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Diatoms are often the dominating group of benthic microalgae living on different types of bottom substrates. Their effects on invertebrate consumers is not well-documented. We here investigate the effects of feeding on another two benthic diatoms, Cocconeis scutellum and Diploneis sp., isolated from leaves of the seagrass Posidonia oceanica, on the sea urchin Paracentrotus lividus. Our results indicate a noxious effect on sea urchin embryos spawned from adults fed on Diploneis sp., with an increasing number of malformed embryos with respect to those spawned from adults fed on Ulva rigida (used as a feeding control). In contrast C. scutellum did not induce any morphological effect on embryos, similar to control non-diatom diets. Moreover, de novo obtained transcriptome indicated that oxidation-reduction process, translation, proton and electron transmembrane transport, ATP/RNA/GTP/heme/calcium and metal ion binding, NADH dehydrogenase activity, cytochrome c oxidase were affected by feeding of sea urchins on Diploneis sp. Our findings have considerable ecological significance considering that diatom biomass ingested by the sea urchin in these experiments is within the range of cell densities characterizing P. oceanica leaves where sea urchins live and spawn.

In order to exploit the rich reservoir of marine cold-adapted bacteria as a source of bioactive metabolites, ethyl acetate crude extracts of thirteen polar marine bacteria were tested for their antiproliferative activity on A549 lung epithelial cancer cells. The crude extract from Pseudoalteromonas haloplanktis TAC125 was the most active in inhibiting cell proliferation. Extensive bioassay-guided purification and mass spectrometric characterization allowed the identification of 4-hydroxybenzoic acid (4-HBA) as the molecule responsible for this bioactivity. We further demonstrate that 4-HBA inhibits A549 cancer cell proliferation with an IC50 value ≤ 1 μg ml−1, and that the effect is specific, since the other two HBA isomers (i.e. 2-HBA and 3-HBA) were unable to inhibit cell proliferation. The effect of 4-HBA is also selective since treatment of normal lung epithelial cells (WI-38) with 4-HBA did not affect cell viability. Finally, we show that 4-HBA is able to activate, at the gene and protein levels, a specific cell death signaling pathway named pyroptosis. Accordingly, the treatment of A549 cells with 4-HBA induces the transcription of (amongst others) caspase-1, IL1β, and IL18 encoding genes. Studies needed for the elucidation of mode of action of 4-HBA will be instrumental in depicting novel details of pyroptosis.

Many organisms have developed defences to avoid predation by species at higher trophic levels. The capability of primary producers to defend themselves against herbivores affects their own survival, can modulate the strength of trophic cascades and changes rates of competitive exclusion in aquatic communities. Algal species are highly flexible in their morphology, growth form, biochemical composition and production of toxic and deterrent compounds. Several of these variable traits in phytoplankton have been interpreted as defence mechanisms against grazing. Zooplankton feed with differing success on various phytoplankton species, depending primarily on size, shape, cell wall structure and the production of toxins and deterrents. Chemical cues associated with (i) mechanical damage, (ii) herbivore presence and (iii) grazing are the main factors triggering induced defences in both marine and freshwater phytoplankton, but most studies have failed to disentangle the exact mechanism(s) governing defence induction in any particular species. Induced defences in phytoplankton include changes in morphology (e.g. the formation of spines, colonies and thicker cell walls), biochemistry (such as production of toxins, repellents) and in life history characteristics (formation of cysts, reduced recruitment rate). Our categorization of inducible defences in terms of the responsible induction mechanism provides guidance for future work, as hardly any of the available studies on marine or freshwater plankton have performed all the treatments that are required to pinpoint the actual cue(s) for induction. We discuss the ecology of inducible defences in marine and freshwater phytoplankton with a special focus on the mechanisms of induction, the types of defences, their costs and benefits, and their consequences at the community level.