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The well-known fatty acids with a Δ5,9 unsaturation system were designated for a long period as demospongic acids, taking into account that they originally occurred in marine Demospongia sponges. However, such acids have also been observed in various marine sources with a large range of chain-lengths (C16–C32) and from some terrestrial plants with short acyl chains (C18–C19). Finally, the Δ5,9 fatty acids appear to be a particular type of non-methylene-interrupted fatty acids (NMA FAs). This article reviews the occurrence of these particular fatty acids in marine and terrestrial organisms and shows the biosynthetic connections between Δ5,9 fatty acids and other NMI FAs.
The well-known notion of demospongic acid appeared for the first time in 1976 in a historical paper from Litchfield and Morales , but at that time only as «demospongiae fatty acids». In another paper published in 1980 , Litchfield et al. used the term «demospongic fatty acids» probably for the first time. Since then and up until now [3–5], this term has widely been used. However, about 35 years after Litchfield’s work on sponge lipids, the notion of demospongic acid seems to no longer have significance, mainly due to their controversial definition and to their wide distribution among marine invertebrates and some terrestrial plants. At the time, it seemed to be of interest to precisely identify the function of demospongic acids, in consideration of their biological activities as topoisomerase inhibitors or against cancer cells as recently reviewed , whereas the biological interests of terrestrial short-chain Δ5,9 fatty acids (FAs) had already been demonstrated [7,8].
The definition of a demospongic acid has never been very clear [1,2]. In their first papers, Litchfield et al. only mentioned very long-chain C24–C30 or C24–C34 acids with the unusual 5,9 unsaturation pattern, but at that time, only fatty acids with an even number of carbons had been found [1,9]. In the 1980s, a lot of work was published on sponge FAs, and it became apparent that “demospongic acids” also contained all odd FAs from C23–C31 . Within this field of research, a consensus was quickly established that agreed that demospongic acids were very long-chain fatty acids, mainly C24–C30, with the atypical 5,9-diunsaturation system, independent of the total number of double bonds. Some years later—and due to many papers being devoted to FAs from sponges—it appeared that:
Currently, the best definition for a demospongic acid would be that of Christie , stating “bis-methylene-interrupted cis-double bonds, ranging in chain-length from C16 to C34 with a cis, cis-dienoic system, either with the double bonds in position 5 and 9, or derived from 5,9-16:2 by chain elongation”.
At present, the question is whether such acids are not at all specific to demosponges, but have been found in other groups of sponges, especially among hexactinellida, in different phyla of marine invertebrates and in several species of terrestrial plants, especially conifers, and in some species of Apocynaceae, Malvaceae, and Ranunculaceae.
Table 1 presents a non-exhaustive list of more than 40 FAs that correspond to Christie’s definition of demospongic acids found in microorganisms, marine invertebrates and terrestrial plants. A particularly interesting point is the presence of 6-Br-5,9-FAs that are very common in demosponges but quite rare in other organisms. To the best of our knowledge only some Cnidaria Hexacorallia were shown to contain these brominated FAs [21–23], which prove the existence of bromoperoxidases in this group of Cnidaria since it has been proved that these brominated demospongic acids are synthesized by the sponge itself in the final stage of biosynthesis .
Demospongic acids represent a particular type of non-methylene-interrupted FA and, according to Christie’s definition, it could be interesting to consider at least three classes of non-methylene-interrupted fatty acids (NMI FAs) depending on the number of methylene groups situated between the two first double bonds. Then, group 1 would contain all bis-methylene-interrupted cis-double bonds and would correspond to the series 5,9; 7,11; 9,13… dienoic or polyenoic acids (demospongic acids). Group 2 would be that of tetra-methylene-interrupted cis-double bonds and would contain the series 5,11; 7,13; 9,15… NMI FAs, such as the acids 7,13-20:2 found in the Brittle star (Echinoderm, Ophiuroidea) Ophiura sarsi  and in the maritime pine Pinus pisaster , or the acid 7,13-22:2 found in the sponge Petrosia ficiformis . Finally, group 3 would contain hexamethylene-interrupted cis-double bonds corresponding to the series 5,13; 7,15; 9,17… NMI FAs, such as the acid 7,15-20:2 found in the sponge Dysidea fragilis . Some other acids of these three groups have been identified in marine invertebrates, especially molluscs and arthropods, and in numerous terrestrial plants, especially gymnosperms, and all of them can be deduced from accepted biosynthetic pathways implying elongases and 5- and 9-desaturases. Figures 1 and and22 give an overview of these putative biosyntheses from palmitic acid (16:0), palmitoleic acid (9-16:1) and linoleic acid (9,12-18:2). These schemes are currently used and have appeared recently in several publications, along with recent reviews on elongases and polyketide synthases [3,9,10,12,40–44].
To end this point of view, we think that the former notion of demospongic acid should no longer be used mainly because bis-methylene interrupted 5,9-diunsaturated FAs and related acids are distributed among several phyla of marine organisms and several classes of terrestrial plants. The former “demospongic acids” can be considered as a particular series of NMI FAs produced by different combinations of elongases and Δ5 and Δ9 desaturases on the most common FAs in nature such as palmitic and palmitoleic acids.