Laboratory culture experiments to study the effect of lignite humic acid fractions on iron solubility and iron uptake rates in phytoplankton

Abstract

Please Cite the article as: H. Hasegawa, Y. Tate, M. Ogino, T. Maki, Z.A. Begum, T. Ichijo and I.M.M. Rahman, Laboratory culture experiments to study the effect of lignite humic acid fractions on iron solubility and iron uptake rates in phytoplankton AbstractThe major fractions of dissolved iron in seawater exist as a complex with organic ligands. A high bioavailability of iron bound to humic acid (HA) compared to the other model ligands, such as desferrioxamine B or ferrichrome, has been reported, which implies the importance of HA to control the geochemical behavior and the transfer of Fe to marine phytoplankton, particularly in estuarine and coastal waters. In the current work, the effect of different HAfractions (>100, 100-30, 30-10, 10-5, and 5-3 kDa), which are extracted from lignite, on the comparative solubility of iron in seawater and the corresponding influence on the iron-uptake and growth rate of the phytoplankton (Prymnesium parvum) has been studied using laboratory cultures. The lower molecular weight (MW) HA fractions, such as 30-10, 10-5 and 5-3 kDa, remained soluble in the simulated seawater medium for a longer time-span compared to the higher MW fractions. The lower MW fractions facilitated higher iron solubility and assisted in achieving a better phytoplankton growth rate. However, a reciprocal impact on phytoplankton growth rates has been observed when the HA concentration increased to a higher range (0.18 to 18 mg-C L -1 ). The highest intracellular Fe uptake in phytoplankton occurred with 30-10 kDa HA in seawater, and the extracellular dissolved Fe concentrations were higher for smaller-sized HA fractions. In a nutshell, our study showed that the controlled addition of lower MW fractions of HA (up to 30-10 kDa) in estuarine waters could ensure the accelerated uptake of Fe in phytoplankton as well as a better growth rate.

Exploring the Potential Role of Terrestrially Derived Humic Substances in the Marine Biogeochemistry of Iron

“…Humic colloids may well facilitate microbial respiration by providing hot spots for (bacteria to congregate as well as a localized source of nutrients. Recent work by Hasegawa et al (2017) provides some support for this hypothesis. They found that the highest intracellular iron uptake by Prymnesium parvum (haptophyte) occurred with the 10−30 kDa MW fraction of humics in seawater medium rather than with the LMW fraction.…”

Binding of proton and iron to lignite humic acid size-fractions in aqueous matrix

“…The comparative logK Fe for the ML species with different HA lignite size-fractions indicated the lowest value for larger MW fractions of HA lignite (>100 kDa). It could be correlated with the increased aggregation rate of corresponding HA lignite size-fraction in the aqueous matrix [35,56], either attributable to intermolecular H-bond formation via the protonation of HA lignite functional groups [57] or to the charge neutralization and cation bridge formation [58].…”

“…The HAs are assumed to be an assorted-assemblage of several smaller molecular fragments that are bound by weak dispersive forces, such as, van dar waals, π-π or CH-π interactions and so forth, to form supramolecular aggregates [33]. The disparity in conformational distribution among the HA lignite size-fractions via molecular characterization [34], or the corresponding impact of HA lignite -fractionation on iron solubility [35] has been reported.…”

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

“…Modifying membrane permeability for various ionic species due to various HS effects, including complexation, generates elevated intracellular levels of redox-active ionic species, such as Fe 2+ , and ionic species acting as secondary messengers, e.g., Ca 2+ [ 67 , 68 ]. The ability of HS to complex ionic species, especially iron ionic species, is important for the HS effect on microalgae [ 69 ].…”

“…The permeability change in the membrane of R. subcapitata depends on pH. The passive diffusion of fluorescent tracers through the membranes of R. subcapitata (synonym used Selenastrum capricornutum ) is higher at pH 5 than at pH 7 [ 67 ].…”