Increase in sedimentary organic carbon with a change from hypoxic to oxic conditions.

In the Seto Inland Sea, Japan, chemical oxygen demand has increased over recent decades, while average dissolved oxygen concentrations in the bottom water have increased. In this study, we investigated responses of organic carbon (OC) in hypoxic sediment to changes of redox conditions using experimental columns containing sediment and overlying water. Surface sediment showed an increase in OC along with the change to an aerobic condition. Microbial community analysis showed a predominance of sulfur-oxidizing bacteria (SOB) such as Sulfurovum sp. in the sediment. This dominance could account for the increased OC. Additionally, the dissolved organic carbon (DOC) concentration in the overlying water increased. Further experiments using sandy sediment showed that biodegradation of Sulfurimonas denitrificans was associated with DOC release. These results show that a change in the sedimentary environment (increase in dissolved oxygen) increased the sedimentary OC and DOC of overlying water by stimulating certain autotrophic bacteria, especially the SOB.

Site-Directed Mutagenesis of Multicopper Oxidase from Hyperthermophilic Archaea for High-Voltage Biofuel Cells.

Enzymes from hyperthermophilic archaea are potential candidates for industrial use because of their superior pH, thermal, and long-term stability, and are expected to improve the long-term stability of biofuel cells (BFCs). However, the reported multicopper oxidase (MCO) from hyperthermophilic archaea has lower redox potential than MCOs from other organisms, which leads to a decrease in the cell voltage of BFCs. In this study, we attempted to positively shift the redox potential of the MCO from hyperthermophilic archaeon Pyrobaculum aerophilum (McoP). Mutations (M470L and M470F) were introduced into the axial ligand of the T1 copper atom of McoP, and the enzymatic chemistry and redox potentials were compared with that of the parent (M470). The redox potentials of M470L and M470F shifted positively by about 0.07 V compared with that of M470. In addition, the catalytic activity of the mutants towards 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) increased 1.2-1.3-fold. The thermal stability of the mutants and the electrocatalytic performance for O2 reduction of M470F was slightly reduced compared with that of M470. This research provides useful enzymes for application as biocathode catalysts for high-voltage BFCs.