Diatoms represent a respectable proportion of the ocean primary production, although their actual biomass varies considerably in time and space, depending on water temperature, salinity and nutrient content. Their abundance can increase rapidly during particularly good conditions. For example during the spring oceanic bloom when, due to their rapid growth, they outgrow the other groups in the phytoplankton.
Phytoplankton constitute the base of the ocean food web. As such, diatoms are in fact eaten by a long list of herbivores and grazers of the seas. Dinoflagellate and ciliates are part of the zooplankton feeding directly on diatoms, the zooplankton itself being then fed upon by invertebrates and so on.
But diatoms are not as defenceless as they might seem. Many species produce defensive chemicals when the cell is disrupted. Those chemicals, called oxylipins*, have a direct impact on the cell reproduction of zooplankton, such as ciliates, decreasing their population and therefore the diatom grazing. Moreover, the decreased density of some grazers allows other smaller species to increase their density. Those species feed on smaller (pico- and nano-) phytoplankton instead of diatoms, making the environment safer.
Oxylipins presence in the environment seems to have an enhancing effect on copepods hunger for ciliates, indirectly decreasing ciliates grazing on diatoms.
Another compound produced by diatoms, possibly as a form of defence, appears to have many interesting properties. It is called marennine, and it is a blue-green pigment produced by a few species in the marine genus Haslea. Marennine, beside being a natural, water-soluble pigment, also has antibacterial, antiviral, antifungal and antioxydant capacities, and significantly affects early developmental stages of some organisms, such as mussels and sea urchins.
In conclusion, diatoms are indeed at the bottom of the ocean food web, but that does not mean that their abundance and species richness is at the mercy of herbivores and upper layers of the web, diatoms still have their say in it.
*Oxylipins is a group of chemicals that derive from the oxygenation of fatty acids, in the case of diatoms the fatty acids constituent of the cell wall, and their oxygenation is triggered by the rupture of the cell wall.
REF
- Falaise, C. et al. (2019). Harmful or harmless: Biological effects of marennine on marine organisms. Aquatic toxicology.
- Franzè, G. et al. (2017). Diatom‐produced allelochemicals trigger trophic cascades in the planktonic food web. Limnology and Oceanography.
- Gastineau, R. et al. (2014). Marennine, promising blue pigments from a widespread Haslea diatom species complex. Marine Drugs.
- Pennisi, E. (1999). Diatoms are Copepods contraceptives, Sciencemag.org.
- Varrella, S. et al.(2014). Molecular Response to Toxic Diatom-Derived Aldehydes in the Sea Urchin Paracentrotus lividus. Marine Drugs.