Protists are the dominant eukaryotes in the biosphere where they play key functional roles. While protists have been studied for over a century, it is the high-throughput sequencing of molecular markers from environmental samples -the approach of metabarcoding -that has revealed just how diverse, and abundant, these small organisms are. Metabarcoding is now routine to survey environmental diversity, so data have rapidly accumulated from a multitude of environments and at different sampling scales. This mass of data has provided unprecedented opportunities to study the taxonomic and functional diversity of protists, and how this diversity is organised in space and time. Here, we use metabarcoding as a common thread to discuss the state of knowledge in protist diversity research, from technical considerations of the approach to important insights gained on diversity patterns and the processes that might have structured this diversity. In addition to these insights, we conclude that metabarcoding is on the verge of an exciting added dimension thanks to the maturation of high-throughput long-read sequencing, so that a robust eco-evolutionary framework of protist diversity is within reach. ll

Siliceous polycystines and phaeodarians are open‐ocean planktonic protists found throughout the water column and characterized by complex siliceous skeletons that are formed, at least partly, through the uptake of silicic acid. These protists contribute to the marine organic carbon (C) and biogenic silica (bSi) pools, but little is known about their contribution to the silica (Si) biogeochemical cycle. Here we report the first measurements of the Si uptake rate of polycystine and phaeodarian cells from samples collected in the Mediterranean Sea using the 32Si‐based method. The elementary composition (bSi, particulate organic carbon and nitrogen) of these organisms was also measured. Combining our results with published data on the distribution and abundance of Polycystina and Phaeodaria in the global ocean, we conclude that these organisms could contribute from 0.2 to 2.2 mmol Si m−2 of the marine standing stock of bSi and from 2 to 58 Tmol Si yr−1 (1% to 19%) of the global oceanic biogenic silica production. The implications for the global marine Si cycle are discussed.

No abstract