Shown below are some pictures of the main nitrogen-fixing genus of phytoplankton, Trichodesmium. These pictures were taken using light microscopes since even the colonies are too small to be seen in detail with the naked eye (they are about 2mm or so across). Each colony is composed of order 100 filaments, and each filament is a string of 100 or so cells. Colonies can occur as more or less parallel sheaves of filaments (known as "tuft" type colonies) or as more randomly-oriented collections of filaments (known as "puff" type colonies).
From the waters off Zanzibar, Tanzania. Photo by Pernilla Lundgren
Photo by Hans Paerl
Photo by Hans Paerl
Photo by Ed Carpenter
Photo by Yasuwo Fukuyo. WESTPAC/IOC/UNESCO Harmful Algal Blooms CD-ROM.
Some other websites on Trichodesmium:
Woods Hole
Oceanographic Institution
University of Queensland