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[1] Though it is clear that plankton in oceanic regions access iron bound to strong organic ligands, the mechanism mediating the release of iron from these complexes remains unresolved. In this study, we aim to elucidate the mechanisms of organic iron acquisition by plankton in subantarctic waters. In particular, we investigated the importance of photochemistry in mediating the reductive dissociation of iron from organic complexes, using naturally occurring ligands, and model iron complexes, with different iron-binding groups and photoreactivities. Our results demonstrate that iron within the model ligands is available for uptake and growth by indigenous plankton, but that photolability of these complexes does not determine iron bioavailability. In contrast, light significantly enhances iron acquisition from the in situ ligands, suggesting that the in situ iron ligands are photolabile, and that photochemistry in surface waters may play a significant role in iron uptake from the dissolved organic iron pool by oceanic plankton.

[1] An improved knowledge of iron biogeochemistry is needed to better understand key controls on the functioning of high-nitrate low-chlorophyll (HNLC) oceanic regions. Iron budgets for HNLC waters have been constructed using data from disparate sources ranging from laboratory algal cultures to ocean physics. In summer 2003 we conducted FeCycle, a 10-day mesoscale tracer release in HNLC waters SE of New Zealand, and measured concurrently all sources (with the exception of aerosol deposition) to, sinks of iron from, and rates of iron recycling within, the surface mixed layer. A pelagic iron budget (timescale of days) indicated that oceanic supply terms (lateral advection and vertical diffusion) were relatively small compared to the main sink (downward particulate export). Remote sensing and terrestrial monitoring reveal 13 dust or wildfire events in Australia, prior to and during FeCycle, one of which may have deposited iron at the study location. However, iron deposition rates cannot be derived from such observations, illustrating the difficulties in closing iron budgets without quantification of episodic atmospheric supply. Despite the threefold uncertainties reported for rates of aerosol deposition (Duce et al., 1991), published atmospheric iron supply for the New Zealand region is $50-fold (i.e., 7-to 150-fold) greater than the oceanic iron supply measured in our budget, and thus was comparable (i.e., a third to threefold) to our estimates of downward export of particulate iron. During FeCycle, the fluxes due to short term (hours) biological iron uptake and regeneration were indicative of rapid recycling and were tenfold greater than for new iron (i.e. estimated atmospheric and measured oceanic supply), giving an ''fe'' ratio (uptake of new iron/uptake of new + regenerated iron) of 0.17 (i.e., a range of 0.06 to 0.51 due to uncertainties on aerosol iron supply), and an ''Fe'' ratio (biogenic Fe export/uptake of new + regenerated iron) of 0.09 (i.e., 0.03 to 0.24).Citation: Boyd, P. W., et al. (2005), FeCycle: Attempting an iron biogeochemical budget from a mesoscale SF 6 tracer experiment in unperturbed low iron waters, Global Biogeochem. Cycles, 19, GB4S20,

The role of heterotrophic bacteria in iron recycling, the influence of complexation on iron remineralization, and iron mobilization rates from lithogenic vs. biogenic particulate iron (PFe) were examined using field experiments and modeling simulations. During summer, we measured the mobilization rate of algal iron by heterotrophic bacteria in the mixed layer at a polar and a subpolar site south of Australia, and conducted shipboard incubations to track the release of dissolved iron (DFe) and iron-binding ligands from subsurface settling particles sampled from 120-m depth. Bacteria mobilized . 25% PFe d 21 in surface waters relative to mobilization at depth (, 2% d 21 ). Our incubations provide the first evidence of the concurrent release of weak iron-binding ligands and DFe from sinking particles. Simulated profiles of PFe remineralization, based on proxies, point to greater dissolution from biogenic PFe than from lithogenic PFe. Together our findings point to different biogeochemical functions for lithogenic vs. biogenic PFe: biogenic PFe is probably the main source of both DFe and ligands, whereas lithogenic PFe may contribute most to DFe scavenging and ballasting of biogenic PFe. The relative proportions of lithogenic vs. biogenic PFe flux vary regionally and set the contribution of scavenging and ballasting vs. dissolution and ligand release, and hence the fate of iron in the water column.

Diatom blooms play a central role in supporting foodwebs and sequestering biogenic carbon to depth. Oceanic conditions set bloom initiation, whereas both environmental and ecological factors determine bloom magnitude and longevity. Our study reveals another fundamental determinant of bloom dynamics. A diatom spring bloom in offshore New Zealand waters was likely terminated by iron limitation, even though diatoms consumed <1/3 of the mixed‐layer dissolved iron inventory. Thus, bloom duration and magnitude were primarily set by competition for dissolved iron between microbes and small phytoplankton versus diatoms. Significantly, such a microbial mode of control probably relies both upon out‐competing diatoms for iron (i.e.,K‐strategy), and having high iron requirements (i.e.,r‐strategy). Such resource competition for iron has implications for carbon biogeochemistry, as, blooming diatoms fixed three‐fold more carbon per unit iron than resident non‐blooming microbes. Microbial sequestration of iron has major ramifications for determining the biogeochemical imprint of oceanic diatom blooms.