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

Abstract

The Rhizaria is a super-group of amoeboid 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 biogenic silica and carbon fluxes, we lack the most basic information about their ecological habitats and preferences. Here, using in situ imaging (Underwater Vision Profiler 5), we characterize the vertical ecological niches of different test-bearing pelagic rhizarian taxa in the southern California Current Ecosystem. We define three vertical layers between 0 and 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 9% 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 abiotic factors influencing populations in the upper 200 m, dissolved silicon concentration is related to the abundance of mesopelagic phaeodarians, though it explains only a portion of the variance. The importance of biotic interactions (e.g., prey availability, predation, parasitism, symbiosis) is still to be considered, in order to fully incorporate the dynamics of testbearing pelagic rhizarians in ecological and biogeochemical models.

Carbon and nitrogen content to biovolume relationships for marine protist of the Rhizaria lineage (Radiolaria and Phaeodaria)

“…Rhizaria are widely distributed in the world oceans (Biard et al 2016). Symbiont bearing Rhizaria, like Collodaria and Acantharia, are most abundant in the upper epipelagic zone, whereas Phaeodaria are predominantly found in deeper water layers (Michaels 1988;Biard and Ohman 2020). Phaeodaria and Radiolaria have shown especially high abundance in locations Table 3.…”

“…Therefore, we have additionally shown C and N content in relation to colony volume. It will be particularly useful for mass estimates in situations where three dimensionally layered central capsules cannot be accurately determined (e.g., in situ optics‐based technologies, like Underwater Vision Profilers or similar plankton recorders (Nakamura et al 2017; Biard and Ohman 2020). In these cases, despite showing weaker correlations, possibly due to the size and quantity variation of the central capsules in the colony, the equations for C : vol (or N : vol) of colonies (Eq.…”

“…Rhizaria are widely distributed in the world oceans (Biard et al 2016). Symbiont bearing Rhizaria, like Collodaria and Acantharia, are most abundant in the upper epipelagic zone, whereas Phaeodaria are predominantly found in deeper water layers (Michaels 1988; Biard and Ohman 2020). Phaeodaria and Radiolaria have shown especially high abundance in locations where the efficiency of the biological carbon pump is high, but also in oligotrophic regions such as the North Pacific and California Current Ecosystem (Stukel et al 2018; Gutierrez‐Rodriguez et al 2019).…”

“…However, Phaeodaria are now considered Cercozoa of the supergroup Rhizaria (Polet et al 2004). Marine Rhizaria can represent up to 33% of large zooplankton community (> 600 μ m) in the upper water column (Biard et al 2016), and are also abundant in deeper layers (Biard and Ohman 2020). Rhizaria are fundamental to many biogeochemical cycles, including silicon (Biard et al 2018; Llopis Monferrer et al 2020), strontium (Bernstein et al 1987), carbon through calcification (Erez 2003), and the sinking of particulate organic matter known as the biological carbon pump process (Lampitt et al 2009; Stukel et al 2018; Gutierrez‐Rodriguez et al 2019).…”

Role of small Rhizaria and diatoms in the pelagic silica production of the Southern Ocean

“…Firstly, we contemplate a large underestimation of Rhizaria production by our sampling strategy. While diatoms are photosynthetic organisms that thrive in the photic zone, Rhizaria inhabit the entire water column (Biard and Ohman 2020), with specific communities and highest abundances occurring at depth (peak at 200–400 m in southern latitudes; Suzuki and Not 2015). Although they present low cell densities, deep‐living species usually inhabit much larger depth intervals, and cell abundance integration results in high standing stocks comparable to those estimated for surface communities (Kling and Boltovskoy, 1995).…”