Plankton, consisting of free-floating diatoms, flagellates, and dinoflagellates, comprises the first stage in the food chain of marine organisms. In order to ensure its survival, defensive strategies are required. Besides mechanical barriers, for example, thick silicified cell walls of diatoms and mere mass occurrence, chemical defense plays an important role (Figure 1). Many diatoms react upon attack with the generation of oxylipins (see Plant Defense Strategies) and the formation of toxic dienals and trienals such as 2-trans-4-cis-7-cis-decatrienal 1 from lipid precursors (wound-activated defense). These Michael acceptors react readily with proteins destroying their function.
Thus the feeding success of the predator is reduced, resulting also in a decreased hatching success. But there are also diatoms (Pseudo-nitzschia) that produce non-lipid-derived toxins such as the neurotoxin domoic acid 2 that activates ionotropic receptors causing influx of Ca2+-ions.
Dinoflagellates, for example, Karenia brevis, use poly-ether polyketides, for example, brevetoxin 3 or maitotoxin as potent lipid-soluble ichthyotoxins (fish poisons). Polyether toxins block ion channels and are highly toxic for many fish and mammals. Severe toxications regularly occur during algal blooms, for example, the so-called red tides (i.e., massive growth of dinoflagellates). Polyether toxins, for example, ciguatoxin, can be isolated from dinoflagellates growing as endophytes on Gambierdiscus toxicus. Polyether toxins are taken up by plankton-filtering organisms such as sponges, corals, mussels, tunicates, and fishes. Apparently, the toxins do not affect such sequesters and they even make use of the polyethers for their own defense. Poisoning with fish that accumulated polyether toxins was named 'ciguatera'. Ciguatera causes long-lasting toxications of humans in the Caribbean, especially during algal blooms which is also combined with massive fish dying.
Some dinoflagellates of the genus Alexandrium produce saxitoxin 4 and related compounds that act as neurotoxins, poisoning, for example, filter-feeding shellfish. Saxitoxin 4 is a potent blocker of sodium channels, thus resulting in paralysis and muscle failure. Plankton-grazing copepods prefer to feed on Alexandrium species that lack saxitoxin 4 but they are able to tolerate it.
In brown algae (Laminaria digitata), the production of toxic reactive oxygen species, for example, H2O2, is initiated upon recognition of own-cell-wall degradation products, oligoalginates. (Oligoalginates are derived from polymeric alginates. Alginates are algal sugar polymers that are important constituents of algal cell walls.) Alginate-degrading enzymes from attackers, both marine mollusks and bacteria, thus activate the defense of brown algae in close analogy to induced defense of terrestrial plants. Also an increase in phlorotannins and polyphenolics was observed after herbivore attack or wounding similar to higher plants.
The invasive green algae (Chlorophyta) of the genus Caulerpales, for example, the fast-growing Caulerpa taxifolia, spread very efficiently because they are well defended against most herbivores with the terpenoid caulerpenyne 5 and related compounds. Caulerpenyne 5 is transformed upon wounding by the action of an esterase into the highly reactive a,/3-unsaturated dialdehyde 6 (Figure 1) (see Plant Defense Strategies, Animal Defense Strategies, and Fungal Defense Strategies). The resulting oxytoxin 2 (7) is prone to react with amino groups of proteins. Thus Caulerpa sp. destroy their own proteins at wounding sites by crosslinking which results in a low food quality rendering the algae unattractive for herbivores. At the same time, rapid wound plug formation protects the
Domoic acid 2
Oxytoxin 2 7
Figure 1 Chemical defense of micro- and macroalgae.
algae from further damage. In addition the highly reactive dialdehyde 7 may also destroy digestive proteins of a feeding herbivore. An analogous defensive strategy is used by the green algae Halimeda that produce halimedate-traacetate, which is also converted after injury by hydrolytic enzymes into a highly reactive defense compound, halimedatrial 8, a compound with a reactive a,/3-unsaturated carbonyl function. Additionally, Halimeda spp. deposit large amounts of CaCO3 in their leaves which render them hard to digest.
Organs of marine organisms are prone to be covered by microbial biofilms because microorganisms use them for settlement (biofouling) and nutrition. Nevertheless the macroalga Delisea pulchra stays clean as indicated by its name, because D. pulchra produces brominated furanones such as 9 (Figure 1) that prevent biofouling by interference with quorum-sensing signal reception of bacteria. Many bacteria use quorum-sensing signals, such as homoserine lactones, to sense their cell density and coordinate their behavior resembling a macroorganism. Brominated furanones from D. pulchra are currently evaluated for their use as lead structures to develop a novel class of antibiotics to fight pathogenic bacteria.
In contrast to most terrestrial organisms, halogenated defense compounds are abundant in marine organisms due to the high amounts of halogenides in seawater. Algae are known for their high content of different brominated phenols. For instance, red algae (Rhodomela) use brominated phenols, for example, lanosol 10, as potent antifeeding agents against marine snails. Halogenated monoterpenoids such as anverene 11 act as antifeedants against amphipods. Additionally, algae use a variety of macrolides to defend
themselves against microorganisms. For example, the common seaweed Lopophora variegata produces the polyketide lobophorolide 12 (containing an epoxide moiety) that is active against marine fungi in very low concentrations.
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