Iron diatoms

Silicic acid (H4Si04) is a necessary nutrient for diatoms, who build their shells from opal (Si02 H20). Whether silicic acid becomes limiting for diatoms in seawater depends on the availability of Si relative to N and P. Estimates of diatom uptake of Si relative to P range from 16 1 to 23 1. Dugdale and Wilkerson (1998) and Dunne et al. (1999) have shown that much of the variability in new production in the equatorial Pacific may be tied to variability in diatom production. Diatom control is most important at times of very high nutrient concentrations and during non-steady-state times, perhaps because more iron is available at those times. 

The results of two successful iron-fertilization experiments in the eastern equatorial Pacific have clearly shown that phytoplankton growth rate is limited by iron at that location (Martin et al., 1994 Coale et al., 1996). The species composition and size distributions of the ecosystem are influenced by iron availability (Landry et al., 1997). In particular, large diatoms do not grow at optimum rates in the absence of sufficient iron. Loukos et al. (1997) used a simple... 

Rich, H. W. and Morel, F. M. M. (1990). Availability of well-defined iron colloids to the marine diatom Thalassiosira weissflogii, Limnol. Oceanogr., 35, 652-662. 

Kuma, K., Tanaka, J. and Matsunaga, K. (1999). Effect of natural and synthetic organic-Fe(III) complexes in estuarine mixing model on iron uptake and growth of coastal marine diatom Chaetoceros sociale, Mar. Biol., 134, 761-769. 

Figure 6.10 The catalytic site of [NiFe] and [NiFeSe] hydrogenases in oxidised inactive (top) and reduced active (bottom) states. Note the three non-protein diatomic ligands to the iron.The site bridging the Ni and Fe is occupied by an oxygen or sulfur species in the most oxidised states and probably by a hydride or molecular hydrogen in the most reduced states.

The binuclear [2Fe]n cluster is believed to be the substrate binding and activation site. It has a most unusual structure with the two iron atoms (Fel and Fe2) bridged by two thiolates of a 1,3-propanedithiol group and a diatomic molecule, probably a CO (Peters et al. 1999 Nicolet et al. 2000). 

Each iron atom is terminally coordinated by one CO and one CN . The coordination of Fel is completed by the additional bridging cysteinyl sulfur. For Fe2, the sixth coordination site may be empty, as in the D. desulfuricans enzyme (Nicolet et al. 1999), or occupied by a solvent molecule, as observed in the C. pasteurianum enzyme (Peters et al. 1999). The assignment for the diatomic ligands is supported by infrared spectroscopic evidence (Pierik et al. 1998X and similar diatomic ligands have also been found for the corresponding binuclear [Ni-Fe] cluster of the nickel-iron hydrogenases (Volbeda et al. 1995). 

The southern flank of the upwelling CDW sinks around the continent of Antarctica to become AABW The northern flank of upwelling CDW is transported by surface currents, first into the polar frontal zone (PFZ) where AAIW forms, and then into the subantarctic zone (SAZ ) where SAMW forms. At these latitudes, dust deposition is high enough to reduce iron limitation of the diatoms. As the surfece waters move northward. 

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