Oh the irony! EPS puts iron back on the microbial menu.

In ocean water, somewhere between particulate and dissolved organic matter, there exist gels of organic compounds, secreted or released by marine micro-organisms, that sink and conglomerate in the water column. These gels have a lot of important functions in the marine system but this blog will focus on how these gels influence the chemistry of dissolved iron.

Iron is a very important nutrient in the water column; required for processes like photosynthesis and nitrogen fixation. Despite this importance, iron is often a limiting factor for many phytoplankton. Organic ligands, such as porphyrins, humic acids and saccharides (exopolymeric substances) can be found in the aforementioned marine gels which bind to iron, making the nutrient more bioavailable. A study published this year by Hassler et al. investigates just how much EPS increases the bioavailablity of iron to phytoplankton. 

By adding two types EPS to populations of Chaetoceros simplex, the researchers found that the bioavailablity of iron was increased by 365% (+/-7) and 437% (+/-8) compared to dissolved iron not bound by organic ligands, showing that dissolved iron is more bioavailable to the model phytoplankton if it is bound in these organic gels. It should be noted that iron bound in bacterial EPS (made by Pseudoalteromonas sp.) was only 28% bioavailable compared to dissolved iron. The study also found that addition of EPS to natural populations stimulated the growth of both Prochlorococcus and Synechococcus, which have very important roles in cycling Carbon in ocean waters.

The study took EPS secreted by two phytoplankton: Phaeocystis antarctica and Emiliana huxleyi and a natural sample from  the Southern Ocean. The two EPSs were mixed with ⁵⁵Fe (a radioactive isotope).Liquid colonies of C.simplex were grown and then enriched with either the EPS or inorganic Carbon. Iron uptake was calculated relative to a form of iron that the scientists and previous studies had shown to 100% bioavailable (FeCl₃). The quench rate of the isotopic iron in the cells was also used to determine cellular concentration of iron.

To me, this is a very interesting study, it reminds me of the studies which seeded the oceans with iron, except this study suggests that the addition of iron bound to organic ligands might be more effective than just inorganic iron. Something that grabs my attention is that one of the types of iron they investigated; FeCl₃ which- to my knowledge- was not organic but was just as bioavailable as that bound in EPS. The iron bound in organic compounds was able to reach higher concentrations in seawater than inorganic iron (3.56nM vs. 2nM before precipitation), could there be a ferrous organic compound that can have an even higher concentration of iron? Furthermore, would it be even more accessible to phytoplankton? It might be worth researching more into finding more bioavailable forms of iron as opposed to increasing the concentration of iron in the seawater.

References:

This paper

Hassler, C.S. Norman,L. Nichols,C.A.M. Clementson,L.A. Robinson,C. Schoemann,V. Watson,R.J. Doblin,M.A. (2015) Iron associated with exopolymeric substances is highly bioavailable to oceanic phytoplankton. Marine Chemistry. Vol.173 pp.136-147.