Abstract
Diatoms are unicellular algae playing a key role as photosynthetic organisms in the world’s ocean food webs. The chemical ecology of planktonic diatoms is well documented, but few studies have reported on the effects of benthic diatoms on their consumers, also due to difficulties in the collection, quantification and massive culturing of benthic species. Here for the first time we investigate the effects of feeding on two abundantly occurring benthic diatoms, Nanofrustulum shiloi and Cylindrotheca closterium, isolated from the leaves of the seagrass Posidonia oceanica, on the sea urchin Paracentrotus lividus. Adult P. lividus were fed for one month on diets of either one of the two diatoms and on the green alga Ulva rigida, used as a feeding control. By combining morphological, metabolomic and de novo transcriptomic approaches, we demonstrate toxigenic effect on embryos generated by females fed with these benthic diatoms. Furthermore, chemical analysis reveal the presence of polyunsaturated aldehydes only for N. shiloi, and a high production of other oxylipins (cytotoxic compounds on their grazers and on cancer cell lines) for both diatoms, including some additional peaks not correlated to the canonic oxylipins commonly observed in planktonic diatoms. These findings open new perspectives in the study of diatom secondary metabolites influencing their grazers.
Highlights
Diatoms are unicellular eukaryotes, representing one of the largest and ecologically groups and exclusively depositing biogenic silica
SEM observation revealed that the first diatom isolated was about 50 μm in length, needle like and thin in shape, the two ends of the cell extended far from the centre of the cell; the cells showed spiral twist of the raphe system, which is characteristic for Cylindrotheca closterium (Fig. 1A)
Feeding experiments reported here revealed a noxious effect of two benthic diatom species (C. closterium and N. shiloi) on embryos spawned from adult sea urchins fed for one month on these diets
Summary
Diatoms are unicellular eukaryotes, representing one of the largest and ecologically groups and exclusively depositing biogenic silica. The siliceous wall is transparent, allowing the entrance of the light, and perforated, making possible the diffusion and excretion of materials[1] They contribute about 20% of global photosynthetic fixation of carbon (about 20 Pg carbon fixed per year), which is more than all the world’s tropical rainforests, playing important roles on earth and in oceans as oxygen synthesizers and biomass sources[2]. Of oxygenated fatty acid derivatives (called oxylipins)[4,5] that affect diatom growth[6,7] or have negative effects on the reproduction and development of several marine invertebrates, such as copepods[8,9], sea urchins[10,11,12,13,14,15] and sea stars[16], polychaete worms[17] and ascidians[16,18]. Other benthic diatoms produce volatile organic compounds (VOCs) similar to those produced by planktonic diatoms that induce behavioural changes in several species of macrinvertebrates[28], but studies on the metabolic effects of fast-growing benthic diatoms on benthic invertebrates are still largely lacking[29,30,31]
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