Abstract

Prochlorococcus is the smallest and most abundant free-living phototrophic organism, dominating the euphotic zone of oligotrophic tropical and subtropical waters. Uncultivated lineages of this cyanobacterium also dominate sub-surface phototrophic microbial communities of Anoxic Marine Zones (AMZs), which is characterized by very low-light and nutrient-rich conditions. A recent metagenomic study revealed that AMZ Prochlorococcus cells have the genetic potential for using different nitrogen (N) sources, including nitrate. In order to determine which of these N sources AMZ Prochlorococcus are actually using in nature, analysis of nitrogen stable isotopes was performed on cells sorted by flow cytometry to assess the cellular 15N natural abundance (δ15Ν) as well as the assimilation rates of different N compounds. Our results showed a high variability in natural δ15Ν (ranging from -6.5‰ to 13‰) in the AMZ Prochlorococcus of the Eastern Tropical South and North Pacific. The highest values could be a signal for the use of nitrate as the main N source at determined stations. Incubation experiments found that assimilation rates were ordered as nitrate < ammonium < cyanate. The high rate of cyanate assimilation was surprising given that the cyanate hydrase (CynS) gene has not been detected in single amplified genomes of the dominant lineage of AMZ Prochlorococcus. However, this confirmed that nitrate as well organic forms of reduced N are being used by AMZ Prochlorococcus different to the lineages inhabiting non-AMZ waters.