Freezing-enhanced oxidation of iodide by hydrogen peroxide in the presence of antifreeze proteins from the Arctic yeast Leucosporidium sp.AY30

Abstract

Ice-binding proteins (IBPs), originating from Arctic or Antarctic microorganisms, have freeze-inhibiting characteristics, allowing these organisms to survive in polar regions. Despite their significance in polar environments, the mechanism through which IBPs affect the chemical reactions in ice by controlling ice crystal formation has not yet been reported. In this study, a new mechanism for iodide (I？) activation into triiodide (I3？), which is the abundant iodine species in seawater, by using hydrogen peroxide (H2O2) in a frozen solution with IBPs was developed. A significant enhancement of I？ activation into I3？ was observed in the presence of Arctic-yeast-originating extracellular ice-binding glycoprotein (LeIBP) isolated from Leucosporidium sp. AY30, and a further increase in the I3？ concentration was observed with the introduction of H2O2 to the frozen solution (25 times higher than in the aqueous solution after 24 h of reaction). The reaction in the ice increased with an increase in LeIBP concentration. The in-situ pH measurement in ice using cresol red (CR) revealed protons accumulated in the ice grain boundaries by LeIBP. However, the presence of LeIBP did not influence the acidity of the ice. The enhanced freeze concentration effect of H2O2 by LeIBP indicated that larger ice granules were formed in the presence of LeIBP. The results suggest that LeIBP affects the formation and morphology of ice granules, which reduces the total volume of ice boundaries throughout the ice. This leads to an increased local concentration of I？ and H2O2 within the ice grain boundaries. IBP-assisted production of gaseous iodine in a frozen environment provides a previously unrecognized formation mechanism of active iodine species in the polar regions.