Trichodesmium

They are found in nutrient poor tropical and subtropical ocean waters (particularly around Australia and in the Red Sea, where they were first described by Captain Cook).

Colonies of Trichodesmium provide a pseudobenthic substrate for many small oceanic organisms including bacteria, diatoms, dinoflagellates, protozoa, and copepods (which are its primary predator); in this way, the genus can support complex microenvironments.

Large gas vesicles (either along the periphery as seen in T. erythaeum or found distributed throughout the cell as seen in T. thiebautii) allow Trichodesmium to regulate buoyancy in the water column.

This process requires a substantial amount of energy (in the form of ATP) in order to break the triple bond between the nitrogen atoms.

[9] Trichodesmium is the major diazotroph in marine pelagic systems[8] and is an important source of "new" nitrogen in the nutrient poor waters it inhabits.

[8] Since the first realization of this enigma, Trichodesmium has been the focus of many studies to try and discover how nitrogen fixation is able to occur in the presence of oxygen production without any apparent structure separating the two processes.

However, Trichodesmium utilises photosynthesis for nitrogen fixation by carrying out the Mehler reaction, during which the oxygen produced by PSII is reduced again after PSI.

This regulation of photosynthesis for nitrogen fixation involves rapidly reversible coupling of their light-harvesting antenna, the phycobilisomes, with PSI and PSII.

[13] Trichodesmium is found primarily in water between 20 and 34 °C and is frequently encountered in tropical and sub-tropical oceans in western boundary currents.

[13] Its presence is more pronounced in nitrogen poor water and can easily be seen when blooms form, trapping large Trichodesmium colonies at the surface.

[10] Nitrogen fixed by Trichodesmium can either be used directly by the cell, enter the food chain through grazers, be released into dissolved pools, or get exported to the deep sea.

[9] Compared to eukaryotic phytoplankton, Trichodesmium has a slow growth rate, which has been hypothesized to be an adaptation to survival in high energy but low nutrient conditions of oligotrophic waters.

[8] Trichodesmium colonies have been shown to have large degree of associations with other organisms, including bacteria, fungi, diatoms, copepods, tunicates, hydrozoans, and protozoans among other groups.

[23] Trichodesmium and the epibiont bacteria within the holobiont colonies may perform mutualistic interactions where limiting nutrients such as iron can be mobilized from dust.

When colonies of Trichodesmium aggregate in large numbers, it is possible for them to produce a phycotoxin that can affect the growth other microorganisms in the local space of the ocean.

Illustration
Trichodesmium erythraeum bloom, between Vanuatu and New Caledonia , SW Pacific Ocean.
Examples of Trichodesmium colonies sorted into morphological classes
(A) radial puffs, (B) non-radial puffs, (C) tufts. [ 11 ]
Colonies of marine cyanobacteria Trichodesmium
interact with other bacteria to acquire iron from dust
a. The N 2 -fixing Trichodesmium spp., which commonly occurs in tropical and sub-tropical waters, is of large environmental significance in fertilizing the ocean with important nutrients.
b. Trichodesmium can establish massive blooms in nutrient poor ocean regions with high dust deposition, partly due to their unique ability to capture dust, center it, and subsequently dissolve it.
c. Proposed dust-bound Fe acquisition pathway: Bacteria residing within the colonies produce siderophores (C-I) that react with the dust particles in the colony core and generate dissolved Fe (C-II). This dissolved Fe, complexed by siderophores, is then acquired by both Trichodesmium and its resident bacteria (C-III), resulting in a mutual benefit to both partners of the consortium . [ 12 ]