Trichodesmium eats Cobalt and B12 for breakfast

This paper follows on from the vitamins in the sea lecture, further detailing how vitamin B₁₂ (cobalamin), is important for uptake and growth and in understanding nitrogen fixation fluctuations, to predict its effects. Phytoplankton growth is regulated by continuous uptake of minor nutrients and trace metals, influencing the community structure. Many studies are based on vitamin B₁₂ use, as it is a Cobalt-containing tetrapyrrole compound synthesised by eubacteria and archaea. Eukaryotes require an exogenous source and there are few to no studies on the effects of trace metals on phytoplankton growth.

Rodriguez et al., (2015) investigated the varying effects of cobalt (Co) and B₁₂ concentrations on the growth and rate of nitrogen fixation of the non-heterocystous diazotroph, Trichodesmium erythraeum, in a laboratory culture study. T. erythraeum is a filamentous cyanobacteria found in the tropical and subtropical oligotrophic oceans, vitamin B₁₂ limiting waters. Dissolved Co concentrations are low in the euphotic zone, 15-81 pmol L^-1 in the North Atlantic, similar to Rodriguez dissolved Co results being 17.1 pmol L^-1, and 26-59 pmol L^-1 in the Southern Ocean. The presence of Co within B₁₂ suggests synthesis and biological functions are influenced by the bioavailability of Co. Original cultures contained bacteria which synthesises B₁₂ for uptake, this importance for phytoplankton is not stated. Batch cultures of the non-axenic strain associated with repeat Fe and P supply were used, as they are successfully cultured using YBC-II culture medium containing Co and B₁₂. Grown in polycarbonate bottles using ethylenediaminetetraacetic acid (20 µmol L^-1). This maintains the consistency of natural trace metal concentrations. They varied inorganic Co concentration, 0, 0.2, 2 and 20 pmol L^-1 and B₁₂ at 0, 40, 100 and 400 pmol L^-1 respectively in different culture mediums. A Beckman Coulter Counter Multisizer 3 used to measure total cellular volume, growth. At the exponential phase, they were single trichomes, not colonies, so transfer of cells, at this stage, to new media occurred for trichrome comparisons. Rates of nitrogen fixation were calculated using an acetylene reduction assay method.

Results achieved were; growth is sustained if a good supply of inorganic Co at concentrations of 20 pmol L^-1 is present. Growth was not affected when no or some B₁₂ was administered at the above Co concentration, growth rates being 0.46 ± 0.01 d^-1 for all vitamin concentrations. Treatments below 2 pmol L^-1 Co saw a decline in the biomass, these limiting Co cultures increased growth if 400 pmol L^-1 of B₁₂ was added. They found a positive correlation to growth. Therefore, in the oligotrophic oceans where Co concentrations are low, these diazotrophs manage to thrive by uptaking B₁₂ to counteract the low levels. Another result in treatments with insufficient B₁₂ administered, 0 and 40 pmol L^-1, the rate of nitrogen fixation was higher, due to the dependence of Co. It could also be higher to compensate for reduced B_12, therefore, nitrogen fixation increases B₁₂ synthesis.

This is the first detailed study on Co influence for Trichodesmium, another being on picocyanobacteria Synechococcus (Saito et al , 2005), therefore engaging my interests, and gives very detailed accounts of their methods. The author states even though it is believed Co controls growth, as it is needed in B₁₂ synthesis, it is not a major factor but, B₁₂ is important in relieving Co limiting stressors, via its break-down. Further study on this species could highlight its possible use in anthropogenic sources where Co is bound by strong ligands and not readily accessible. However, nothing stated enzymatic roles or function of T. erythraeum. Therefore, in my view, future studies with this aim, comparing species, could aid understanding on nutrient cycling, blooms and pollutant fluctuations occurring due to us.

Referenced Article

Rodriguez, B. I., & Ho, T. (2015). Influence of Co and B₁₂ on the growth and nitrogen fixation of Trichodesmium. Frontiers in Microbiology, 6(623)

https://www.frontiersin.org/articles/10.3389/fmicb.2015.00623/full

Further Reading

Saito, M. A., Rocap, G., & Moffett, J. W. (2005). Production of cobalt combining ligands in a Synechococcus feature at the Costa Rica upwelling dome. Limnology and Oceanography, 50(1), 279-290