Nitrogen excretion by copepods and its contribution to the ammonium oxidizing activity in the upwelling zone off central-southern Chile (36ºS)

Zooplankton play a pivotal role in the nitrogen cycle by sustaining phytoplankton and heterotrophic bacterial growth through N excretion. This biogeochemical interaction between microbial community and zooplankton has been poorly studied in the ocean. In this work we explored the interaction between the nitrogen compounds excreted by dominant copepods in the upwelling zone off central Chile and the activity of ammonium oxidizing microbial community. For doing so, two experiments were conducted in May and early September 2010 off Concepción (36ºS) in central-southern Chile upon non-upwelling condition. Among organic and inorganic nitrogen compounds excreted by copepods (NH₄⁺, DON, NO₃^- and NO₂^-), DON was the most abundant. In the first experiment, DON excretion rate was <0.6 µmol L^-1 h^-1 in the first hours of incubation, while in the second experiment we found values of ~ 0.9 µmol L^-1 h^-1 in the last hour of incubation. NH₄⁺ excretion rates were lower than those of DON, with values ca. 0.02 and < 0.4 µmol L^-1 h^-1 in autumn and winter, respectively. When assessing the response of microbial ammonium oxidizing groups to the input of these excreted products, it was found that NH₄⁺ was significantly consumed in the first four hours of incubation in the autumn experiment, while bacterioplankton abundance incremented to ~ 8 x10⁵ to 1.2 x10⁶ cells mL^-1 after 6 hours. The activity of AOB and AOA amoA transcript copies increased from 1,539 to 5,669 copies mL^-1 and 1,510 a 3,763 copies mL^-1, respectively. In the second experiment, NH₄⁺ showed complete consumption in the first two hours of incubation and specific groups of AOA and AOB transcript were below <10,000 copies mL^-1. Our findings confirm, that NH₄⁺ is not the main compound excreted by copepods, and this can be directly used by bacterioplankton and AOB followed by AOA response to available NH₄⁺. Both are able to utilize a large proportion of ammonium excreted by the copepods during austral autumn and winter.