Determination of Lipid Hydroperoxides in Marine Diatoms by the FOX2 Assay

Silencing of a Pseudo-nitzschia arenysensis lipoxygenase transcript leads to reduced oxylipin production and impaired growth

Because of their importance as chemical mediators, the presence of a rich and varied family of lipoxygenase (LOX) products, collectively named oxylipins, has been investigated thoroughly in diatoms, and the involvement of these products in important processes such as bloom regulation has been postulated. Nevertheless, little information is available on the enzymes and pathways operating in these protists. Exploiting transcriptome data, we identified and characterized a LOX gene, PaLOX, in Pseudo-nitzschia arenysensis, a marine diatom known to produce different species of oxylipins by stereo- and regio-selective oxidation of eicosapentaenoic acid (EPA) at C12 and C15. PaLOX RNA interference correlated with a decrease of the lipid-peroxidizing activity and oxylipin synthesis, as well as with a reduction of growth of P. arenysensis. In addition, sequence analysis and structure models of the C-terminal part of the predicted protein closely fitted with the data for established LOXs from other organisms. The presence in the genome of a single LOX gene, whose downregulation impairs both 12- and 15-oxylipins synthesis, together with the in silico 3D protein modelling suggest that PaLOX encodes for a 12/15S-LOX with a dual specificity, and provides additional support to the correlation between cell growth and oxylipin biosynthesis in diatoms.

Lipoxygenase Pathways in Diatoms: Occurrence and Correlation with Grazer Toxicity in Four Benthic Species

Oxygenated derivatives of fatty acids, collectively called oxylipins, are a highly diverse family of lipoxygenase (LOX) products well described in planktonic diatoms. Here we report the first investigation of these molecules in four benthic diatoms, Cylindrotheca closterium, Nanofrustulum shiloi, Cocconeis scutellum, and Diploneis sp. isolated from the leaves of the seagrass Posidonia oceanica from the Gulf of Naples. Analysis by hyphenated MS techniques revealed that C. closterium, N. shiloi, and C. scutellum produce several polyunsaturated aldehydes (PUAs) and linear oxygenated fatty acids (LOFAs) related to the products of LOX pathways in planktonic species. Diploneis sp. also produced other unidentified fatty acid derivatives that are not related to LOX metabolism. The levels and composition of oxylipins in the benthic species match their negative effects on the reproductive success in the sea urchin Paracentrotus lividus. In agreement with this correlation, the most toxic species N. shiloi revealed the same LOX pathways of Skeletonema marinoi and Thalassiosira rotula, two bloom-forming planktonic diatoms that affect copepod reproduction. Overall, our data highlight for the first time a major role of oxylipins, namely LOFAs, as info-chemicals for benthic diatoms, and open new perspectives in the study of the structuring of benthic communities.

Density-dependent oxylipin production in natural diatom communities: possible implications for plankton dynamics

Oxylipins are important signal transduction lipoxygenase-derived products of fatty acids that regulate a variety of physiological and pathological processes in plants and animals. In marine diatoms, these molecules can be highly bioactive, impacting zooplankton grazers, bacteria and other phytoplankton. However, the ultimate cause for oxylipin production in diatoms is still poorly understood, from an evolutionary perspective. Here we analysed production of particulate linear oxygenated fatty acids (LOFAs, previously named non-volatile oxylipins) from natural phytoplankton collected weekly for 1 year. We demonstrate for the first time that diatoms are the main LOFA producers in natural phytoplankton assemblages. Interestingly, LOFA-per-cell production decreased with increasing diatom density and was not due to major changes in diatom community composition. An inverse relation was confirmed at a global scale by analysing diatom lipoxygenase unigenes and metagenomes from Tara Oceans datasets. A network analysis suggested that different LOFAs could contribute to modulate co-variations of different diatom taxa. Overall, we offer new insights in diatom chemical ecology, possibly explaining the evolution of oxylipin synthesis in diatoms.

Spectrophotometric versus spectrofluorometric assessment in the study of the relationships between lipid peroxidation and metabolic dysregulation

Reactive oxygen species are crucial to normal cell function, but are also part of the pathogenesis of multiple modern maladies. As such, sensitive, fast, and reliable methods of appreciating redox status are needed. We aimed to optimize the Amplex Red (AR) and ferric-xylenol orange (FOX) methods using human serum samples, rat tissue homogenates, and mitochondrial preparations. For AR, we intended to reduce probe concentration, maintaining method sensitivity, as well as extending its use from isolated lipoproteins samples, and readjust it for a high-throughput application. Also, we evaluated the usefulness of a modified xylenol orange-based spectrophotometric protocol, comparing and contrasting these methods in terms of clinical relevance and suitability for their further use in assessing redox status of various biological samples in different pathological conditions. Our results show that these optimized protocols are suitable for complex in vivo studies, as they require low quantities of sample and reagents, and are sensitive, rapid, and economical, with the option of adapting them for high-throughput analysis. For a better assessment of oxidative status of serum-derived samples, the two methods can be used concurrently, while for tissue-derived ones, either can be employed for the measurement of a global redox status.