Relationship between vitamins and iron limitation in diatoms

Biosynthesised molecules such as B vitamins are irreplaceable co-factors in enzymes helping cellular metabolism. They can be growth limiting for those unable to synthesise or acquire from the environment, possibly influencing microbial community composition, where in bloom events seawater may be depleted or enriched. B7 is required in carboxylation enzymes involved in fatty acid synthesis. Four genes are involved in the biosynthetic pathway and all genes are found in algae except for dethiobiotin synthase (BIOD). Diatoms in culture are able to grow with B7 suggesting there is a BIOD homologue present. It is iron dependant so it is hypothesised that iron limitation in marine diatoms could alter production, function and activity.

In yeast saccharomyces cerevisiae iron limitation causes a decrease in B7 synthesis and activates a transporter that functions in the acquisition of external sources of B7. if the same happens in diatoms it would allow intracellular iron in short supply to redistribute towards other necessary iron dependant processes. B12 plays a major role in methionine synthesis  and some plankton possess B12 independent of METE, there being a strong correlation between absence of METE and B12 auxotrophy.

Iron, and B12 co limitation on phytoplankton has been documented in the southern ocean. It has been previously seen in studies such as those by Bertrandt et al 2011 who looked at the Ross sea, which has been shown to be iron limited and B12 colimited in the summer. B12 was limiting only when bacterial abundance was low with iron addition enhancing bacterial growth. Iron additions enhanced B12 uptake in phytoplankton maybe due to it increasing bacterial growth and consequently B12 production.

Cohen et al 2017 looked into the iron and vitamin interactions further with marine diatoms. Exploring the dynamics between iron and vitamins can lead to better understanding of how iron fertilisation events could influence B vitamin production and consumption. The influence of iron status on B7 production, B12 utilisation and METE activity in diatom isolates in the Pacific ocean was studied. They quantified the transcriptional response of BIOB under variable iron and B7 growth conditions through gene expression in the diatom P. granii . The expression of METH and METE in Grammonema cf islandica with B12 requirement was also measured in response to the variables.

P. granii in iron replete cultures grew at a rate higher than those in iron limiting conditions, there being a 65% reduction in growth rate. There was no significance in growth rate between cultures with or without B7 in both iron conditions. P. granii had a reduced growth rate of 50% when iron limited with B12. G. cf islandica under the same conditions of either iron or none with B12 experienced 67% reduction in growth rate when iron limited. B12 absence would decrease growth rate by 35% and the combination of both variables limited reduced growth rate and METH expression by 3 fold. Diatom populations when incubated with iron increased appreciably 40 fold higher in density, the highest abundance being exhibited when this was combined with B12. Vitamin B7 metabolism was affected by iron addition increasing 2 fold in iron treatments. Laboratory studies with G. islandica under iron/B12 conditions demonstrate METE gene expression is primarily regulated by B12 status. Any increase in METE following iron addition is due to B12 concentrations becoming limiting, so there is insufficient acquisition of B12 from the environment causing METE possessing diatoms to produce B12 independently.

It is probable that iron limitations reduced the amount of diatom produced vitamin B7 available to other auxotrophs. Following iron enrichment emergence of METE containing diatoms provide evidence for B12 limitation. Iron limited diatoms produce fewer vitamin synthesis transcripts compared to cells enriched with iron either they require less vitamins when growing or they lack essential resources for fuel production. Cohen et al 2017 concluded that subarctic North East ocean diatom community examined was driven into B12 limitation following the iron addition.

paper reviewed: Cohen, N., A. Ellis, K., Burns, W., Lampe, R., Schuback, N., Johnson, Z., Sañudo-Wilhelmy, S. and Marchetti, A. (2017). Iron and vitamin interactions in marine diatom isolates and natural assemblages of the Northeast Pacific Ocean. Limnology and Oceanography, 62(5), pp.2076-2096.

reference: Bertrand, E., Saito, M., Lee, P., Dunbar, R., Sedwick, P. and DiTullio, G. (2011). Iron Limitation of a Springtime Bacterial and Phytoplankton Community in the Ross Sea: Implications for Vitamin B12 Nutrition. Frontiers in Microbiology, 2.