Nitrospina–like bacteria are the main drivers of nitrite oxidation in the seasonal upwelling area of the Eastern South Pacific (central Chile ~36° S). Marine nitrite-oxidizing bacteria

Nitrite oxidation is the second step of the nitrification process and plays a key role in marine nitrate regeneration. In spite of its environmental importance, bacteria involved in this process have been seldom studied in coastal waters. We herein report for the first time the presence, activity and diversity of nitrite-oxidizing bacteria (NOB) in the seasonal upwelling and oxygen deficient area off central Chile. DNA- and cDNA-based tag pyrosequencing of 16S-rRNA genes were combined with specific qPCR assays in two contrasting periods (upwelling v/s non upwelling). We found that NOB were exclusively represented by Nitrospina-like bacteria, showing a high degree of ribotypes microdiversity related to sequences retrieved from coastal upwelling, deep-sea water and sediment environments. Their abundance and richness increased with depth regardless of the studied period and may account for <5 % in our pyrolibraries at 80m. DNA- and cDNA-based qPCR of 16S-rRNA genes confirmed this trend in the water column, while cDNA/DNA gene copies ratios indicated enhanced Nitrospina-like NOB activity during the upwelling-favorable period, especially in the first 30m depth where ammonium –oxidizers are also active. These results demonstrate that Nitrospina-like bacteria are a dominant active nitrite oxidizing microorganism which probably plays a significant role in nitrification in this seasonal upwelling area.