Bioavailability of natural colloid‐bound iron to marine plankton: Influences of colloidal size and aging

Abstract

Iron (Fe) is mostly complexed with the organic ligands or colloids that are very abundant in natural seawater. In this study, natural colloids were isolated by ultrafiltration and radiolabeled with 59 Fe. The biological uptake of radiolabeled colloid-bound Fe of different sizes (1-10 kDa and 10 kDa-0.2 m) and ages (1 and 15 d) by diatoms (Thalassiosira pseudonana) and copepods (Acartia spinicauda) was then determined. The uptake of radiolabeled colloid-bound Fe was compared with the uptake of low molecular weight (LMW) complexed Fe (Ͻ1 kDa) or that of EDTA-Fe (at a ratio of 1 : 2 for Fe : EDTA). Our study demonstrates that the colloid-bound Fe of different sizes and ages was bioavailable to the diatoms. The uptake of colloidal bound Fe was, however, 6-31 times lower than the uptake of LMW Fe, suggesting that colloidal binding reduced Fe bioavailability to diatoms. Fe bound with small colloids (1-10 kDa) was taken up at a higher rate than Fe bound with large colloids (10 kDa-0.2 m) at typical colloidal organic carbon concentrations. The uptake of colloid-bound Fe was also much higher when the Fe had been bound with the colloids for 1 d rather than for 15 d. Differences in the colloidal organic carbon concentration did not appreciably affect the uptake of colloid-bound Fe by the diatoms. Similarly, copepods accumulated colloid-bound Fe at a much higher rate when the Fe was associated with small colloids rather than with large colloids. Direct ingestion of colloidal particles by the copepods appeared to be negligible. In both diatoms and copepods, Fe uptake may involve its dissociation from the colloids before being accumulated by the organisms. The study therefore demonstrates that colloid-bound Fe is less available for marine phytoplankton and that colloidal size and thus the colloidal speciation may be critical in controlling colloid-bound Fe uptake by aquatic organisms.

The Iron Biogeochemical Cycle Past and Present

“…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.…”

PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies

“…Both forms of iron are assumed to be equally susceptible to consumption by phytoplankton despite recent observations suggest that this may be not the case (Nishioka and Takeda, 2000;Chen and Wang, 2001;Chen et al, 2003). In other words, the total bioavailable concentration of iron is equal to the total dissolved iron concentration (Fe).…”

Iron biogeochemistry across marine systems – progress from the past decade

“…The major proportion of the dissolved Fe in open ocean seawater (here defined as 0.4 µm filtered) was found to be in the colloidal form (here defined as >0.02 µm-0.4 µm) (Wu et al, 2001), with continuing debate about the bioavailability of this fraction. Chen and Wang (2001) showed that freshly precipitated colloids were available to phytoplankton but aging processes (15 days) reduced markedly their availability. Wang and Dei (2003) demonstrated that Fe availability from colloidal matter to cyanobacteria (Synechococcus, Trichodesmium) is largely dependent on the size and origin of the material, with the tendency of Fe bound to smaller colloids and biogenic colloidal material derived from the same species being more available.…”