Trichodesmium is the most important nitrogen-fixing marine cyanobacteria, and much more

9th July 2023

Translated from the original article in Catalan (14th June 2023)

Trichodesmium is one of the most important cyanobacteria genera from several points of view: it is prolific in the oceans, it fixes half of the atmospheric nitrogen needed for the entire food chain, it is photosynthetic, it is colonial and forms flakes that move vertically between the surface and down to 200 m, contributing to the biological cycles of nitrogen, carbon, phosphorus and iron.

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CYANOBACTERIA ARE KEY ORGANISMS IN THE EVOLUTION AND EARTH ECOLOGY

They were formerly known as “blue-green algae” or cyanophyceae, due to their resemblance to algae, being photosynthetic autotrophs and filamentous in appearance, and are still commonly referred to as a type of microalgae. However, cyanobacteria are prokaryotes, and therefore they have nothing to do structurally and phylogenetically with algae, since these are eukaryotes. They have been recently renamed as Cyanobacteriota, in the revision of taxonomic names for bacterial phyla (Oren & Garrity 2021).

They are one of the main bacterial phyla, within the super-taxon Terrabacteria. You can read my previous post about italso in this blog. In fact, Cyanobacteria are some of the oldest bacteria, with fossils from 3.5 billion years ago. They “invented” oxygenic photosynthesis, with non-cyclic photophosphorylation, with 2 photosystems and chlorophyll, unlike other anoxygenic photosynthetic bacteria with only one photosystem and bacteriochlorophylls. The oxygen produced by the cyanobacteria, especially between 2.4 and 2 billion years ago, generated the current atmosphere with 21% O₂. In fact, cyanobacteria are the evolutive origin of the first chloroplasts and other plastids present in algae and plants, the eukaryotes appearing later, since the cyanobacteria were incorporated by endosymbiosis into the first eukaryotes.

They are gram-negative and have internal membranes, called thylakoids, where pigments accumulate and where photosynthesis takes place. Cyanobacteria are a large and very diverse phylum, with many types of forms, both unicellular and colonial and many filamentous (Figure 1). Cyanobacteria thrive in a wide variety of habitats across Earth and are major contributors to global biogeochemical cycles. Many of them, in addition to being photosynthetic, are atmospheric N₂ fixers—diazotrophs—converting it to ammonium and/or nitrites and nitrates. Some do nitrogen fixation in specialized cells, such as the heterocysts of Anabena and other genera (Figure 1). There are many free-living N₂ fixers but there are also symbiotic ones with plant roots, called cyanobionts, such as Anabaena itself.

Apart from being found in terrestrial environments and surface waters, cyanobacteria are ubiquitous in marine environments, where they play an important role as primary producers, as part of the phytoplankton. The main open sea cyanobacteria are Crocosphaera, Trichodesmium, Synechococcus and Prochlorococcus. These last 2 genera are of the so-called picobacteria, because they are very small, only 0.5-0.8 microm, and despite their size they are the most abundant organisms on Earth: about 10⁵ bacteria for every mL of seawater, especially in all oligotrophic regions (with few nutrients) of the oceans. Prochlorococcus alone is estimated to produce 20% of atmospheric oxygen (Partensky et al 1999).

Marine cyanobacteria are also present in the so-called atmospheric aeroplankton that forms with the splash of the waves, through which these bacteria are transported to other regions.

Figure 1. Morphological diversity of cyanobacteria, including unicellular, colonial and filamentous forms. Small bars are to scale, 10 microm. Abbreviations: h, heterocyst; s, spore; ho, hormogony. Image from AEDA, Freshwater Biological Association, original drawings by Allan Pentecost.

Some cyanobacteria can form harmful algal blooms —due to excess nutrients from fertilizers or waste with high temperatures— which in turn cause imbalances in the ecosystem and can produce toxins, called cyanotoxins (Huisman et al 2018).

Biotechnologically, cyanobacteria are important as model organisms for the study of photosynthetic mechanisms and potential applications. In addition, they are known —besides some products of secondary metabolism— for their use as food, that is the so-called “single cell protein” (SCP) or microbial protein. Genera such as Arthrospira (formerly Spirulina) or Aphanizomenon are used as dietary supplements and have positive effects on health. You can read this in my post on alternative meat.

