Vertical niche definition of test-bearing protists (Rhizaria) into the twilight zone revealed by in situ imaging

The Rhizaria is a super-group of ameoboid protists with ubiquitous distributions, from the euphotic zone to the twilight zone and beyond. While rhizarians have been recently described as important contributors to both silica and carbon fluxes, we lack the most basic information about their ecological preferences. Here, using the in situ imaging (Underwater Vision Profiler 5), we characterize the vertical ecological niches of different test-bearing rhizarian taxa in the southern California Current Ecosystem . We define three vertical layers between 0-500 m occupied, respectively, by 1) surface dwelling and mostly symbiont-bearing rhizarians (Acantharia and Collodaria), 2) flux-feeding phaeodarians in the lower epipelagic (100-200 m), and 3) Foraminifera and Phaeodaria populations adjacent to the Oxygen Minimum Zone. We then use Generalized Additive Models to analyze the response of each rhizarian category to a suite of environmental variables. The models explain between 13 and 93% of the total variance observed for the different groups. While temperature and the depth of the deep chlorophyll maximum, appear as the main factors influencing populations in the upper 200 m, silicic acid concentration is the most important variable related to the abundance of mesopelagic phaeodarians. The relative importance of biotic interactions (e.g., predation, parasitism) is still to be considered, in order to fully incorporate the dynamics of test-bearing pelagic rhizarians in ecological and biogeochemical models.