Effect of Acid Species on iCryptococcus/isp. T1 and Construction of Its Bioreactor System for Continuous Neutralization of Acidic Media

摘要

Generally, the tolerance of yeasts to acid is stronger than that of bacteria. In addition, some of the yeasts are able to neutralize acidic media, which is called acid-neutralizing yeast. An acid-neutralizing yeast, Cryptococcus sp. strain T1, was previously isolated from Lake Tazawa in Japan. Here we investigated the effect of several types of acid on the neutralizing ability of strain T1, and we improved an existing bioreactor model to effectively neutralize acidic water. First we carried out a neutralizing test with strain T1 using casamino acid solution adjusted to pH 3.0 - 5.0 with each inorganic or organic acid. The solutions adjusted to pH 3.0 with nitric acid, phosphoric acid, sulfuric acid, and citric acid were neutralized by strain T1. The pH 3.0 solutions with lactic acid, formic acid, and acetic acid were not neutralized. These results demonstrated that some types of organic acid avoid neutralization by strain T1, and we hypothesized that this might be due to the sterilizing function of non-dissociative organic acid. We then constructed a bioreactor system with a column filled with T1-immobilized alginate beads. The previous research identified a problem in this bioreactor system: the high level of ammonium ions (NH+4) in the neutralized water might pollute water environments, and aluminum ions (Al3+) included in acidic water prevent the neutralization. Here, we used zeolite to get rid of the NH+4 and Al3+, and the addition of a zeolite reactor enabled the bioreactor system to neutralize the acidic water with a decrease of NH+4 in the water.