TRICHODESMIUM

It is a genus of filamentous cyanobacteria, found in nutrient-poor tropical and subtropical ocean waters, such as the Red Sea and the Australian seas, where it was first described in the 18th century by the English explorer Captain James Cook. In fact, the Red Sea has this name because of the reddish colour that sometimes is observed due to the proliferation of T. erythraeum and other similar species. T. erythraeum is the most studied and the only sequenced so far of this genus. Its genome of 7.75 Mb is one of the largest of all sequenced bacteria. Another species well known for surface blooms is T. thiebautii.

Trichodesmium filaments are chains of cell units separated by septa (Figure 2). The filaments are called trichomes —the same name (hair in Greek) as the fine appendages or hairs of plants and algae—. Trichomes are grouped together to form flakes of up to 2 mm, visible to the naked eye (Figure 3), and their accumulation on the sea surface in the form of bands is visible from space (Figure 4). For this aspect they are also called “sea sawdust” or also “sand roads”. In fact, the name of this genus comes from the Greek Trichos = hairs, and Desmos = bands, i.e. bands of hair.

Trichodesmium colonies are a preferred substrate for numerous other oceanic organisms, including other bacteria, protists such as diatoms, dinoflagellates, protozoa, and especially copepods (small crustaceans). Therefore, this genus is the substantial pillar of complex marine microenvironments.

Figure 2. Optical microscope image of trichomes of Trichodesmium sp. H9-4 members of a flock (Image from Annette Hynes, WHOI 2023)

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Figure 3. Trichodesmium filaments or trichomes grouped together forming a tuft. (From Le Page 2023)

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Figure 4. Trichodesmium blooms with a sea sawdust appearance near Australia’s Great Barrier Reef. Image from Wexcan, Creative Commons, Wikimedia Commons.

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NITROGEN FIXATION BY TRICHODESMIUM

Research carried out in several studies in recent years has revealed the substantial contribution of the diazotrophic cyanobacterium Trichodesmium to atmospheric nitrogen capture in global marine ecosystems and the key ecological importance of this genus (Capone et al 1997; Bergman et al 2013).

With its great ability to fix atmospheric N₂, Trichodesmium is the main diazotroph among cyanobacteria, and also the main diazotroph in marine pelagic systems. It is an important source of “new” nitrogen in the nutrient-poor waters it inhabits, producing half of the nitrogen needed for primary production in the oceans (MicrobeWiki 2010). The global contribution of nitrogen fixation by Trichodesmium is approximately 60-80 Tg —Teragrams (10¹² g), that is megatons— of N per year.

As you know, the biological fixation of atmospheric nitrogen can only be done by bacteria, and consists in the reduction of N₂ —very inert, it takes a lot of energy to reduce it— to 2 ammonium molecules by means of an enzyme complex, dinitrogenase, in a sequence of reactions similar to an electron transport chain with pyruvate as an electron donor, involving a flavodoxin oxidoreductase and the dinitrogenase complex itself, with a requirement of about 24 moles of ATP per mole of fixed N₂. Since nitrogenase is inhibited by oxygen, diazotrophic organisms have several mechanisms to be able to fix nitrogen without the presence of oxygen:

• Being anaerobic, like some clostridia, and also some anoxygenic photosynthetic bacteria.
• Aerobic bacteria that quickly consume O₂ —called respiratory protection—, such as Azotobacter, which has a very active cytochrome oxidase, so there is no free oxygen.
• Aerobic bacteria with specialized cells that contain nitrogenase, such as the heterocysts of many cyanobacteria (e.g., Anabaena, Figure 1) that prevent the entry of oxygen. Also, since cyanobacteria are oxygenic photosynthetic, they are producing oxygen, and therefore, they need to have nitrogenase isolated in these heterocysts, which fix nitrogen and do not photosynthesize.
• Aerobic symbiotic bacteria of plants, such as Rhizobium of leguminous nodules where the compound leghemoglobin sequesters oxygen.

The paradox is that Trichodesmium, despite being an oxygenic photosynthetic cyanobacterium, and therefore producing oxygen and fixing N₂, does not have heterocysts, although it is not the only genus with these characteristics (Bergman et al 1997). Instead of heterocysts, nitrogen fixation in Trichodesmium takes place in relatively specialized cells, diazocytes, which contain nitrogenase but lack thickened cell walls (Figure 5). As we can see, the area of diazocytes does not have granules, it is clearer, it stains differently from the rest and where the presence of the nitrogenase protein NifH is detected by immunofluorescence (Bergman et al 2013).

