HNLC

Over 20% of the world s open ocean surface waters are replete in light and major nutrients (nitrate, phosphate, and silicate), yet chlorophyll and productivity values remain low. These so-called "high-nitrate low-chlorophyll" or HNLC regimes (Chisholm and Morel, 1991) include the sub-arctic North Pacific (Martin and Fitzwater, 1988 Martin et al, 1989 Miller et al, 1991), the equatorial Pacific (Murray et al, 1994 Fitzwater et al, 1996) and the southern Ocean (Martin et al.,... 

Fluxes of continental dust preserved in ice cores of Greenland and Antarctica suggest a 30-fold increase in dust flux during the last Glacial Maximum. Dramatic increases in new biological production in the HNLC regions may have resulted, resulting in the draw-down of atmospheric CO2 (Martin, 1990). 

The 4F complex is particularly important in controUing the rate of protein translation. As described above, 4F is a complex consisting of 4E, which binds to the m G cap strucmre at the 5 end of the mRNA, and 4G, which serves as a scaffolding protein. In addition to binding 4E, 4G binds to elF-3, which hnlcs the complex to the 40S ribosomal subunit. It also binds 4A and 4B, the ATPase-hehcase complex that helps unwind the RNA (Figure 38—7). 

The few studies that exist on Fe control of bacterial degradation in HNLC waters agreed that bacterio-plankton is first C-limited as a consequence of low primary production due to Fe limitation and not directly limited by Fe (Hutchins et al. 1998 Church et al. 2000 Kirchman et al. 2000). As reported above, the amount of TOC, potential OC available for bacteria, was similar in our HFe and LFe experiments and therefore bacteria could be colimited by two major factors the availability of Fe and the quality of the organic matter. These factors cannot be discriminated in our experiments. HFe bioassays contained, in addition to increased Fe, a better quality of OM than the LFe bioassays. Moreover, as the distribution in dissolved and particulate organic carbon was affected by Fe availability in the phytoplanktonic cultures, the response of bacteria to different size pool of organic matter (DOM-TOM) was also investigated. 

Fe concentrations measured at the start of the bioassays, and based on the estimated half-saturation constant fQ of 0.09 nM Fe (by employing MichaeHs—Menten type kinetics), the availability of Fe during that time (ambient dissolved Fe concentration of 0.03—0.04 nM) would appear to limit NO uptake by phytoplankton. These studies aU demonstrate a clear enhancement of specific and absolute NO3 uptake rates by Fe during short-term incubations, and are supported by similar results conducted in other HNLC regions including the equatorial Pacific Ocean (Coale et al., 1996 Cochlan, 2001 Price et al., 1991, 1994) and the eastern (Boyd et al., 1996, 1998 Marchetti et al., 2006) and western (Kudo et al., 2005, in review) subarctic Pacific Ocean. 

Franck, V. M., Bruland, K. W., Hutchins, D. A., Brzezinski, M. A. (2003). Iron and zinc effects on silicic acid and nitrate uptake kinetics in three high-nutrient, low-chlorophyU (HNLC) regions. Mar. Ecol. Prog. Ser. 252, 15-33. 

Of particular interest in relation to Si N interactions and diatoms is the relatively recent finding that iron provides a strong regulating influence on the species composition of phytoplankton assemblages, as well as on the elemental ratios of diatoms. Fe and Si supply appear to be particularly important in several large areas where high concentrations of other macro nutrients (N, P) are present in surface waters. These HNLC areas (Minas et al, 1986) are characterized by low phytoplankton biomass... 

---