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Discovery
The cyanobacteria genus Trichodesmium was first described in 1830 based on morphological features, from samples collected in marine waters surrounding Egypt and Syria. In 1892, approximately sixty years following the initial discovery of the genus, Gomont (1892) described two new species, T. thiebautii and T. hildebrandtii, based on specific morphological characteristics, particularly the shape of the trichomes. T. theibautii was first cultured in a lab in 1993 from water samples collected in North Carolina coastal waters using a sterilized oligotrophic seawater solution with an addition of 25 mg liter-1 Tricine buffer and adjusted to a pH of 8.17 (Prufert-Bebout et al. 1993).

Phylogeny
The highly diverse colonial and cellular morphologies among Trichodesmium species have caused much debate about the phylogeny of the genus. Upon initial discovery by Ehrenberg in 1830 the genus Trichodesmium was placed in the family Oscillatoriaceae. More recently, an examination of several key morphological characteristics including colony formation associated with sheath production, cell differentiation along the trichome, hair-like projections or appendages that extend out of the colony, and fatty acid composition led to the placement of Trichodesmium thiebautii into the family Phormidiaceae and order Oscillatoriales. Many species originally placed into the family Phormidiaceae, including Trichodesmium spp., were taxonomically relocated in 2005 by two researchers, J. Komárek and K. Anagnostidis, into the family Microcoleaceae, where they remain today.

Analysis of the 16s rRNA from Trichodesmium sp. strain NlBB 1067 indicated that its closest phylogenetic neighbor is Oscillatoria PCC 7515 with 94.9% sequence similarity. This close sequence similarity did not resolve the debate on separation of Trichodesmium into a separate genus from Oscillatoria. However, a genetic analysis of the nitrogenase nifH gene sequences of Trichodesmium spp, including T. thiebautii, revealed a distinct cluster within the cyanobacteria clade with very deep branches indicating an early evolutionary radiation. Capone et al. (1997) suggests that the large genetic distance of the nifH gene between Trichodesmium spp. and other species of cyanobacteria, including those in the genus Oscillatoria, may be due to the structural requirements of aerobic N2 fixation.

Physical Characteristics
Members of the Microcoleaceae family have a distinct radial arrangement of their thylacoids, flattened sac-like structures found inside of chloroplasts, that distinguishes them from other closely related families of cyanobacteria. Trichodesmium thiebautii is usually composed of a few to hundreds of cells in a colony and has trichomes that appear to be twisted together much like a rope with radiating ends. Janson et al. (1995) also observed the rope-like twisted trichomes mentioned by Gomont, under the scanning electron microscope (SEM). Gomont (1892) described T. thiebautii cells as twice as long as they are wide. More than 100 years later, Prufert-Bebout et al. (1993) was able to cultivate T. theibautii and saw various colony morphologies ranging from solitary cells to spherical and fusiform (spindle-shaped) aggregates. T. thiebautii’s most distinct physical cellular structures are a series of gas vesicles found within the cell that allow it to be naturally buoyant and remain at the ocean’s surface (Capone et al. 1997). In lab cultures T. theibautii exhibits growth of 0.23 division per day and individual cells are 4-6 μm with trichomes ranging in width from 8 to 10 μm. (Prufert-Bebout et al. 1993).