Abstract

Anaerobic ammonium oxidizers contribute to the removal of fixed nitrogen in oxygen-deficient marine ecosystems such as oxygen minimum zones (OMZ). Here we surveyed for the first time the occurrence and diversity of anammox bacteria in the Colombian Pacific, a transition area between the prominent South and North Pacific OMZs. Anammox bacteria were detected in the coastal and oceanic areas of the Colombian Pacific in low oxygen (<22M), high nitrate (25-35M) and low nitrite (<0.07M), and ammonium (<1M) waters. In these waters, anammox bacteria were rich [approximate to 7 operational taxonomic units (OTUs), 98% cut-off) and microdiverse (Shannon index H<1.24), in comparison with the observed at the prominent OMZ of the Eastern Tropical South Pacific, Arabian Sea and Black Sea. Anammox bacteria-like sequences from the Colombian Pacific were grouped together with sequences retrieved from the distinct OMZ's marine subclusters (Peru, Northern Chile and Arabian Sea) within Candidatus Scalindua spp'. Moreover, some anammox bacteria OTUs shared a low similarity with environmental phylotypes (86-94%). Our results indicated that a microdiverse anammox community inhabits the Colombian Pacific, generating new questions about the ecological and biogeochemical differences influencing its community structure.