Research
interest & previous works of Dr. Tilmann Harder
Research
projects in the Environmental Biochemistry Group
Marine
Sex-Pheromones
Marine Biofouling
Chemical Ecology of Larval Settlement
I
seek to understand ecological phenomena on the basis of their
underpinning chemical interaction between organisms. My research
interest is particularly directed towards chemical communication
systems in the marine environment. Research in this area requires
unveiling sender-receiver relationships, locating the sources
of chemical signals involved and the development as well as the
design of suitable bioassays in order to measure and monitor the
efficacy of bioactive crude samples. Since the bioactive target
compounds are structurally unknown, their bioactivity needs to
be conserved in each purification step. Thus, the bioassay-guided
purification of bioactive targets represents a highly challenging
task to the analyst. The isolation and structural elucidation
of analytes involves the whole spectrum of instrumental chemical
analysis of natural products in trace amounts, such as chromatography,
spectroscopy and coupled analytical methods. Hence, my research
interest effectively frames my core expertise in Analytical Chemistry
of natural products with Marine Biology, Larval Biology, Environmental
Microbiology and Molecular Biology.
Marine
sex-pheromones
A long-standing interest of mine has been directed towards the
investigation of the highly synchronized reproductive behavior
of marine polychaetes by sex-pheromones. We have successfully
isolated the chemical signals that coordinate species-specific
mate recognition and regulate the synchronous release of gametes
in these nereids by bioassay-guided HPLC. The pheromones were
identified as purine and pyrimidine derivatives by NMR and MS
and represent a novel class of pheromones both in the terrestrial
and the marine environment. TOP
Marine
Biofouling
My recent research interest has been focussed on marine biofouling
and fouling control. In the marine environment, any submerged
material is quickly colonized by bacteria, protozoa and algae
resulting in so-called "biofilms". Marine biofilms can
provide various chemical signals to the larvae of sessile invertebrates
in search for suitable sites for permanent attachment and metamorphosis.
Once settled, these invertebrates cause tremendous problems for
shipping industries and mariculture. The annual costs associated
with marine biofouling, e.g. due to drag, repainting, dry-docking
and environmental compliance, are estimated in the 1b dollar range.
Present antifouling technology is based on organotin- and copper-containing
coatings. Due to the toxicity, non-specificity, persistence and
bioaccumulation of organometals in the marine environment, the
current technology is being outlawed by the International
Maritime Organization and will be globally banned by 2008.
Thus, the demand for environmentally benign substitutes is high.
One rational approach towards this goal is to understand chemical
ecological principles of natural antifouling defense. Marine organisms
such as sponges, soft corals and certain macroalgae are frequently
observed to lack overgrowth and are therefore hypothesized to
employ allelochemical strategies to prevent epibiosis (i.e. the
overgrowth by fouling organisms).
I felt intrigued by this subject since it sensibly augments chemical
analysis and elucidation of bioactive marine natural products
with an area of direct commercial application. In case of seaweeds
and soft corals we have isolated and identified secondary metabolite
with high efficacy to inhibit larval attachment and metamorphosis
of polychaetes, barnacles and bryozoans, all of which are dominant
troublesome fouling organisms in temperate and tropical waters.
TOP
Chemical
ecology of larval settlement
An alternative approach towards novel environmentally benign antifouling
strategies is to resolve chemical cues involved in the induction
of larval settlement. According to the current state of knowledge,
marine biofilms signal the appropriateness of a substratum to
the surface-exploring larva. Since marine bacteria represent the
major component of marine biofilms, bacteria-derived cues are
hypothesized to mediate the larval settlement response in addition
to chemical signals from conspecifics.
In a collaborative effort of Marine Biologists and Microbiologist,
we have determined significant differences in the settlement induction
activity of a large number of bacterial isolates from marine biofilms
in bioassays with common fouling organisms and have analyzed both
surface-associated and waterborne bacteria-derived chemical signals.
These works led to the first experimental evidence that bacterial
metabolites indeed evoke the larval settlement response. The experiments
comprised the bioassay-guided isolation of volatile and nonvolatile
bacterial metabolites and their purification and analysis by HPLC
and GC-MS. We have isolated and analyzed larval settlement inducers
from a large pool of marine bacteria in order to understand the
complexity of chemical signals involved in a variety of fouling
organisms. A comprehensive knowledge of the induction of larval
settlement on the molecular level is crucial for the design of
novel antifouling strategies. The search for common patterns in
structural similarities among these chemical signals may serve
to design and model broadband antagonists of larval receptors.
TOP
