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.

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