Response of pico-nano-eukaryotes to inorganic and organic nutrient additions

Abstract

Organic nutrient loading in coastal waters has considerably increased due to anthropogenic activities over the last decades. In particular, the increase in the proportion of organic vs. inorganic nitrogen may have an impact on microbial plankton communities, particularly within the small size classes, where mixotrophy is likely to be widespread. Here we explore the change in taxonomic composition of small eukaryotes (<20 μm) populating a productive coastal system in response to inorganic and/or organic nutrient amendments using 18S rRNA gene sequencing. Major changes in the community were observed in the mixed treatment, containing both inorganic and organic nutrients, which mostly affected phytoplankton taxa. Adding mixed nutrients had a positive effect on pigmented groups, when compared to additions of inorganic nutrients. The responding pigmented groups included dinoflagellates, cryptophyceans, chrysophyceans, bolidophyceans or dictyochophyceans, which comprise mixotrophic species, but also groups that are considered to be strictly autotrophs, such as diatoms or mamiellophyceans. Our results highlight the importance of organic nutrients for growth of small phytoplankton. The combination of both, autotrophic and heterotrophic capabilities, is possibly an advantage for small phytoplankton to thrive when inorganic nutrients or light are limiting or suboptimal.