Phytoplankton proteins "concentrate" to increase iron uptake.

In 1982, it was hypothesised that phytoplankton in the oligotrophic open ocean overcame the problem of iron uptake by first binding it to their membranes before metabolising it. They suggested it was a hypothetical protein “phytotransferrin” that carried out this process but lacked the technology to test for such a protein. In 2014, Morrissey et al. identified novel proteins deemed “ubiquitous in marine phytoplankton” that concentrate dissolved iron around the surface of the cell, resolving the hypothesis that was put forward some 30 years ago.

The main finding of this study is the discovery of the Iron Starvation Induced Proteins (ISIPs), in the diatom Phaeodactylum tricornutum. The study focused on ISIP2a, which metatranscriptomics show to be “ubiquitous” in marine phytoplankton, being expressed in species of Pelagophyceae, Haptophyta, Cryptophyta, Chlorophyta, Dinoflagellata, Rhodophyta and Bacillariophyta. These proteins are transmembrane and composed of two subunits, ISIP-N and ISIP-C. ISIP-N and C are highly conserved in brown and green macroalgae but it’s the combination and arrangement of the two in phytoplankton that makes them novel.

ISIP2a functions by binding to ferric iron (Fe³⁺) in the water column, facilitating the activity of other proteins, such as ferrireductases that metabolise the iron once it is localised to the cell surface. Quantitative PCR found that ISIP2a is sharply upregulated when the cell first experiences iron deprivation. Ferrireductase and other “more conventional” iron uptake proteins are expressed a while after ISIP2a, suggesting its expression is an immediate response to low iron levels.

ISIP2a is also inhibited by chelating agents (another way phytoplankton are able to acquire dissolved iron from the water column) of the same oxidative state of iron (Fe³⁺) I think this is interesting, why is this protein inhibited by chelated ferric iron? Perhaps it is not energetically viable for the diatom to express proteins that acquire the same nutrient simultaneously. Or, seeing as ISIP2a binds to iron, chelated ferric iron competitively inhibits its function. It would be interesting to see how the expression of ISIP2a relates to expression of chelating agents for ferric iron as I would have thought the diatoms would want to be able to acquire as much iron as possible and would express both.

This study presents a novel protein that upon investigation is revealed to be prevalent in many 

phytoplankton and gives new insight into phytoplankton response to iron deprivation. Whilst it may not be a novel idea that they are able to concentrate iron around their cell surface, this may well be the first evidence to support that hypothesis. Another paper I have recently read (and will blog about) states that different phytoplankton use the same proteins for different purposes, so I would be interested to see studies on the function of the “ubiquitous” ISIP2a in the rest of the phytoplankton it is expressed in, there may be analogues that concentrate other oxidation states of iron, or other nutrients entirely.

Reference:

Morrissey, J. Sutak, R. Paz-Yepez, J. Tanaka, A. Moustafa, A. Veluchamy, A. McQuaid, J.B. Tirichine, L. Allen, A.E. Lesuisse, E. and Bowler, C. (2015) A Novel Protein, Ubiquitous in Marine Phytoplankton, Concentrates Iron at the Cell Surface and Facilitates Uptake. Current Biology, 25, 364-374.