Abstract. Bacteria with low nucleic acid content (LNA) and high nucleic acid content (HNA) are widely distributed in aquatic environments. Most of the current understanding of these two subgroups is derived from studies in marine environments. In comparison, information on the spatial distribution of these two subgroups in freshwater environments is very limited. The present study analysed the biogeographical pattern of those two groups on a large-river scale (i.e. the Songhua River catchment, .1000 km). The results showed that the concentrations of LNA and HNA bacteria were distributed over a wide range from 5.45 Â 10 4 to 4.43 Â 10 6 cells mL À1 , and from 1.35 Â 10 5 to 4.37 Â 10 6 cells mL À1 respectively. The two groups have almost equal proportions in the Songhua River, with the average contribution of LNA bacteria reaching 47.0%. In comparison, the abundance of LNA bacteria in the mainstream was significantly higher than in the tributaries. The cytometric expressions (green fluorescence and side scatter) within LNA and HNA were strongly covaried, which implies that these two subgroups are intrinsically linked. Multivariate redundancy analysis indicated that both the abundance and cytometric characteristics of co-occurring LNA and HNA bacteria were regulated differently in the Songhua River. This suggests that LNA and HNA bacteria play different ecological roles in river ecosystems.

Curvibacter sp. strain PAE-UM, isolated from river sediment, is a typical ultramicrobacterium capable of phthalate ester degradation. The genome of Curvibacter sp. PAE-UM consists of 3,284,473 bp, and its information will provide insights into the molecular mechanisms underlying its degradation ability.