Natalie Cohen’s Research

Investigating co-limitation of iron and biotin in marine microorganisms

Biotin synthase monomer. The iron-sulfer clusters are highlighted in orange and yellow. (Jarret 2005)

About 30-40% of our oceans are characterized as high-nutrient, low-chlorophyll (HNLC), meaning that although these regions are replete in macronutrients, primary productivity is limited by the supply of the micronutrient, iron. In order to understand how diatoms respond to the availability of iron and the implications of increases in iron supply (such as over short timescales during iron fertilization experiments or over longer timescales during glacial periods) it is useful to identify both direct and indirect consequences of iron limitation on phytoplankton and the other microorganisms they commonly interact with. Iron limitation may influence diatom production of the essential vitamin biotin (also known as vitamin B7) and thus regulate the extent to which biotin is found in the surface ocean. This may have a tremendous effect on other microorganisms that are reliant upon diatoms for their primary source of biotin.

I am currently investigating the effects of iron limitation on the production of biotin in the oceanic diatoms of the genus Pseudo-nitzschia. Biotin is required for enzymes involved in fatty acid synthesis, amino acid metabolism, and cell growth. The iron requirement is due to the iron-sulfur enzyme, biotin synthase, which is involved in the last step of the biotin synthesis pathway. My research goals are to determine if iron-limitation influences their ability to synthesize biotin and to evaluate if diatoms are able to potentially reduce their cellular iron demands by utilizing external sources of biotin from the environment when iron is not readily available and biotin is (such as in HNLC regions).

Previously, it was shown that diatoms are dependent on bacteria for a source of cobalamin (vitamin B12) (Croft et al. 2005) . Similarly, although diatoms are one of the groups able to produce biotin, most bacteria are not known to posses the necessary genes for biotin synthesis. It has been well documented that an intimate and sometimes symbiotic relationship exists between diatoms and certain bacteria. Another goal of my research is to explore whether a vitamin exchange is occurring between these two groups of microorganisms and how this exchange is affected by iron limitation.

Pseudo-nitzschia multiseries with attached bacteria (Kaczmarska et al., 2005)

References:

Croft M.T., et al. (2005) Algae acquire vitamin B12 through a symbiotic relationship with bacteria. Nature 438:90-93.
Jarrett J.T., (2005) Biotin Synthase: enzyme or reactant? Chemistry & Biology 12: 409-415.
Kaczmarska I., et al. (2005) Diversity and distribution of epibiotic bacteria on Pseudo-nitzschia multiseries (Bacillariophyceae) in culture, and comparison with those diatoms in native seawater. Harmful Algae 4: 725-741.