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). Photoammonification experiments were carried out using exudates obtained from representative diatom species (Chaetoceros muelleri and Thalassiosira minuscule) and natural marine DOM under simulated solar radiation conditions. Additionally, we evaluated the use of photoproduced ammonium by natural microbial communities and separated ammonium 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 occurrence of abiotic phototransformation and biological remineralization of DOM into NH₄⁺ (referred as gross photoproduction of NH₄⁺). The maximum rates of abiotic ammonification reached 0.057 μmol L⁻¹ h⁻¹, whereas maximum rates of gross photoproduction reached 0.746 μmol L⁻¹ h⁻¹. Our results also suggest that ammonium oxidizing archaea could dominate the biotic remineralization induced by photodegradation 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 NH₄⁺ demand. Surprisingly, gross ammonium photoproduction (remineralization induced by abiotic ammonification) might support 50 to 180% of spring-summer phytoplankton NH₄⁺ assimilation.