Diatoms are microscopic algae which secrete a skeleton of silica. These skeletons are often preserved in sediments, sometimes for millions of years, and they are extremely useful to oceanographers and geologists alike as they are sensitive indicators of environmental change, and some species are restricted in time and so are useful for age-dating ancient sediments.
As primary producers at the base of the food chain, they record changes in nutrient fluxes and thus indicate the productivity of ocean environments. Other species depend on diatoms, either directly or indirectly (including some species of whales- so the largest organisms in the oceans depend on the smallest!), so what happens to diatoms is of great importance to marine ecosystems and ultimately the planet, as they also absorb carbon dioxide and release oxygen.
Bursts of diatom productivity in the geological past have been linked to changes in climate, such as the periods outlined in this paper, and these help us to recognise how similar changes will affect marine ecosystems and climate change in our warming world.
Dr Mitlehner's full paper can be read free of charge at: https://www.j-micropalaeontol.net/38/67/2019.