Microbial plankton, with sizes ranging from 0.2 to 2.0 microns.
To research what's going on in the ocean, biologically speaking, you could dip in a net, and haul up a batch of organisms, and study those. Suppose you wanted to catalog plankton-- you need a net with a very fine mesh. Phytoplankton might be 0.1 millimeters across (100 microns), so you need holes smaller than that. Suppose you made a net with holes just 10 microns across. You would likely catch lots of traditional phytoplankton. You could put it under the microscope, catalog it, report on it, and call it a day. Well, that's plankton.
In the 1970s and 1980s, molecular biologists made a startling discovery: a heretofore unknown fauna was floating in the ocean, tinier than the imagination of... well, of the folks who made plankton nets. These tiny organisms were also notoriously difficult to see via light microscopy at that size... it was epifluorescence microscopy, the dyeing of DNA in larger organisms, along with culturing of these cells, which led to these discoveries.
Although there are approximately 4000 known species of traditional plankton (you know, the big boys, a whole 100 microns across), as of this writing, only 30 or so species of picoplankton have been identified, and, based on DNA analysis... they've got gene sequences that can't be linked to other species (So they'll keep marine biology graduate students busy for some time).
How tiny are these one-celled organisms? The big guys at 2 microns across you could line up 500 of them to reach the millimeter mark on your ruler. The little guys are one tenth that size. (Cf. nannoplankton, larger at 2-20 microns in size; and femtoplankton, the next smaller scale)
Picoplankton is a classification merely based on size: taxonomically, the group reaches across kingdoms, including bacteria (bacterioplankton), archaea, and eukaryotes. But what this group lacks in size they make up for in abundance: dip in a thimble, okay, a mini-thimble, haul in a milliliter. You'll find as many as a million organisms in surface water. Picoplankton are the most abundant living things in the world's oceans, and constitute 20-50% of marine primary productivity, making them the keystone of the oceanic food webs. Also, via photosynthesis or other chemical transformations (fixing of carbon, nitrogen, or sulfur), they have a key role in the global climate (via the carbon cycle) and the recycling of living matter in the seas.
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