Trichodesmium and Fe-cycling the possibilities are endless…

Iron is a nutrient that is associated with several processes, for instance nitrogen fixation, photosynthesis, DNA biosynthesis and electron transfer. It is considered a limiting factor because it is scarce in surface waters. Iron can be present in the open waters as organic iron, colloidal and soluble inorganic iron or even as complexes. Organic ligands which are molecules that binds to a central metal (in this case dissolved Fe) and makes a complex. These iron-ligand complexes are believed to be an important source of iron. There have also been observations of dust particles migrating into Trichodesmium colonies, suggesting it is an iron source.

Trichodesmium is an abundant cyanobacteria, found in open waters. They are responsible for a large majority of nitrogen cycling and have been related to the formation of new nitrogen pools. Trichodesmium lives in colonies that create a microbial consortium which is a mixture of different organisms with unique roles. The organisms commonly associated with Trichodesmium are bacteria, dinoflagellates, amoeba, ciliates and diatoms. Bacteria in particular are common and are known to be present in Trichodesmium colonies. Heterotrophic bacteria can produce siderophores (molecules released by bacteria to obtain iron) under iron limiting conditions, Trichodesmium have not been shown to produce any siderophores however it does up regulate an iron deficiency-induced protein an IdiA homologue under conditions of reduced iron availability.

In this study the rate of iron cycling was tested using laboratory cultures of Trichodesmium and two strains of Trichodesmium-associated bacteria. The two strains representative of Trichodesmium-associated bacteria used for the iron uptake experiments were an alpha proteobacteria from Roseobacter lineage Silicibacter TrichCH4B and a bacteriodetes Microscillia marina. The bacteria strains were supplied with various iron sources after acclimation to PC media where iron was the limiting nutrient. The cultures were non-axenic, which means they were not contaminated with any other living organism. The genomes of both M.marina and TrichCH4B have been sequenced through the Moore Microbial Genome Sequencing Project.

For determining Trichodesmium growth rates in Fe-low and Fe-High medium, aliquots of a single Fe-high Trichodesmium culture was gravity filtered onto a 3.0µm filter, rinsed twice and resuspended with either Fe-low or Fe-high R medium. The Fe-high and Fe-low cultures were then sampled over 3 weeks of incubation.

To insure that the Trichodesmium cells were healthy the authors measured the photosynthetic efficiency (Fv/Fm) after 2 weeks. A FRRFII sensor was used to measure the kinetics of chlorophyll fluorescence induction and decay in Trichodesmium.

The Fe uptake in Trichodesmium was measured using a labelled 55^Fe solution, which was added to the cultures and then incubated at 24ºc with a 12h light/dark cycle. To examine Trichodesmium Fe uptake during the diel cycle, uptake experiments were initiated by rinsing and resuspending all cultures at the beginning of the corresponding light or dark periods.

The Trichodesmium was gravity filtered onto a 3.0µm polycarbonate filter and rinsed. The amount of radioactivity on the filter was analysed. The Fe concentration was determined by converting 55^Fe activity to molar concentrations using a standard curve.

Cultures were maintained for a month but the first 16 days were used for the growth curves to show exponential growth. After day 6 the Fe-high and Fe-low cultures start to diverge and there was a Fe stress showed in Fe-low cultures. The exponential growth calculated for Fe-high was 0.12 and for Fe-Low was 0.08, which was clearly slower. The Fe-low culture was however able to survive longer (1 week) than the Fe-high culture. The decreases in Trichodesmium growth rates show that there is an indication of Fe stress in the Fe-low cultures. This was supported by the observations of morphological differences present in the Fe-low cultures compared to Fe-high cultures. The morphological features were longer trichomes containing more cells per trichome and individual cells that were longer and narrower.

To make up for bacterial Fe uptake in each sample, samples were size fractionated at each time point. In all the Trichodesmium experiments the filtration reduced the abundance of bacteria by 80% in the Trichodesmium fraction. Active Trichodesmium Fe uptake could then be calculated by subtracting the 3.0µm bacterial Fe uptake and the 3.0µm glutaraldehyde killed control.

The Trichodesmium associated bacteria were grown on PC medium which were grown in room temperature. They were under iron limiting conditions, created by transferring the culture 3 times through iron poor PC media. The iron limitation was confirmed by comparing the growth rate of subcultures of M. marina (36h) and TrichCH4B(20h).

The bacteria were rinsed and resuspended 3 times with chelexed artificial seawater . Bacterial 55^Fe uptake experiments were set up in triplicate with a 0.01% glutaraldehyde killed control and cultures were incubated at room temp in the dark.

Trichodesmium and the two bacteria strains were also analysed for carbon content.

The authors concluded that Trichodesmium was rationed to inorganic iron (FeCl₃) or iron in association with weak organic ligands (FeCit) in iron limiting conditions. However the bacteria strains were able to acquire iron from all of the Fe sources provided, although the uptake of iron by TrichCH4B was somewhat slower than M.marina. M.marina’s preference for iron complexes over FeCl₃ could be the result of siderophore receptors being upregulated in low Fe conditions, because it increases the efficiency of ferric siderophore uptake. Several theories were provided for the contrast between the bacteria and Trichodesmium, for instance because Trichodesmium is a diumal diazotroph its ability to acquire and transport iron might be expected to vary temporally. Other iron acquisition mechanisms were also considered such as iron reduction in trichodesmium or possibility of cell lysis to cycle within a colony.

The field colonies of Trichodesmium would contain bacteria; this environment could impact the nutrient and iron cycling. Diazotrophs need high concentrations of iron and combined with the limited iron accessibility of Trichodesmium it is possible a mutualistic relationship is present. This relationship would not have been represented in the laboratory experiments because the cultures were non-axenic. It is also possible that the Trichodesmium-associated bacteria which acquire iron by using siderophores may compete with Trichodesmium.

The authors created a model to understand the possible relationship between the Trichodesmium and the bacteria. It showed that bacteria directly competed with Trichodesmium during the 3 hour uptake incubations. Although the model shows competition, mutualism cannot be ruled out because even though Trichodesmium cannot access from strong siderophore sources they can acquire it elsewhere, for instance mineral dust. Trichodesmium and its colony-associated microbial community may differ substantially in terms of iron acquisition strategy.

I was interested in this paper because it focused on Trichodesmium, which in my last summary was used in a comparison with diatom-cyanobacteria complexes. It was interesting to look at its connection to nitrogen fixation through iron cycling. This paper was very detailed into the different methods of iron acquisition and the idea that trichodesmium-associated bacteria like TrichCH4B and M.marina are either competing or mutualistic would be very important in terms of colonial densities of Trichodesmium. If it is mostly competing, the possible knock on effect on the formation of new nitrogen pools in the open oceans, should be considered.

Roe, K.L, Barbeau K, Mann E.L, Haygood M.G, (2012) Acquisition of iron by Trichodesmium and associated bacteria in culture, Environmental Microbiology, 14(7), 1681-16951..