoglossans) can overcome the chemical defense of Caulerpa spp. is a major focus.
We also adress the generally different wound plug formation of Dascyclardus spp. and Halimeda spp. with focus on mechanistic and evolutionary aspects but also on practical applications.
We investigate the alga/pathogen interaction in the system of the red alga Chondrus crispus (left on top) and the green algal pathogen Acrochaete operculata (below left), which is able to grow within the tissue of the red algal host. This process, called endophytism, can be inhibited during certain developmental phases of C. crispus. During the resistant, gametophytic, phase of the life cycle, C. crispus can recognise the attacker and kill it by an immediate release of hydrogen peroxide. We found that carrageenans from the red alga induce the release of asparagine from the green algal parasite. The free amino acid itself acts as a substrate for an amino acid oxidase of the host that releases micromolar amounts of hydrogen peroxide, sufficient to contain the attacker. The aim of a related collaborative project (European Union EPIFIGHT) is to understand the biological bases of the interactions between the red alga Gracilaria chilensis and its main epiphytic pests. We are elucidating the defensive chemistry of the host with special emphasis on the induction of reactions upon pathogen stress. Both, the signals and hormones involved in the up-regulation of the production of defensive metabolites are investigated using analytical chemical and biochemical approaches.
The cover picture of ChemBioChem (3/2006) an epifuorescence-microscopic view(excitation: 360 nm, emission: 460 nm) of a microalgal epiphyticcommunity growing on a field-collected branch of the red seaweed Gracilaria chilensis. Host cells and membranes are often damaged by penetrating epiphytes such as Acrochaetiumsp. (shown as up to 2.5 mm-long filaments in the picture). In our article on p. 457 ff, we provide evidence that thehydroxylated eicosanoids shown in yellow are generated through twodistinct biosynthetic pathways after wounding of G. chilensis. These compounds activate resistance against spore settlement of A. operculata.
In an ongoing survey we investigate the oxylipin chemistry of mosses. These organisms are of particular interest since they are able to combine biosynthetic aspects found in mammals, plants and algae to synthesize a plethora of oxylipins.