Fundamental studies on acid-tolerant chironomids in Japan

Abstract

The distribution, evolutionary relationships, and acid-resistance level (including mechanisms behind it) were studied in acid-tolerant chironomid species in Japan. All six species studied showed a tendency of restricted distribution. Chironomus acerbiphilus showed the highest acid tolerance with 50% survival at pH 0.75, whilst C. cf. riparius and C. solicitus showed the 2nd highest tolerance. Tolerance mechanisms, including enzymatic protein digestion into smaller molecules, was strongly observed, even at pH 2.0 in C. sulfurosus. Additionally, C. cf. riparius displayed the highest H + -ATPase activity; however, the same activity level was observed in both acid-tolerant and non-tolerant species. Phylogenetic analysis did not result in any exclusive acid-tolerant clades, although C. cf. riparius and C. solicitus formed an exclusive small clade within the genetic tree based on their cytochrome oxidase I (CO I) region. Other acid-tolerant species were grouped within clades with non-tolerant species. C. cf. riparius and C. solicitus, distributed only in northern Japan, were estimated to have diverged from ancestral species earlier, while C. fusciceps and C. sulfurosus, distributed only in southern Japan, diverged later. These results point to the possibility of Japanese acid-tolerant chironomids originating in northern area and later expanding their range to southern areas.