Variables determinantes de los costos de Transporte Marítimo

access Fe-siderophore complexes via their membrane transporter systems (Maldonado and Price, 2001; 1999; Amin et al., 2009). Most experimental work has shown that the addition of ferrihydrite enhances the growth of diatoms in seawater at pH ~8, where the initial cultures initially contained no aqueous Fe species. These results are valuable because diatoms form algal blooms in HNLC areas (Maher et al., 2010). However, growth rates decreased with aged ferrihydrite, and other iron minerals (goethite, haem- atite) have been found to be only poorly bioavailable (Wells et al., 1983; Rich and Morel, 1990), probably because of their lower solubilities and dissolution rates in seawater (Kuma and Matsunaga, 1995). Yoshida et al. (2006) studied the effects of ferrihydrite aging on iron uptake by coastal diatoms. Decreasing cell yields with more aged material were attributed to age-related chemical and structural changes that affected the ability to supply bioavailable Fe by dissolution (Fig. SI-1). Solubility measurements of ferrihydrite showed that the aqueous Fe content of solutions in contact with ferrihydrite decreased over 20 days due to the progressive growth of larger and more stable particles, consistent with the aggregation observed by Gilbert et al. (2009), Raiswell et al. (2010) and Bligh and Waite (2011).

There is thus good evidence that synthetic ferrihydrite can support plankton growth, but do natural ferrihydrites behave in the same way? Visser et al. (2003) found that iron present as ferrihydrite in soil dust, and the iron dissolved from the dust, were able to increase the growth rate of diatoms. The ferrihydrite was not completely bioavailable and, more surprisingly, neither was the dissolved iron completely utilised. However dissolved Fe was measured on solutions passing a 0.2 µm filter and may thus contain colloids which were not readily bioavailable. Experimental work suggests that iron-bearing colloids harvested from seawater are only partially bioavailable to eukaryotic organisms. Chen and Wang (2001) carried out experimental studies of diatom growth in seawater spiked with colloids/nanoparticles harvested from seawater and separated into two size fractions (approximately <5 nm and 5 nm to 0.2 µm), each labelled with 56_{Fe. Uptake of iron from the smaller size fraction exceeded that from}

the larger, but both fractions were used less readily than Fe present as low molecular weight aqueous complexes. No data were presented on the colloid composition but Chen et al. (2003) used the same approach to follow the variations in uptake between colloids harvested from coastal, oceanic and estuarine environments. Over a 35 day period, uptake increased to maximum levels of ~20% from the coastal colloids and ~15% from the oceanic material, as compared to ~25% from the low molecular weight Fe complexes. The estuarine colloids were only weakly bioavailable (~8%), and uptake remained uniform with time. Experiments in which three different saccharides (poorly defined, polyfunctional carbohydrate compounds) were added to eukaryotic plankton assemblages produced an increase in iron uptake by forming highly bioavailable species (probably by adsorption

rather than complexing) with iron and by increasing the solubility of colloidal iron. (Hassler et al, 2010). These relatively weak iron-saccharide species are easily able to exchange iron with the more strongly-binding iron transporters used by eukaryotes to supply Fe(III) to the cell surface, where reduction to Fe(II) is followed by entry into the cell (see Supplementary Information SI-2).

In addition to these processes it has been suggested that colloidal iron is rapidly recycled where phytoplankton are grazed by heterotrophic protozoans but detailed mechanisms are poorly understood (Breitbarth et al., 2010). Ingested iron minerals are subjected to low pH processing in the gut that enhances bioavailability. Nodwell and Price (2001) showed that some species of mixotrophic flagellates were able to use goethite, haema- tite and magnetite but not ferrihydrite, whilst other species could only use ferrihydrite. In all cases uptake appeared to involve ingestion and thus represents the direct use of colloidal iron by eukaryotic organisms. Barbeau

et al. (1996) have also shown that ingestion by grazing decreased the grain- size of colloidal ferrihydrite, increased the labile Fe content and increased the bioavailability to marine diatoms (Fig. SI-1). Faecal aggregates may also supply Fe(II) where microbial reduction of Fe (oxyhydr)oxides occurs inside organic C-bearing marine aggregates (Balzano et al., 2009), although rapid sinking of aggregates would suggest that this source is more effec- tive for deep waters than surface waters. To summarise; it seems clear that eukaryotes can, at least partially, take up Fe from natural and synthetic ferrihydrite and from natural Fe-bearing organic colloids.

Attachment Growth and

Aggregation TransformationMineralogical Aging Decreases Reactivity and Bioavailability through Physical and Chemical Transformations

Increasing Reactivity and Bioavailability through Rejuvenation

Goethite Hematite Clay Grain

Ferrihydrite

Figure SI-1 The aging-rejuvenation cycle (from Raiswell, 2011a, with permission

Iron Uptake by Prokaryotic Organisms. Siderophore transport systems are utilised by prokaryotic organisms to chelate and solubilise aqueous iron species and iron minerals. Siderophores are low molecular weight organic ligands with a high affinity and specificity for iron chela- tion. The stability constants of Fe(III) siderophores are 1023 _{to 10}52_{, signifi-}

cantly higher than for common laboratory complexing agents such as EDTA (which has a stability constant of 1020_{). Siderophores can be synthesised}

by many prokaryotic organisms, including cyanobacteria (Haygood et al., 1993; Sunda, 2001). However, organisms that are unable to produce sidero- phores may still be able to take up iron bound to siderophores produced by other organisms (Granger and Price, 1999).

The influence of siderophores on iron dissolution and complexation has been studied using thermodynamic models (Kraemer et al., 2005). These models show that the solubility of ferrihydrite and goethite increases with increasing concentrations of the siderophore desferrioxamine B (DFOB) in seawater at pH 8, but significant (order of magnitude) increases in the solu- bility of ferrihydrite only occurred at siderophore concentrations above ~1 nM. Kraemer et al. (2005) also show that variations in siderophore stability constant (log KFe3+L from 18 to 25) have little effect on ferrihydrite solubility

but large effects on goethite and haematite solubility. Experimental work described below has supported the modelling results. Yoshida et al. (2002) showed that siderophores extracted from marine organisms increased both the dissolution rate and solubility of ferrihydrite aged for 24 hrs. Dissolution rates increased as siderophore concentrations increased but rates slowed with increasing pH and the dissolution rates at pH 8 were too slow to measure over 100 days. Siderophore concentrations were not directly meas- ured but were in the micromolar range. This implies that there would be limited kinetic enhancement of siderophore-controlled dissolution of aged ferrihydrite in seawater with much lower concentrations of siderophores (Kraemer, 2006). Fresh ferrihydrite (aged for 1 minute) is however dissolved an order of magnitude more readily by the siderophore DFOB at pH 8 than material aged for 24 hrs (Rose and Waite, 2003). Overall, siderophores are clearly able to produce Fe from iron (oxyhydr)oxides that is bioavailable to prokaryotic organisms but fresh ferrihydrite may represent a more viable resource in seawater than aged ferrihydrite.