Animal-like prostaglandins in marine microalgae

Abstract

Diatoms are among the most successful primary producers in ocean and freshwater environments. Deriving from a secondary endosymbiotic event, diatoms have a mixed genome containing bacterial, animal and plant genes encoding for metabolic pathways that may account for their evolutionary success. Studying the transcriptomes of two strains of the diatom Skeletonema marinoi, we report, for the first time in microalgae, an active animal-like prostaglandin pathway that is differentially expressed in the two strains. Prostaglandins are hormone-like mediators in many physiological and pathological processes in mammals, playing a pivotal role in inflammatory responses. They are also present in macroalgae and invertebrates, where they act as defense and communication mediators. The occurrence of animal-like prostaglandins in unicellular photosynthetic eukaryotes opens up new intriguing perspectives on the evolution and role of these molecules in the marine environment as possible mediators in cell-to-cell signaling, eventually influencing population dynamics in the plankton.

Figure 1

The Prostaglandin pathway: expression and abundance of the prostaglandin metabolites synthesized by Skeletonema marinoi FE7 and FE60 strains. (a) Schematic representation of prostaglandin synthesis pathway, starting from the three main precursors: eicosatrienoic, arachidonic and eicosapentaenoic acids. The most abundant precursor PUFAs in diatoms is shown in red. Square box contains the prostaglandins identified in our study in relation to their respective PUFA precursors. Colored rectangles or ovals indicate presence of each enzyme in the annotation table of Skeletonema marinoi strains FE7 or FE60 (see legend). (b–d) Expression levels of enzymes responsible for PGs synthesis and of the prostaglandin transporter measured by Real-time qPCR in exponential, stationary and senescent phases of growth. Results were analysed with the REST software and reported as 2 log expression ratio in the exponential phase (control) ±s.d. (b, FE7 strain; c, FE60 strain) or as 2 log expression ratio of FE60 with respect to FE7 strain (d). Statistical analysis (N=3) was performed using the Pair Wise Fixed Reallocation Randomization test by REST. Relative expression ratios above two fold were considered significant. (e) Abundance of the prostaglandin metabolites identified in our Skeletonema marinoi strains, quantified with LC/MSMS in the three different phases of growth (N=3). Results are means of three technical replicates (b–e) for each biological replicate (N=3).

Figure 2

Phylogenetic analysis of the enzymes involved in the prostaglandin pathway. (a–c) Bayesian mid-point routed phylogenetic trees of COX-1 (a), PTGE Synthase (b), PTGD Synthase (c) proteins from pennate and centric diatoms. Sequences from other organisms are also included as outgroup. Sequences used for phylogenetic analyses are listed in Supplementary Table S3. The trees were built on scale in the branch length (scale bars reported). Sequences are identified by the species name and annotation (where applicable), followed by their MMETSP ID deprived of the taxon ID for simplicity or GenBank accession number. Posterior probability (PP) values are represented with symbols at the nodes: star represents PP above or equal to 0.95; square PP between 0.75 and 0.95; triangle below 0.75. Branch coloring refers to PP, color code is reported in figure. (a) COX and COX-like proteins (2 000 000 generations, split frequency standard deviation (s.d.)=0.002); (b) PTGE synthase proteins (5 000 000 generations, split frequency s.d.=0.01 (c) PTGD synthase (10 000 000 generations, split frequency s.d.=0.04). For simplicity, large sequence clusters were condensed and identified by numbers and the nine main clades were identified by letters highlighted with vertical bars on the right side of the picture. Numbers do not correspond to a rank (see details in Supplementary Information and Supplementary Figure S7).