First Morphological and Molecular Evidence of the Negative Impact of Diatom-Derived Hydroxyacids on the Sea UrchinParacentrotus lividus

Abstract

Oxylipins (including polyunsaturated aldehydes [PUAs], hydoxyacids, and epoxyalcohols) are the end-products of a lipoxygenase/hydroperoxide lyase metabolic pathway in diatoms. To date, very little information is available on oxylipins other than PUAs, even though they represent the most common oxylipins produced by diatoms. Here, we report, for the first time, on the effects of 2 hydroxyacids, 5- and 15-HEPE, which have never been tested before, using the sea urchin Paracentrotus lividus as a model organism. We show that HEPEs do induce developmental malformations but at concentrations higher when compared with PUAs. Interestingly, HEPEs also induced a marked developmental delay in sea urchin embryos, which has not hitherto been reported for PUAs. Recovery experiments revealed that embryos do not recover following treatment with HEPEs. Finally, we report the expression levels of 35 genes (involved in stress, development, differentiation, skeletogenesis, and detoxification processes) to identify the molecular targets affected by HEPEs. We show that the 2 HEPEs have very few common molecular targets, specifically affecting different classes of genes and at different times of development. In particular, 15-HEPE switched on fewer genes than 5-HEPE, upregulating mainly stress-related genes at a later pluteus stage of development. 5-HEPE was stronger than 15-HEPE, targeting 24 genes, mainly at the earliest stages of embryo development (at the blastula and swimming blastula stages). These findings highlight the differences between HEPEs and PUAs and also have important ecological implications because many diatom species do not produce PUAs, but rather these other chemicals are derived from the oxidation of fatty acids.

Combined Effects of Diatom-Derived Oxylipins on the Sea Urchin Paracentrotus lividus

“…By contrast, HEPEs induced developmental malformations at much higher concentrations when compared to PUAs. Interestingly, HEPEs also induced a marked developmental delay in sea urchin embryos, which is also evident at higher concentrations, and has not hitherto been reported for PUAs [31]. Our current results indicate that mixtures of these compounds induce a synergistic effect, confirming that PUAs induce malformations and HEPE developmental delay but with a stronger effect with respect to individual compounds [46,47].…”

“…These findings can be explained in light of previous results demonstrating a reversible effect for individual PUAs and irreversible effects for HEPEs. P. lividus embryos can recover after exposure to PUAs [30] but not HEPEs [31]. Our morphological results are also linked to the molecular response of sea urchin embryos to PUAs that occurred later (at the pluteus stage) compared to HEPEs (at the blastula stage).…”

“…We then extended the exposure time of PUA + HEPE mixtures to 1 week post-fertilization (wpf) to follow the fate of delayed embryos in the mixture with 0.3 µM decadienal, 0.7 µM heptadienal, and 1.0 µM octadienal for PUAs and 1.8 µM for HEPEs. Observations under the light microscope indicated that embryos had reached the pluteus stage but were malformed (degraded arms and apex) because they did not display the characteristic ampoule-like shape, as observed in the controls ( Figure S2) [31].…”

“…However, very few studies are available on natural toxins and on the possible effects of mixtures of these toxins on marine organisms. Oxylipins are a well-known class of diatom secondary metabolites that have been studied for their capability to induce reproductive failure in several marine invertebrates [10,[24][25][26][27], including sea urchin Paracentrotus lividus [28][29][30][31][32]. Oxylipins represent a group of diverse compounds deriving from the oxidation of polyunsatured fatty acids that are released after cell damage or senescence [11,12,14].…”

“…Oxylipins have multiple biological and biochemical effects inducing the disruption of gametogenesis, gamete functionality, fertilization, embryonic mitosis, and larval fitness and competence in both planktonic and benthic grazers [45]. Previous studies by our group have demonstrated toxigenic effects on sea urchin P. lividus embryos exposed to individual polyunsaturated aldehydes (PUAs) and hydroxyacids (HEPEs) [30][31][32] or mixtures of these compounds belonging to the same chemical group [46,47].…”