Abstract
Brown algae represent a major component of littoral and sublittoral zones in temperate and subtropical ecosystems. An essential adaptive feature of this independent eukaryotic lineage is the ability to couple oxidative reactions resulting from exposure to sunlight and air with the halogenations of various substrates, thereby addressing various biotic and abiotic stresses i.e., defense against predators, tissue repair, holdfast adhesion, and protection against reactive species generated by oxidative processes. Whereas marine organisms mainly make use of bromine to increase the biological activity of secondary metabolites, some orders of brown algae such as Laminariales have also developed a striking capability to accumulate and to use iodine in physiological adaptations to stress. We review selected aspects of the halogenated metabolism of macrophytic brown algae in the light of the most recent results, which point toward novel functions for iodide accumulation in kelps and the importance of bromination in cell wall modifications and adhesion properties of brown algal propagules. The importance of halogen speciation processes ranges from microbiology to biogeochemistry, through enzymology, cellular biology and ecotoxicology.
Highlights
Most organisms seem to have the ability to produce organohalogens, and the reported chemodiversity of this class of compounds (3,800 in 2003) has jumped 20-fold in the last 30 years [1].Marine halocarbon producers encompass an extensive phyletic range, and sessile life forms use a rich repertoire of diffusible and mucus-bound molecules, many of which are halogenated, for communication in the largest sense
Less than 1% of the secondary metabolites from brown algae contain bromine or chlorine compared with 7% of green algal compounds and 90% of those reported for red algae [5]
We describe the specificity of the evolution of the halogen metabolism in brown algae, pointing towards the emergence of strict specificity for the oxidation of iodide in the most evolved orders of Phaeophyceae, such as the Laminariales, and, the emergence of novel physiological adaptations with their ecological and atmospheric significance
Summary
Most organisms seem to have the ability to produce organohalogens, and the reported chemodiversity of this class of compounds (3,800 in 2003) has jumped 20-fold in the last 30 years [1]. It is likely that the halogen chemistry of marine algae reflects an episode of their polyphyletic evolutionary history. On one hand, the latter has involved a primary endosymbiosis (the engulfing of a prokaryotic cyanobacterium by a heterotrophic host cell), which lead to the emergence of the Plantae, the green lineage (green algae and land plants) and its sister group the red algae. Less than 1% of the secondary metabolites from brown algae contain bromine or chlorine compared with 7% of green algal compounds and 90% of those reported for red algae [5]. Iodination is more frequent and chlorination is less frequent in brown algae metabolites than in the red and green lineages. We describe the specificity of the evolution of the halogen metabolism in brown algae, pointing towards the emergence of strict specificity for the oxidation of iodide in the most evolved orders of Phaeophyceae, such as the Laminariales, and, the emergence of novel physiological adaptations with their ecological and atmospheric significance
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