Mixotrophs dominate in surface waters of the subtropical North Atlantic: insights for a warming ocean

With global warming, oligotrophic subtropical ocean biomes are predicted to expand in a near future. In these zones, marine mixotrophic protists are suspected to significantly contribute to primary production and bacterivory. Yet, their diversity and distribution are still poorly characterized, making it difficult to assess their role in the microbial food web in a warmer future. We sampled protistan community composition (18S rRNA gene) alongside a stratification (horizontal) and depth gradient (vertical; 5 – 125 m) across a 500 km North-South transect from the Azores into the sub-tropical gyre, in August 2022. Following taxonomic assignment of amplicon sequence variants, trophic modes were assigned to taxa in which these are known. We highlighted a clear trophic niche separation between mixotrophs, autotrophs and heterotrophs. Mixotrophs were found to be dominant in offshore surface waters, where nutrients are limited and light is plentiful, while autotrophs were more important in the deep chlorophyll maximum and heterotrophs in deeper waters. Among the mixotrophs, constitutive mixotrophs were by far the most abundant and increased in relative abundance in offshore surface waters, highlighting their ubiquity but also their relevance in the oligotrophic ocean. Interaction network analysis revealed that they play a crucial role in the microbial food web in the stratified layers. With global warming and the expansion of oligotrophic conditions, we expect the relevance of mixotrophs for food web functioning and biogeochemical processes to increase in future oceans.

Presentation Preference: Oral

Primary Presenter: Thomas Trombetta, University of Amsterdam - IBED (t.trombetta@uva.nl)

Authors:

Thomas Trombetta, University of Amsterdam  (t.trombetta@uva.nl)

Shai Slomka, University of Amsterdam  (s.slomkadeoliveira@uva.nl)

Sebastiaan A. M. van Beusekom, University of Amsterdam  (s.a.m.vanbeusekom@uva.nl)

Jolanda M. H. Versapgen, University of Amsterdam  (j.m.h.verspagen@uva.nl)

Susanne Wilken, University of Amsterdam  (s.wilken@uva.nl)