Oceans were warmer last month than any other May on record Copyright GETTY IMAGES NORTH AMERICA/AFP POOL
Protists are outcasts of sorts, a diverse, catchall group of mostly singlecelled organisms that have features that disqualify them from the other life groups: animals, plants, fungi, bacteria, and archaea.
Protists are found in most environments, including the ocean, but their contribution to marine food webs has been difficult to discern, partly because scientists cannot coax them to grow in the laboratory.
To get around this problem, Alexandra Worden of the Marine Biological Laboratory, Woods Hole (linked to the University of Chicago), has developed a method for examining individual protist cells fresh out of the ocean using equipment brought onboard research vessels.
The protists studied are among the smallest in the ocean, measuring two to five microns, approximately the size of a dust particle or small fungal spore. This includes choanoflagellates—the closest living unicellular relatives of animals. The name refers to the characteristic funnelshaped “collar” of interconnected microvilli and the presence of a flagellum.
Choanoflagellates are found globally in marine, brackish and freshwater environments from the Arctic to the tropics, occupying both pelagic (open sea) and benthic (towards the bottom of the sea) zones.
Choanoflagellates envelop and digest still smaller cells. Ocean ecosystems depend on these predators to consume plantlike phytoplankton and bacteria. In turn, protists become food for the tiny marine animals that fish eat. While protists consume bacteria, not all bacteria are food sources.
The success of the methods deployed by the researchers has led to a new discovery: Many protists in the ocean’s upper layer have symbiotic bacteria hitching a ride along with them.
It is also of interesting that some of these symbionts discovered are close relatives of bacteria pathogenic to animals, including humans.
Of concern to people, one of these lineages identified is closely related to Coxiella, a group that contains the bacteria responsible for the flulike Q fever.
Another set belongs within a group that includes Rickettsia, members of which can cause Rocky Mountain spotted fever, typhus, and other infections.
An important application of the research could provide insights that could help researchers understand disease vectors and the infections they cause in people.
The study is published in the journal Cell & Host Microbe. The research is titled “Symbionts of predatory protists are widespread in the oceans and related to animal pathogens.”
