[shardakar]

I'm having an disagreement with my friend,
 
He contends that Iron does not have to be soluble because certain bacteria will secrete siderophores and chelate the iron and thus make it soluble and then algae can use up the siderophore chelated iron. Either that or the bacterium will absorb the chelated iron and eventually die releasing iron and then be used up by the algae
 
My understanding is that iron first has to be in a soluble form before that kind of chelation can occur. Iron has to be dissolved and its ions freely floating in the water before chelation by bacteria can happen. If its just sitting as a precipitate at the bottom, no chelation will occur. Further more, Siderophores form some of the strongest chelates and most algae would be unable to break the chelating molecule to absorb the iron ions.
 
Could someone help solve this argument? We're both probably wrong.

[Arkcon]

Here's a brief technical note by a researcher on the subject of siderophores.  http://www.princeton.edu/~cebic/chelbindintro.html  Briefly, you're right, those chleates are tough to extract the iron from.  But living things use reductases, to alter the charge on the bound iron, to get them out.  Iron does have to be soluble tho, especially for algae.  The implication from online sources is that siderophore-bound iron is the major source of iron for all algae, and what there is available in the ocean is mostly in that form.  There is probably not much free form iron ions in the slightly alkaline ocean water.  I guess bacterial colonies are associated with iron sediments in the ocean as a source.

[shardakar]

Thanks Arkcon, your reply was helpful.

Leading on from my original question,
So must iron be in a soluble form before chelation can occur?
For example if I were to add something insoluble like ferrous fumarate or Iron(II)/(III) oxide into a biotic environment(the ocean for example), can bacteria chelate an insoluble compound?

[Arkcon]

Its worth looking up to be absolutely sure, but I'd suspect insoluble iron compounds in ocean sediments are well colonized by siderophore-forming marine bacteria.  http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6941.1999.tb00593.x/abstract

Terrestrial plants might be able to use their root cells to dissolve and absorb iron compounds from soil, but even the largest algae don't have specialized, absorbing root cells.