Iron uptake by Algae in seawater?

[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.
If we used a water soluble form of Iron in the first place the algae would just absorb the free floating iron ions.

Could someone with some sort of chemistry/biology background please help explain what is actually happening and if both of us are wrong. This is of course in the context of a marine environment

[Randy Holmes-Farley]

To Reef Central

Thas is a complex question, and the answer is not going to fully satisfy either of you.

First of all, while it is theoretically possible for an organic chelator of iron to take iron off of the surface of an insoluble species, like iron oxide or hydroxide, it is not clear how rapidly (and effectively) that can happen. This is an issue for folks using GFO, for example. Is simple use of GFO adequate to supply iron, or is extra soluble iron useful. We do not know, but I use both GFO and a soluble form of iron.

Second, even if you have an insoluble form of iron, such as GFO, there will necessarily be some soluble Fe+++ and Fe++ present in the water in equilibrium with the insoluble iron oxide/hydroxide. So there is some free iron, and organic chelators may come along and bind that.

Third, it is not clear if algae can use chelated iron. If it is chelated strongly enough, such as with EDTA, the literature suggests that such complexes need to be broken apart by UV before the iron is available to organisms. There are, however, a whole range of chelators present in a reef tank, some of which are strong annd some of which are weak.

Finally, it isn't clear to me exactly how algae take up iron. That is, what forms they actually take up.

I discuss these various issues more in several articles, including:

First Iron Article: Macroalgae and Dosing Recommendations
http://www.advancedaquarist.com/issues/aug2002/chem.htm

Iron Oxide Hydroxide (GFO) Phosphate Binders
http://reefkeeping.com/issues/2004-11/rhf/index.htm

Reef Aquaria with Low Soluble Metals
http://reefkeeping.com/issues/2003-0...ture/index.htm

[shardakar]

Thanks for the reply randy.

Yes I have actually read all of your articles before posting this.
The bigger part of the discussion I was having with my friend was with regards to Nitrate removal on a commerical scale recirculating Aquaculture system.

The idea is that macroalgae provides an excellent method of nitrate that is all natural.
One will never be short on waste organics that algae will use for growth.
Iron is likely to be the key element limiting growth.
Commerical Iron supplements aren't exactly cost effective on a large scale and his idea was to simply dump waste iron from factories.

To me this sounds like a bad idea for a number of reasons.
1) We have no control over the rate at which the waste iron will dissolve and subsequently be absorbed by the algae. We have no idea if it'll just sit there and never provide a significant amount of Iron to the algae.
2) There is no control over other unwanted heavy metals that could be a part of the waste iron. We also have no idea of the quantities of these unwanted heavy metals.
3) Even if the Iron does eventually get used up by the algae the other components of the waste iron will eventually accumulate to toxic levels as these other heavy metals might not be used for other biological functions.

My question is, are there any merits to using waste iron as a way to promote macroalgae growth? Are there any merits that would outweigh the cons of using waste iron?

[Randy Holmes-Farley]

I'm not sure that most algae can use waste organics. Bacteria will.

I think it a bad idea as well. Waste iron may be heavily polluted with other metals, as you note. Whether enough iron will dissolve in a way that algae can take up is not at all clear to me. One would have to experiment under the exact conditions you'd be experiencing in the real setting, including pH, salinity, bacterial speciation, etc.

[HighlandReefer]

Too much inorganic iron is a problem. I agree with Randy, heavy metals are a concern in waste iron.

FWIW:


Black reefs: iron-induced phase shifts on coral reefs

Linda Wegley Kelly1, Katie L Barott1, Elizabeth Dinsdale1, Alan M Friedlander2, Bahador Nosrat1, David Obura3, Enric Sala4, Stuart A Sandin5, Jennifer E Smith5, Mark J A Vermeij6,7, Gareth J Williams5, Dana Willner8 and Forest Rohwer1

1Department of Biology, San Diego State University, San Diego, CA, USA
2Hawaii Cooperative Fishery Research Unit, Department of Zoology, University of Hawaii, Honolulu, HI, USA
3CORDIO East Africa, Mombasa, Kenya
4National Geographic Society, Washington, DC, USA
5Center for Marine Biodiversity and Conservation, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA
6Carmabi Foundation, Piscaderabaai z/n, Curaçao, Netherlands Antilles
7Aquatic Microbiology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, The Netherlands
8Australian Centre for Ecogenomics, University of Queensland, St Lucia, QLD, Australia

Correspondence: LW Kelly, Department of Biology, San Diego State University, 5500 Campanile Dr, San Diego, CA 92182, USA. E-mail: lwegley@gmail.com

Received 4 March 2011; Revised 13 July 2011; Accepted 15 July 2011; Corrected online November 2011; Published online 1 September 2011.

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Abstract

The Line Islands are calcium carbonate coral reef platforms located in iron-poor regions of the central Pacific. Natural terrestrial run-off of iron is non-existent and aerial deposition is extremely low. However, a number of ship groundings have occurred on these atolls. The reefs surrounding the shipwreck debris are characterized by high benthic cover of turf algae, macroalgae, cyanobacterial mats and corallimorphs, as well as particulate-laden, cloudy water. These sites also have very low coral and crustose coralline algal cover and are call black reefs because of the dark-colored benthic community and reduced clarity of the overlying water column. Here we use a combination of benthic surveys, chemistry, metagenomics and microcosms to investigate if and how shipwrecks initiate and maintain black reefs. Comparative surveys show that the live coral cover was reduced from 40 to 60% to <10% on black reefs on Millennium, Tabuaeran and Kingman. These three sites are relatively large (>0.75 km2). The phase shift occurs rapidly; the Kingman black reef formed within 3 years of the ship grounding. Iron concentrations in algae tissue from the Millennium black reef site were six times higher than in algae collected from reference sites. Metagenomic sequencing of the Millennium Atoll black reef-associated microbial community was enriched in iron-associated virulence genes and known pathogens. Microcosm experiments showed that corals were killed by black reef rubble through microbial activity. Together these results demonstrate that shipwrecks and their associated iron pose significant threats to coral reefs in iron-limited regions.

This article has been corrected since Advance Online Publication and a Corrigendum is also printed in this issue