PRJDB2201 Alteration of the Structure of Humic Acids by Bacteria from the Deep Subsurface Miocene and Pliocene Formations in Northernmost Japan. The goal of this study is to elucidate the mechanism how organic matter such as humic acids (HAs) would be utilized by bacteria in the subsurface environment. The purpose of this study is to obtain bacteria capable of utilizing HAs in the subsurface environment in Horonobe and elucidate the mechanisms of HA utilization. First of all, we examined the bacterial community structure in groundwater which was used in enrichment culture to obtain HA-utilzing bacteria to know the initial state of the groundwater sample. Biotransformation of subsurface organic matter to smaller molecules.