Anaerobic ammonium oxidation (anammox) process is regarded as a promising nitrogen removal technology to treat ammonium wastewaters in a wide concentration range. Oligotrophic anaerobic ammonia oxidation bacteria (O-AnAOB) culture has been successfully achieved from a new anammox system to treat superlow ammonium concentration wastewaters. In this work, the O-AnAOB culture was compared with the eutrophic AnAOB (E-AnAOB) culture to reveal its physiological, morphological, and ecological features. Results showed that the specific anammox activity (SAA) of O-AnAOB culture was 0.07 kgN/(kgVSS center dot d) with the nitrogen removal rate (NRR) of 0.20 kgN/ (m(3) d) in the reactor, while the SAA of E-AnAOB culture was 2.11 kgN/(kgVSS center dot d) with the NRR of 11.10 kgN/(m(3) d). The hzs gene transcription levels (hzs-mRNA) of O-AnAOB and E-AnAOB cultures were 1.32 x 10(9) copies/gVSS and 1.51 x 10(10) copies/gVSS, respectively. Morphologically, the O-AnAOB culture took on the unique brown color rather than the typical red color of E-AnAOB. The O-AnAOB cells lived in a disperse pattern in the culture. The cells were seriously deformed with deep craters on the cell wall. The size of anammoxsome and paryphoplasm compartments inside the O-AnAOB cells was smaller than that inside the E-AnAOB cells. Ecologically, the O-AnAOB culture had special microbial community with a higher bacterial diversity than the E-AnAOB. The most dominant genera in O-AnAOB were Anaerolineaceae (33.7%, fermentative bacteria), Candidatus Kuenenia (17.4%, anammox bacteria), and Nitrospira (7.3%, nitrite oxidizing bacteria). This study provided an insight into the new anammox process for deep nitrogen removal from wastewaters.