かび臭物質産生ラン藻類Anabaena macrospora, Phormidium tenue, Oscillatoria tenuis 及びOscillatoria brevisの鉄吸収

近年、琵琶湖の南湖をはじめ、霞々浦や相模湖など多くの湖沼や水道水源地において、人間活動に付随すると富栄養化問題となり、ラン藻類、特にかび臭物質を産生するラン藻の発生が憂慮されている。琵琶湖を水源とする大津、京都、大阪、神戸など関西の各都市の水道では、同湖で毎年発生するラン藻、すなわちAnabaena macrospora｛ジオスミン（geosmin）を産生｝やPhormidium tenue, Oscillatoria tenuis｛2-メチルイソボルネオール（2-methylisoborneol, MIB）を産生｝などが作り出すこれらのかび臭物質（ジオスミンまたはMIB）による水道水の異臭味が大きな問題となっている。こうした水圏環境における異臭味は世界各地でも問題となり、近年この問題に関する国際シンポジウム（International Symposium on Off-Flavors in the Aquatic Environment）が開かれるようになり、1982年6月には第1回（ヘルシンキ）、1987年10月には第2回（鹿児島）、そして1991年3月には第3回（ロサンゼルス）のシンポジウムがそれぞれ開催された。しかしこうしたラン藻が異常増殖する要因はまだ明らかではない。そのためかび臭物質を産生するラン藻類の発生を予測あるいは抑制する情報に関する研究は強い要請があるにもかかわらずあまり進展をみていない。

A recent problem for water utilities is musty-odor, which is caused by geosmin or 2-methylisoborneol produced by cyanobacteria. In order to clarify the mechanism of growth of musty-odor producing cyanobacteria, the effect of iron on the growth of Anabaena macrospora, Phormidium tenue, Oscillatoria tenuis and Oscillatoria brevis was studied. The growth of A. macrospora, P. tenue and O. tenuis was suppressed at low levels of iron (colloidal iron, 1 μM as Fe) in the EDTA-free CT medium. However, at high levels of iron (18 μM as Fe) using colloidal iron kept in a refrigerator, A. macrospora and P. tenue were able to grow although O. tenuis could not. When autoclaved colloidal iron was used as the iron source, none of the cyanobacteria except O. brevis were able to grow. The effect of spongy or powdery iron on the growth of cyanobacteria was also investigated. A. macrospora, P. tenue and O. brevis were capable of growing in media containing spongy or powdery ion, which release Fe(Ⅱ) continuously, but the growth of O. tenuis was suppressed. The culture experiments and photoreduction of Fe(Ⅲ) to Fe(Ⅱ) show that A. macrospora, P. tenue and O. brevis can utilize iron even in the absence of EDTA if there are large amounts of iron present in a form photoreducible to Fe(Ⅱ). However, O. tenuis was found to require chelated and photoreducible iron for its growth. The present results and previous studies {Nogaku Kenkyu, 62:253-270(1991)} indicate that O. brevis has the ability to utilize a wide variety of iron forms such as Fe(Ⅲ)-EDTA, Fe(Ⅱ)-BPDS, Fe(Ⅲ)-Desferrioxamine B(microbial siderophore) chelates, colloidal iron, iron oxides (Fe2O3, Fe3O4), spongy iron and powdery iron. On the other hand, O. tenuis seems to have very specific requirements for the from of iron it can utilize for growth.