Figure 5. Morphological characteristics of Trichodesmium trichomes. (a) T. erythraeum colony with aligned trichomes; scale bar 25 μm. (b) Trichome with the DNA of the cells dyed by fluorescent blue diamidino-phenyl-indole; the central zone of diazocytes is marked, with absence of yellow polyphosphate granules; scale bar 20 μm. (c) Lugol-stained trichomes where the least stained central parts are diazocytes where carbohydrate reserves have been consumed; scale bar 20 μm. (d) Fluorescent immunolocalization of nitrogenase protein NifH in diazocytes, in the central parts of trichomes; scale bar 10 μm (Image taken from Bergman et al 2013).

In cyanobacteria with heterocysts, nitrogen fixation and photosynthesis occur simultaneously, during the day with light, because the two processes are separated in space by using heterocysts to protect the oxygen-sensitive nitrogenase enzyme. The energy needed for the heterocysts is provided by the other cells that carry out photosynthesis. Some other cyanobacteria, the lesser ones, do the separation in time by photosynthesising during the day and fixing N₂ at night.

In contrast, the diazocyte zones of Trichodesmium lack the structural protection of heterocysts and the almost non-existent temporal separation between the two processes is peculiar and unique among diazotrophs. Thus, the nitrogenase of diazocytes is active during daylight hours, but especially with a maximum at noon, coinciding with a drop in CO₂ capture and O₂ production, and an increase in O₂ sequestering mechanisms. These would be, on the one hand, the aforementioned respiratory protection —with increased cytochrome c oxidase activity—, and also the so-called Mehler reaction, also observed in plant chloroplasts, which is the reduction of O₂ to H₂O₂ by electrons from photosystem I, where this is reversibly coupled to photosystem II. H₂O₂ is then reduced by one of the own antioxidant systems. Therefore, surprisingly N₂ fixation depends on photosystem activity in Trichodesmium (Bergman et al 2013).

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THE AGGREGATES OF TRICHODESMIUM

This cyanobacterium is found in the oceans either in the form of dispersed multicellular filaments, or in the form of aggregates comprising hundreds of filaments, with the final appearance discussed as flakes or colonies (Figures 3 and 5a). The aggregation of filaments occurs mainly during the day with light, after the exponential phase of growth, and also in stressful situations due to changes in environmental conditions, such as the lack of phosphate or iron. The aggregates, as flakes that they are, allow rapid vertical migration towards the bottom, especially in order to capture phosphate. On the surface they assimilate and accumulate C and N and once at the bottom, about 100-200 m, they capture the phosphorus that is missing on the surface (Rodríguez 2017). The aggregates simultaneously create suboxic microenvironments that reduce the fixation of N₂ and CO₂, but also facilitate the capture of iron which at the same time allows the recovery of N₂ fixation simultaneously with photosynthesis (Pfreundt et al 2023).

It has recently been seen that the formation of aggregates from individual filaments can be very fast and due to sudden changes in light such as the passage of clouds (Figure 6), and that aggregations are due to changes in motility of the individual filaments, in order to arrange themselves in contact and overlapping with each other, which is known as thigmotaxis or orientation by contact (Figure 5a). When two filaments touch, one slips over the other, and in the opposite direction of the other in order to increase the size of the aggregate, and the more frequent the reversals, the denser the flake (Pfreundt et al 2023).

Figure 6. Density change of a flake-shaped Trichodesmium aggregate as a function of light (taken from Pfreundt et al 2023).

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NOT ONLY NITROGEN, ALSO IRON AND CARBON

The bioavailability of iron (Fe) limits the growth of phytoplankton in the open sea of the vast oceans. The dust transported by the atmosphere from the deserts to the surface of the oceans is rich in Fe but this is not very bioavailable because it is not very soluble in seawater and the dust sinks quickly below the photic zone where the phytoplankton is.

The exception is Trichodesmium, because its colonies or flakes capture the particles of this dust and symbiotically with other associated bacteria promote the dissolution of the dust and the uptake of Fe through siderophores (Figure 7) (Basu et al 2019). Regarding the contribution of dust from the Sahara to the Atlantic Ocean, the relationship with the proliferation of Trichodesmium around the Canary Islands (Figure 8) has been demonstrated (Ramos et al 2005).

