The role of natural organic ligands in transformations of iron chemistry in seawater and their effect on the bioavailability of iron to marine phytoplankton
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It is widely accepted that the complexation of iron (Fe) with organic compounds is the primary factor that regulates Fe reactivity and its bioavailability to phytoplankton in the open ocean. Despite considerable efforts to unravel the provenance of the many organic ligands present in the ‘ligand soup’ much of this pool remains largely unresolved and the ligands remain grouped into either strong (L₁) or weak (L₂) types. The Tasman Sea and Southern Ocean are areas of particular interest as both regions are subject to Fe limitation or co-limitation and are likely to be severely affected under climate change scenarios. The predictions of dryer conditions in central Australia suggest that the Tasman Sea may be subject to changes in the intensity and frequency of atmospheric dust deposition and, in consequence, enhanced Fe deposition into the surface waters. This thesis aims to improve our knowledge of a) how natural organic ligands affect Fe solubility, chemistry, and bioavailability, and b) which forms of Fe are available to phytoplankton. Natural seawater samples (surface and profiles to 1000m) revealed that electrochemically detected HS-like material, which are thought to make up a proportion of the weaker L₂ class of ligands, account for a very small fraction of the Fe-binding organic ligand pool. The distribution of HS-like material in coastal, shelf and offshore regions associated with the EAC does not exhibit a nearshore to offshore (high to low) concentration gradient, likely because of low riverine HS-like input. Higher concentrations of HS-like material were generally found at, or adjacent to, the chlorophyll maximum (Cmax). However, little correlation with chlorophyll-a (Chl-a) was observed and so these higher concentrations are more likely linked to degraded algal material and microbial activity rather than direct primary productivity. Perturbation experiments using water collected offshore in the EAC and a cold core cyclonic eddy (CCE) indicated that the in situ utilisation and production of HS-like material, and its character, differ depending on the phytoplankton and microbial communities present, and reflect the biological activities of these different communities, as well as photochemical transformations. The addition of a model HS (Suwannee River fulvic acid) enhanced Chl-a concentration in both communities, particularly in the EAC, likely due to the remineralisation of Fe and other nutrients via photochemical and bacterial transformation of this material. Seawater depth profiles from the northern and southern Tasman Sea indicate Fe limitation (or co-limitation) at the stations sampled. Dissolved Fe (dFe), organic ligand concentrations and conditional stability constants were consistent with previous studies (showing the presence of mostly L₂ ligands) with higher ligand concentrations and conditional stability constants close to the Cmax. Ligand concentration, as previously reported, is in excess of dFe throughout the water column, although no correlation between dFe and ligand concentration was observed. Fe-enrichment experiments using two contrasting phytoplankton communities investigated how the communities respond, in terms of biomass and community structure, to inorganic Fe delivered alone or bound to an organic ligand (siderophore, saccharides, bacterial exopolymeric substances (EPS)) or dust-borne Fe from two dust samples (D1 and D2) originating from the Australian continent. Overall, Fe bound to a strong Fe-binding siderophore was much less available to both phytoplankton communities; whereas, Fe bound to bacterial EPS (lowest conditional stability constant) induced the greatest increase in overall phytoplankton biomass. Dust D1 did not have the highest rate of dFe uptake, or result in the greatest increase Chl-a, but did induce the greatest shift in community structure. Whilst one ligand (L₂) was measured in most incubations, both L₁ and L₂ ligands were detected in the D1 and inorganic Fe incubations, indicating in situ biological production of Fe-binding ligands (i.e. siderophores or EPS) in response to Fe addition and an added ligand component from the dust. The greater response of the phytoplankton to the EPS and D1 led to further laboratory experiments. Analysis of 4 EPS isolates (1 bacterial, 1 mixed natural community, and 2 microalgal laboratory cultures) showed that both bacterial and algal EPS contain functional components known to bind Fe (uronic acid, saccharides). The bacterial EPS was made up of mainly high molecular mass components, whereas the algal EPS were of low molecular mass. Most EPS contained components that were measured as both L₁ and L₂ ligands, with the L₁ ligands having an affinity for Fe close to that of bacterial siderophores. EPS greatly enhanced Fe solubility in seawater, however, it may also accelerate Fe(II) oxidation, and thus, Fe(II) removal from the system. Other trace elements and macronutrients were associated with the EPS that may be accessible to phytoplankton and could help to relieve nutrient limitation. Bioaccumulation experiments indicated that Fe bound to all EPS used was highly bioavailable to the Southern Ocean diatom C. simplex (50 to > 100%) relative to the bioavailability of inorganic Fe (assumed 100% bioavailable). This enhanced bioavailability was likely due to increased Fe solubility, and possible formation of more bioavailable forms of Fe. Further experiments using dust D1, and rainwater collected in the Tasman Sea, revealed that despite low fractional solubilities (< 1%), the dust represents, potentially, an important source of Fe and other vital macronutrients and trace elements. Both the rainwater and dust were associated with ligands in the L2 class that helped to maintain the solubility of Fe. Light exposure, particularly UV, can a) have a substantial effect on the Fe chemistry of the Fe-laden dust, lowering the conditional stability constant and altering the size distribution of both Fe and ligands (including saccharides and HS-like material), and b) improve the bioavailability of dust-borne Fe to C. simplex. The perturbation experiments in the EAC, CCE and north and south Tasman Sea demonstrated that organic ligands play an important role in regulating the nutrient dynamics of marine systems. They show that the bioavailability of Fe to phytoplankton is dependent on the various Fe species and Fe sources (i.e. inorganic Fe, organically bound, dust-borne), and that this differs between phytoplankton size fractions and from one bacterio- or phytoplankton species to another. The Tasman Sea and Southern Ocean receive, possibly increasing, periodic inputs of atmospheric dust from the source region of D1, which initiated a substantial community shift in perturbation experiments. However, the impact that dust-borne Fe will have on a natural phytoplankton community will be dependent on the duration and intensity of the dust deposition event, and the nutritive state and community structure of the resident phytoplankton. Bacterial siderophores have previously been suggested as key players in Fe biogeochemistry, however, in remote regions bacterial and algal EPS could play a significant role in the biogeochemical cycling of Fe and other nutrients, and their contribution should also be considered to further our understanding of the dynamics of Fe-limited oceans.
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