Marine diatom uptake of iron bound with natural colloids of different origins

Abstract

Natural colloids are abundant in seawater and are an intermediary in the fate, transport and bioavailability of many trace elements. Knowledge of the pathways and mechanisms of the biological uptake of colloidal Fe and other Fe species is of paramount importance in understanding Fe limitation on marine phytoplankton and thus carbon sequestration in the ocean. Whether the natural colloids serve as a source for the biological Fe requirements of marine phytoplankton, or just as a sink for particle-reactive metals in the oceans remains largely unknown. This study examined the bioavailability of Fe bound with colloids from different regions to a coastal diatom (Thalassiosira pseudonana). Natural colloids were isolated by cross-flow ultrafiltration and radiolabeled with ⁵⁹Fe before being exposed to phytoplankton. Control experiments were conducted to ensure that ⁵⁹Fe radiolabeled onto the colloids remained mostly in the colloidal phase. Both the natural oceanic and coastal colloidal organic matter complexed Fe (1 nm–0.2 μm) can be biologically available to the marine diatom even though its uptake was lower than the low molecular weight counterparts. By comparing the measured Fe internalization fluxes and the calculated maximum diffusive uptake fluxes, it is evident that ligand exchange kinetics on the cell surface may control the internalization of macromolecular Fe. The calculated concentration factors under dark and light conditions were generally comparable. Colloidal Fe, as an important intermediary phase, can be actively involved in the planktonic food web transfer through biological uptake and regeneration processes. The bioavailable fraction of Fe may be substantially underestimated by only considering the truly dissolved Fe or overestimated when using the external fluxes, such as aerosol Fe, as the bioavailable fraction.

Introduction

The importance of iron in marine ecology and oceanography has received substantial attention because Fe limits primary production in several parts of the ocean Martin and Fitzwater, 1988, Martin et al., 1994, Coale et al., 1996, Behrenfeld and Kolber, 1999, Boyd et al., 2000. Iron is essential for nitrogen utilization and metabolism, chlorophyll biosynthesis, and numerous cellular respiratory functions in phytoplankton and, therefore, plays a critical role in the ocean ‘biological pump’. The biological acquisition of various forms of Fe is important for assessing the actual degree to which Fe limits primary production in the ocean. Previously, the bioavailability of Fe was thought to be a function of the concentration of free or inorganic Fe species Anderson and Morel, 1982, Harrison and Morel, 1986, Campbell, 1995, Sunda and Huntsman, 1997. However, cathodic stripping voltammetric studies have indicated the presence of Fe complexing ligands in natural seawater Gledhill and van den Berg, 1994, Rue and Bruland, 1995, Rue and Bruland, 1997, Wu and Luther, 1995, van den Berg, 1995, Witter and Luther, 1998. Binding with organic ligands increases Fe solubility in seawater and may play an important role in Fe bioavailability and its geochemical cycling Sunda et al., 1991, Kuma et al., 2000, Wu et al., 2001. The organic ligands may have included siderophores produced by microorganisms in response to Fe stress Trick et al., 1983, Granger and Price, 1999, or the release of other intracellular materials (e.g., porphyrin complexes, Hutchins et al., 1999a).

Iron may also be complexed by macromolecular organic ligands, such as natural organic colloids, operationally defined as size range between 1 nm and 0.2 μm (Buffle and Leppard, 1995). Marine colloids are very abundant in the surface oceans Wells and Goldberg, 1991, Benner et al., 1992, Guo et al., 1995, and are mostly organic in nature. Recent measurement showed that >90% of Fe in the traditionally defined dissolved phase (<0.2 μm) was in fact in the colloidal phase (>1 kDa), particularly in the larger colloidal size spectrum (8 kDa–0.2 μm) Wen et al., 1999, Wells et al., 2000, Wu et al., 2001. Fe in the colloidal fraction has been shown to be the most dynamic size fraction during the growth of marine diatoms (Nishioka and Takeda, 2000).

There has been a longstanding interest in the biological availability of colloidal Fe by marine phytoplankton. Although several studies have shown that Fe bound with synthetic inorganic colloids was related to the thermodynamic stability and kinetic liability of colloids as sources for uptake by marine phytoplankton Wells et al., 1983, Rich and Morel, 1990, Kuma and Matsunaga, 1995, the applicability of these studies to natural organic colloids has yet to be explored. Recent evidence revealed that artificially synthetic organic Fe (such as porphyrin, ferrioxamines B and E) are biologically assessable for marine phytoplankton Soria-Dengg and Horstmann, 1995, Maldonado and Price, 1999, Kuma et al., 1999, Kuma et al., 2000. Several studies demonstrated that Fe bound with fungal siderophore desferriferrioxime B (DFB) can indeed be utilized by marine diatoms and natural phytoplankton communities in the subarctic Pacific Soria-Dengg and Horstmann, 1995, Maldonado and Price, 1999, whereas other studies have indicated that the addition of DFB reduced the biological uptake of Fe and the growth rate of phytoplankton Hutchins et al., 1999a, Hutchins et al., 1999b, Wells, 1999. Hutchins et al. (1999a) further suggested the specificity of Fe uptake, e.g., Fe bound with siderophore was more bioavailable to prokaryotic picoplankton, whereas porphyrin-complexed Fe was more bioavailable to eukaryotic phytoplankton. Both photoreduction and biological reduction (such as plasma membrane ferrireductase) have been proposed to convert the organic-complexed Fe to inorganic bioavailable Fe species Jones et al., 1987, Rich and Morel, 1990, Johnson et al., 1994, Weger, 1999, Gerringa et al., 2000, Maldonado and Price, 2000.

Despite the intensive interest in the biological availability of colloidal Fe to aquatic organisms (particularly marine phytoplankton), the biological uptake of Fe bound with colloids isolated from natural waters remains largely undetermined (Chen and Wang, 2001). It remains less certain whether the colloidal Fe (between 1 kDa and 0.2 μm) is available for phytoplankton. In this study, we examined the bioavailability of Fe bound with natural colloids isolated by cross-flow ultrafiltration from different regions, including estuarine, coastal and oceanic waters. Previously, we had compared the bioavailability of Fe bound with different sized and aged colloids to the marine diatom (Chen and Wang, 2001). Here, we quantified the intracellular biological uptake by phytoplankton using the established Ti-citrate–EDTA washing technique (Hudson and Morel, 1989). Control laboratory experiments were first carried out to monitor the physico-chemical behavior of radiolabeled colloids in our experimental system. Partitioning of Fe in different phases was monitored after the exposure experiment. The cellular distribution of Fe following exposure to natural colloids was also quantified.