Abiotic ammonification and gross ammonium photoproduction in the upwelling system off central Chile (36° S)

Abstract

We investigated the production of ammonium via photodegradation of dissolved organic matter (DOM) in the coastal upwelling system off central Chile (36 • S). Photoam-monification experiments were carried out using exudates obtained from representative diatom species (Chaetoceros muelleri and Thalassiosira minuscule) and natural 5 marine DOM under simulated solar radiation conditions. Additionally, we evaluated the use of photoproduced ammonium by natural microbial communities and separated am-monium oxidizing archaea and bacteria by using GC-7 as an inhibitor of the archaeal community. We found photoammonification operating at two levels: via the transformation of DOM by UV radiation (abiotic ammonification) and via the simultaneous oc-10 currence of abiotic phototransformation and biological remineralization of DOM into NH + 4 (referred as gross photoproduction of NH + 4). The maximum rates of abiotic am-monification reached 0.057 µmol L −1 h −1 , whereas maximum rates of gross photopro-duction reached 0.746 µmol L −1 h −1. Our results also suggest that ammonium oxidizing archaea could dominate the biotic remineralization induced by photodegradation 15 of organic matter and consequently play an important role in the local N cycle. Abiotic ammonium photoproduction in coastal upwelling systems could support between 7 and 50 % of the spring-summer phytoplankton NH + 4 demand. Surprisingly, gross ammonium photoproduction (remineralization induced by abiotic ammonification) might support 50 to 180 % of spring-summer phytoplankton NH + 4 assimilation.