Difference between revisions of "Chemical ecology"

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(Chemical ecology and seaweeds)
(Chemical ecology and seaweeds)
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===Chemical ecology and seaweeds===
 
===Chemical ecology and seaweeds===
  
Seaweeds have been shown to produce a large variety of metabolites with highly variable structures (such as terpenoids, acetogenins, amino-acid derivates and polyphenols). Many of these compounds can act as antimicrobial and [[antifouling agent|antifouling]] or ultraviolet screening
+
Seaweeds have been shown to produce a large variety of metabolites with highly variable structures (such as terpenoids, acetogenins, amino-acid derivates and polyphenols). Many of these compounds can act as antimicrobial and [[antifouling agent|antifouling]] or ultraviolet screening agents, as well as herbivore deterrents.
agents, as well as herbivore deterrents.
 
  
 
Most marine herbivores are generalist grazers that consume many different seaweeds, although some herbivore species can be specialised on one or a few algal species.
 
Most marine herbivores are generalist grazers that consume many different seaweeds, although some herbivore species can be specialised on one or a few algal species.
Line 51: Line 50:
 
Smaller grazers use plants both as food and habitat, and they consume individual algae over a more extended period of time. It has been hypothesised that smaller grazers may select for inducible rather than constitutive defences (i.e., defences that are produced in response to specific environmental cue).
 
Smaller grazers use plants both as food and habitat, and they consume individual algae over a more extended period of time. It has been hypothesised that smaller grazers may select for inducible rather than constitutive defences (i.e., defences that are produced in response to specific environmental cue).
  
The hypothesis that sessile or slow-moving
+
The hypothesis that sessile or slow-moving organisms, without obvious escape mechanisms
organisms, without obvious escape mechanisms
+
and physical protection, are likely to be chemically defended has been explored
and physical protection, are likely to be
+
in the marine environment. Of these organisms,
chemically defended has recently been explored
+
[http://www.marinespecies.org/aphia.php?p=taxdetails&id=382226 opisthobranch] molluscs appear to be particularly well endowed with secondary metabolites. In these gastropods, the reduction of the protection offered by the shell is compensated by the development of complex defence strategies that include use of chemicals. Opisthobranchs can feed
with greater frequency in the marine
+
upon [http://www.marinespecies.org/aphia.php?p=taxdetails&id=558 sponges], algae, [http://www.marinespecies.org/aphia.php?p=taxdetails&id=19494 hydroids], [http://www.marinespecies.org/aphia.php?p=taxdetails&id=146142 bryozoans], [http://www.marinespecies.org/aphia.php?p=taxdetails&id=146420 tunicates] and soft corals. In some cases they are not only capable to accumulate dietary molecules but can also transform or even produce new chemical mediators (see D on figure).
environment. Of these organisms,
+
 
opisthobranch molluscs appear to be
+
[http://www.marinespecies.org/aphia.php?p=taxdetails&id=140672 Oxynoe olivacea], a green sea snail that lives camouflaged upon algae ([http://www.marinespecies.org/aphia.php?p=taxdetails&id=143816 Caulerpa]), is able to transform a major algal metabolite, caulerpenyne, to oxytoxins, which is 100 times more toxic (see E on figure).
particularly well endowed with secondary
 
metabolites. In these gastropods, the reduction
 
of the protection offered by the shell is
 
compensated by the development 30 of complex
 
strategies of defence that include use of
 
chemicals. In sea slugs (Nudibranchia), the shell
 
is lost, and these species tend to show high
 
specialised behaviour. Opisthobranchs can feed
 
upon sponges, algae, hydroids, bryozoans,
 
tunicates and soft corals. In some cases they are
 
not only capable of accumulating dietary
 
molecules but also transform or even produce
 
chemical mediators de novo (Fig. 7d).
 
Oxynoe olivacea, a green sea snail
 
that lives camouflaged upon algae
 
(Caulerpa), is able to transform a major
 
algal metabolite, caulerpenyne, to
 
oxytoxins, increasing the toxicity of the
 
algal metabolite 100 times (Fig. 7e).
 
 
Despite its emphasis on the integration of
 
Despite its emphasis on the integration of
 
researchers, MarBEF has also served as a
 
researchers, MarBEF has also served as a

Revision as of 12:57, 28 August 2009

The importance of chemical ecology

Chemical ecology, has helped to understand terrestrial ecosystems. How bees pollinate flowers, how birds find their nests and human attractiveness to a partner are some of the many examples of interactions which are mediated by chemicals. It is not difficult to imagine the catastrophic consequences of the absence of such crucial relationships. Imagine a similar scenario without chemical interactions in the marine environment. Species would no longer be able to identify their food, locate their pray, recognize mates,... . Species-specific chemicals can shape community processes such as seasonal succession, niche structure, selective feeding and population dynamics.

The MarBEF ROSEMEB (Role of Secondary Metabolites in Ecosystem Biodiversity) project has provided a better understanding of the roles of these chemicals in maintaining marine biodiversity and driving ecosystem functionality. Some of these findings are discussed bellow.


Chemical ecology and microbes

Microbes sense their environment via cell-associated and diffusible molecules such as AHL (N-acylhomoserine lactones). Such molecules are constantly produced by many bacteria and diffuse through membranes into the surrounding environment.

When a certain cell density (a threshold or quorum) of the bacterial population and a corresponding concentration of AHL is reached, the expression of certain target genes is initiated. These may include the proteins for light emission in luminous bacteria or pathogenic factors that cause disease.

This quorum-sensing typically controls processes, such as swarming (coordinated movement), virulence (coordinated attack) or conjugation (gene transfer between cells), which require high cell densities for success and that are essential for the survival of the organisms which produce the molecules.


