Iron fluxes and algae blooms

Southwest Florida HABs or “red tides” are due to blooms of the dinoflagellate, Karenia brevis. Brevetoxins produced by the organism poison finfish, shellfish, mammals, and birds, and can have negative human health impacts(2004) and economic effects from medical care, decreased tourism, and losses to fisheries.(2014). The initiation of blooms is complex and approximately two dozen pathways have been proposed for the oligotrophic WFS (Vargo, 2009).  In addition to examining macronutrients for the chemical ecology of blooms (Dixon et al., 2014a; Dixon et al., 2014b; Heil et al., 2014),some older research demonstrated that Fe could potentially be a catalyst for bloom formation (Kim and Martin, 1974). A few recent studies have considered the role of micronutrients such as Fe or other trace metals (Lenes et al., 2001; Lenes et al., 2012). Deposition of Fe-rich from Africa has been demonstrated to contribute to Trichodesmium blooms in the WFS (Lenes et al., 2001). Trichodesmium requires Fe for nitrogenase enzymes to fix atmospheric N₂ to NH₄⁺. This source of fixed nitrogen then alleviates observed N-limitation for K. brevis (Walsh et al., 2006).  As K. brevis blooms still occur during low-dust years, however, unexpected sources of Fe are thought to play a role in bloom formation.

Although abundant,  Fe is present primarily as Fe(III) in the presence of O₂, is highly insoluble near circumneutral pH (e.g. marine environments), and any Fe(II) is rapidly oxidized to Fe(III). Organisms then experience a physiological challenge of using a solid phase mineral for assimilatory or dissimilatory purposes(Boukhalfa and Crumbliss, 2000; DiChristina, 2005). To remain in the photic zone, Fe must be dissolved and chelated by autochthonous or allochthonous ligands (Rue and Bruland, 1995; Witter and Luther, 1998; Wu and Luther, 1995). Rivers may deliver Fe in both particulate and dissolved form, but it is often assumed that most dissolved Fe is rapidly removed to estuarine sediments by flocculation upon mixing with seawater(Boyle et al., 1977; Sholkovitz et al., 1978). Particulate Fe can be resolubilized during the dissimilatory reduction of Fe(III) to Fe(II), coupled to organic-C oxidation. Dissolved Fe(II) generated in the sediments and transported vertically was usually discounted as a bioavailable source, however, as it is expected to oxidize and reprecipitate near the sediment surface(Elrod et al., 2004; Homoky et al., 2012; Lohan and Bruland, 2008; Severmann et al., 2010).

I have demonstrated that in the sediments of organic- and iron-rich “blackwater” (high CDOM) rivers, soluble organically-complexed Fe(III) is formed from the remobilization of flocculated dissolved Fe(III) and contributes significantly to the total portion of dissolved Fe at the mouth of the estuary(Beckler et al., 2015a; Jones et al., 2011). The soluble organic-iron(III) is bioavailable and can exported to coastal waters. The potential effect upon annualized global estimates of Fe loading to coastal margins is substantial. Little is known about the dependence of organic-Fe(III) flux on high frequency estuarine physical and chemical variations or the potential for this flux to contribute to phytoplankton blooms on the WFS. However, at least a dozen rivers draining into the WFS are considered blackwater streams, and effects of pulsed Fe inputs on primary production may be magnified because of the oligotrophic coastal waters.  Elucidating the physical and chemical factors which regulate the flux of soluble organic-Fe(III) across the sediment-water interface will require high frequency observations and subsequent statistical analyses to capture flux, river discharge and salinities, temperature, CDOM concentrations, sediment deposition, and pore water flushing or aeration.

We are currently analyzing sediments in selected rivers to locate regions with high concentrations of solid phase reactive Fe (ascorbate extractable), the precursor to soluble organic-Fe(III) (Fig. 1a)(Beckler et al., 2015a; Beckler et al., 2015b).  Future work will determine flux rates in cores using bench-top electrochemical profiling with Au/Hg voltammetric microelectrodes (e.g. Fig 1c). This technique allows for rapid measurement of concentration gradients of respiratory analytes (e.g O_2(aq), soluble organic-Fe(III), dissolved Fe(II) and Mn(II), ΣH₂S), and subsequently, the modeled vertical flux of these analytes. We then plan to observe an extended time series of organic-Fe(III) flux across the sediment/water interface with an in situ electrochemical analyzer and a novel stationary array of individually-addressable microelectrodes (IAME; Fig 1b).  Hourly depth profiles, concentration gradients and modeled flux rates of soluble organic-Fe(III) will be compared with discrete flux measurements obtained from a benthic lander platform by collaborators at Georgia Tech using more conventional in situ techniques (in situ sediment microprofiling, pore water gradients, and concentration changes of benthic chambers), to characterize other co-factors related to Fe flux. Statistical analyses and a diagenetic reaction-transport model will unravel the controls of Fe flux.  We plan to apply the results locally to estimate flux over larger spatial scales using  colocated continuous chemical and physical data provided by our collaborators. Finally, time series analyses will be used to investigate fine scale variations in algal biomass (chlorophyll) with respect to measured Fe flux at both near and far-field locations.

Figure 1 – a) Idealized pore water profiles demonstrate how soluble organic-Fe(III) depends on reactive Fe concentrations; b) A custom array with 8 individually-addressable Au/Hg microelectrodes will be positioned  in the sediments so that the electrodes capture concentration gradients similar to (c), an example  microelectrode profile of Satilla River sediments demonstrating a flux of soluble organic-Fe(III).