Functional metabolites in phytoplankton
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Allelopathy and functional metabolites in phytoplankton
Allelopathy is the study of chemical interactions among neighboring plants and the chemicals responsible for such interactions. The word allelopathy derives from two separate words: allelon which means "of each other", and pathos which means "to suffer". In the phytoplankton, the release of chemicals by microalgae that induce negative effects on growth of other microalgae has mainly been studied in toxin-producing species such as cyanobacteria, diatoms, dinoflagellates and flagellates, and has been suggested to influence phytoplankton competition, succession, and bloom formation or maintenance. The mode of action of allelochemicals can be quite diverse, and the chemical nature of these compounds is largely unknown. The most common effect is to cause cell lysis, blistering, or growth inhibition. The factors that affect allelochemical production have not been studied much, although nutrient limitation, pH, and temperature appear to have an effect. The evolutionary aspects of allelopathy remain largely unknown, but it has been suggested that the producers of allelochemicals should gain a competitive advantage over other phytoplankton. A recent line of research is highlighting the role of these compounds for cell-to-cell communication. This is the case for diatom unsaturated aldehydes, which are involved in a stress surveillance mechanism based on fluctuations in calcium and nitric oxide levels. When stress conditions during a bloom and cell lysis rates increase, aldehyde concentrations may exceed a certain threshold, and possibly function as a diffusible bloom-termination signal that triggers an active cell death. Diatom-derived aldehydes also have an allelopathic role, since they have been shown to affect growth and physiological performance of diatoms and other phytoplankton species.
Phytoplankton-zooplankton chemical interactions
Herbivory is very intense in the plankton. Copepods and other planktonic crustaceans are predominantly herbivorous, grazing on large quantities of phytoplankton cells. Herbivory is therefore an important pressure for the evolution of defensive compounds in marine phytoplankton, seaweeds and macroalgae, and for shaping prey- predator relationships in the pelagic environment. However, the organism has to pay a price for this ecological advantage since the chemical pathways that generate these metabolites are often complex and significant amounts of metabolic energy are expended to generate their production. This may be the case of constitutive metabolites that are always present within the cells as opposed to induced defenses that are only produced when the predator is present. In the case of diatoms, for example, some compounds (oxylipins) are not constitutively present in the cells but are only produced when the cell is damaged as would occur during grazing. Thus, the cost for the production of these metabolites is expected to be lower than for other microalgal toxins which are always present in the cell, such as the saxitoxins, gonytoxins and other chemically complex neurotoxic compounds produced by dinoflagellates. Diatom defense relies on primary metabolites such as storage lipids, which are transformed by lipase and lipoxygenase enzymes after wounding or ingestion. The cost of defense would therefore be negligible and the evolution of such defenses could thus be driven by the need for processes involved in primary metabolism together with the need for feeding pressure reduction.
Due to the teratogenic nature of diatoms oxylipins, the mechanism of chemical defense in diatoms functions by reducing grazing effects of subsequent generations of copepods. Hence, these compounds differ from those that act as feeding deterrents, the purpose of which is not to intoxicate the predator but discourage further consumption, or those that lead to physical incapacitation such as paralysis and death of the predator. Feeding deterrence would not protect the individual ingested cells but the community as a whole and the defense compounds would not target the predator but its offspring. In the end, grazing pressure would be reduced allowing blooms to persist when grazing pressure would otherwise have caused them to crash.
Another activated enzyme-cleavage mechanism of defense in the plankton is found in the bloom-forming coccolithophorid, Emiliana huxleyi, which produces dimethylsulfoniopropionate (DMSP) found in several marine phytoplankton species, seaweeds and some species of terrestrial and aquatic vascular plants. DMSP is cleaved by DMSP-lyase enzymes into the gas DMS and the feeding deterrent acrylate by protistan and zooplankton grazers. DMS released into sea water, and eventually into the atmosphere, can have profound effects on global climate processes. Seabirds such as petrels respond behaviorally to DMS and use the gas to track areas where phytoplankton and zooplankton accumulate. DMS and acrylate are also produced in another bloom-forming alga, the prymnesiophyte Phaeocystis globosa, which is thought to be a poor food source for a variety of zooplankton grazers. When copepods feed on P. globosa, this alga suppresses colony formation since individual cells are too small for the copepod to attack. However when ciliates attack this alga, it shifts to the colonial form which is too large to be grazed.
Dinoflagellate toxins are also often assumed to act as chemical defenses against herbivory. Effects on predatory copepods range from severe physical incapacitation and death in some species to no apparent physiological effects in others. This variability indicates that some copepods are more resistant to these compounds and may have evolved counter-defenses and detoxification mechanisms. Some copepod species seem capable of concentrating toxins in their body tissues, as occurs in bivalve molluscs, and ingested toxins may then act as defenses to deter predation by fish and other zooplanktivorous consumers.
See also
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