Chemical ecology and phytoplankton

Many plankton species use a chemical defence against their predators, either through toxin production or feeding deterrence. Diatoms are key players at the base of the marine food web and have always been assumed to be a good food source for herbivores. Some species however use chemicals as a defence against being grazed. The discovery that these unicellular algae produced chemicals, such as polyunsaturated aldehydes (PUAs) and other oxidised products of fatty acid metabolism (collectively termed oxylipins), that induced abortions, birth defects, poor development and high offspring mortality to their grazers has changed our view of plant-animal interactions in the plankton. (C on the figure)


The sponge Aplysina aerophoba produces the anti-microbial and anti-tumour compound Aplysinin-1. Staining with DAPI reveals the rich microbial fauna associated with the sponge. B. The effect of diatom-derived unsaturated aldehydes on diatoms (a) Phaeodactylum tricornutum and (b) Thalassiosira weissflogii. C. The effect of the diatom-derived unsaturated aldehydes 2-trans-4- cis-decatrienal (A1), 2-trans-4-trans-7-cis-decatrienal (A2) and 2-trans-4trans-decadienal (A3) on copepod hatching success (Miralto et al., 1999; Ianora et al., 2004a). D,E. Transformation of caulerpenyne in the allomonal oxytoxins by lipolytic enzymes, named Lip-1 (Lipase 1) and Lip-2 (Lipase 2), in Oxynoe olivacea (from Cutignano et al., 2004).


Although the effects of these toxins are less catastrophic than of those which cause death, they can have ecological effects. They can sabotage future generations of grazers which might allow diatom blooms to persist when grazing pressure would normally have caused them to crash. This defence mechanism is new and specific for the marine environment because most of the known negative plant–animal interactions are related to poisoning, repellence or feeding deterrence instead of to reproductive failure.

In fact, the production of PUAs mainly affects future generations of grazers and have less effect on the direct adult grazers. PUAs have also been shown to negatively impact other phytoplankton cells and might function as a signal to trigger active cell-death. (B on the figure)

So, these compounds may have multiple functions within plankton communities. They can act as defence molecules against predators and competitors, as well as signal molecules to drive diatom bloom dynamics and species succession patterns. More information can be found here.

Other phytoplankton groups such as the dinoflagellates produce neurotoxins that can be transferred (biomagnify) up the marine food chain and have been responsible for mass fish-kills, both wild and farmed, as well as for the deaths of sea birds and marine mammals, including whales and sea lions. See also here.

In humans, consumption of shellfish containing high levels of such toxins can induce paralytic, neurotoxic, diarrhetic and amnesic shellfish poisoning. Records of human poisoning by at least two of these syndromes date back hundreds of years, yet the discovery and characterisation of the responsible molecules happened quite recent.

Many benthic invertebrates can use compounds, from the food they consume, as defensive molecules against predators. Some plankton species might possibly do the same. Lots of research still needs to be conducted on the effects of toxins on gamete, embryonic and larval development of herbivorous grazers. It also remains unknown why zooplankton don't consume certain metabolites and what happens to them when they do.

Chemical ecology and seaweeds

Seaweeds have been shown to produce a large variety of metabolites with highly variable structures (such as terpenoids, acetogenins, amino-acid derivates and polyphenols). Many of these compounds can act as antimicrobial and antifouling or ultraviolet screening agents, as well as herbivore deterrents.

Most marine herbivores are generalist grazers that consume many different seaweeds, although some herbivore species can be specialised on one or a few algal species. Grazing pressure highly depends on the specific seaweed and herbivore involved. Grazing pressure is however generally considered be higher in tropical coral reefs than in temperate habitats. Large mobile grazers, such as fish, crabs and sea urchins, can have a more drastic negative effect on seaweed production and fitness than smaller ones. Due to their ability to rapidly consume large amounts of algal tissues, they are thought to select for constitutive defences (i.e., defences which are continuously produced and present within the algae).

Smaller grazers use plants both as food and habitat, and they consume individual algae over a more extended period of time. It has been hypothesised that smaller grazers may select for inducible rather than constitutive defences (i.e., defences that are produced in response to specific environmental cue).

The hypothesis that sessile or slow-moving organisms, without obvious escape mechanisms and physical protection, are likely to be chemically defended has been explored in the marine environment. Of these organisms, opisthobranch molluscs appear to be particularly well endowed with secondary metabolites. In these gastropods, the reduction of the protection offered by the shell is compensated by the development of complex defence strategies that include use of chemicals. Opisthobranchs can feed upon sponges, algae, hydroids, bryozoans, tunicates and soft corals. In some cases they are not only capable to accumulate dietary molecules but can also transform or even produce new chemical mediators (see D on figure).

Oxynoe olivacea, a green sea snail that lives camouflaged upon algae (Caulerpa), is able to transform a major algal metabolite, caulerpenyne, to oxytoxins, which is 100 times more toxic (see E on figure). Despite its emphasis on the integration of researchers, MarBEF has also served as a catalyst in a remarkable range of new discoveries. New species were found and characterised from across the range of marine life from bacteria to crustaceans, polychaetes and echinoderms. In addition, the application of new tools, which we recognise as one of the fundamental drivers of scientific progress over the last millennium, has generated insight into the astonishing functional diversity of microbial life in the oceans. Molecular tools have allowed MarBEF scientists to probe the functioning of marine microbial communities in novel ways – and their results have reinforced the emerging view that improved understanding of the dynamics of marine life at all scales is the key to developing a predictive model of the Earth Systems. This understanding will be built by integrating knowledge of which organisms are involved (taxonomy), how they are involved in ecosystem processes (ecology, chemical ecology and functional genomics) and their history of involvement (phylogeny and co-evolutionary history). The discoveries we have made in MarBEF are vital steps in this process.

See also