Figure 7. Schematic of the uptake system of Fe bound to dust particles by Trichodesmium colonies associated with other bacteria. a Environmental relevance of Trichodesmium, which fertilizes the oceans. b Trichodesmium forms extensive proliferations in nutrient-poor oceans by dissolving deposited dust. c Interactions of Trichodesmium colonies with bacteria that produce siderophores which capture Fe from the dust and this is used by the cyanobacterium and other bacteria (Figure taken from Basu et al 2019).

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Figure 8. Left: Saharan dust dispersing across the Atlantic (NASA NOAA-20 satellite image, June 2020). Right: Chlorophyll (green) detected by satellite on 1 August 2004, showing the current carrying T. erythraeum blooms from the outcrop off the coast of Africa towards Canary Islands (image from Ramos et al 2005)

And what’s more, the cyanobacterium Trichodesmium contributes directly to the export and sequestration of carbon in the seabed. Until now it was known that these microorganisms recycled the CO₂ captured from the atmosphere through photosynthesis, but not that they also took it with them to the depths when they die and sink, as the rest of the phytoplankton do, it is the eukaryotes. This is called the biological carbon pump, and in this way roughly twice as much carbon is stored in the seabed as is currently in the atmosphere. Until now this has been attributed almost exclusively to eukaryotic phytoplankton, and global biogeochemical models, i.e. the tools used to make predictions about the evolution and fluxes of carbon on the planet, do not account for the direct contribution of diazotrophs such as Trichodesmium in this process (Cornejo 2022, Bonnet 2022).

CONCLUSION

To conclude, it can be said that the cyanobacterium Trichodesmium is one of the most abundant organisms in marine environments, it is a substantial part of the phytoplankton and therefore a very important primary producer. It is the most relevant nitrogen fixer in the sea, and one of the most important for the global biogeochemical cycles of nitrogen, carbon, phosphorus and iron. And in addition, it has a variable and complex structure of groupings, and is still little known from the point of view of the functioning of its metabolism and regulation of its large genome, despite being prokaryotic.

BIBLIOGRAPHY

Basu S et al (2019) Colonies of marine cyanobacteria Trichodesmium interact with associated bacteria to acquire iron from dust. Comm Biol 2, 28

Bergman B et al (1997) N₂ fixation by non-heterocystous cyanobacteria. FEMS Microbiol Rev 19, 139–185

Bergman B et al (2013) Trichodesmium –a widespread marine cyanobacterium with unusual nitrogen fixation properties. FEMS Microbiol Rev 37, 286–302

Bonnet S et al (2022) Diazotrophs are overlooked contributors to carbon and nitrogen export to the deep ocean. The ISME Journal, 1-12

Capone DG et al (1997) Trichodesmium, a Globally Significant Marine Cyanobacterium. Science 276, 1221-1229

Cornejo FM (2022) Els bacteris marins diazòtrofs, petits grans aliats contra el canvi climàtic. Institut de Ciències del Mar ICM-CSIC, 18 oct. 2022

Huisman et al (2018) Cyanobacterial blooms. Nature Rev Microbiol 16, 471-483

Le Page M (2023) Ocean-fertilising bacteria work together to adapt to light levels. New Scientist, 25 May 2023

MicrobeWiki (2010) Trichodesmium. A Microbial Biorealm page, Kenyon College, 6 Aug 2010

Partensky F et al (1999) Prochlorococcus, a marine photosynthetic prokaryote of global significance. Microbiol Mol Biol Rev 63, 106-127

Pfreundt U et al (2023) Controlled motility in the cyanobacterium Trichodesmium regulates aggregate architecture. Science 380,830-835

Ramos AG et al (2005) Bloom of the marine diazotrophic cyanobacterium Trichodesmium erythraeum in the Northwest African Upwelling. Marine Ecol Progress Series 301, 303-305

Rodríguez F (2017) Más respuestas sobre Trichodesmium. Blog Fitopasión, 1 agosto 2017

Wikipedia contributors (2023, May 28). Cyanobacteria. Wikipedia, The Free Encyclopedia

Wikipedia contributors (2022, July 18). Trichodesmium. Wikipedia, The Free Encyclopedia

WHOI, Woods Hole Oceanographic Institution (2023) The many faces of Trichodesmium