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

Abstract

We examined biogenic silica production and elementary composition (biogenic Si, particulate organic carbon and particulate organic nitrogen) of Rhizaria and diatoms in the upper 200 m along a transect in the Southwest Pacific sector of the Southern Ocean during austral summer (January–February 2019). From incubations using the 32Si radioisotope, silicic acid uptake rates were measured at 15 stations distributed in the Polar Front Zone, the Southern Antarctic Circumpolar Current and the Ross Sea Gyre. Rhizaria cells are heavily silicified (up to 7.6 nmol Si cell−1), displaying higher biogenic Si content than similar size specimens found in other areas of the global ocean, suggesting a higher degree of silicification of these organisms in the silicic acid rich Southern Ocean. Despite their high biogenic Si and carbon content, the Si/C molar ratio (average of 0.05 ± 0.03) is quite low compared to that of diatoms and relatively constant regardless of the environmental conditions. The direct measurements of Rhizaria's biogenic Si production (0.8–36.8 μmol Si m−2 d−1) are of the same order of magnitude than previous indirect estimations, confirming the importance of the Southern Ocean for the global Rhizaria silica production. However, diatoms largely dominated the biogenic Si standing stock and production of the euphotic layer, with low rhizarians' abundances and biogenic Si production (no more than 1%). In this manuscript, we discuss the Antarctic paradox of Rhizaria, that is, the potential high accumulation rates of biogenic Si due to Rhizaria in siliceous sediments despite their low production rates in surface waters.

Planktonic protist diversity across contrasting Subtropical and Subantarctic waters of the southwest Pacific

“…Acantharea), provides substantial mineral ballast (Takahashi 1983) conferring them a key role in vertical export that is supported by their common presence in sediment traps (Bernstein et al 1987;Gutierrez-Rodriguez et al 2019;Michaels et al 1995;Preston et al 2020) and their enrichment in suspended and sinking particles (Duret et al 2020). Despite their abundance and their important role in biogeochemical processes (Biard et al 2016;Guidi et al 2016) little is known about the vertical distribution of Radiolaria particularly in the meso-and bathypelagic ocean (>500 m) (Biard and Ohman n.d.;Boltovskoy 2017;Llopis Monferrer et al 2021;Ollison et al 2021). In our study, we found an opposite distribution between Acantharea and RAD-B, which showed preference for the upper (<500 m) and deeper (>500 m) mesopelagic samples, respectively (Figure 7).…”

Siliceous Rhizaria abundances and diversity in the Mediterranean Sea assessed by combined imaging and metabarcoding approaches

“…The contribution of Rhizaria to the biogenic silica biomass can be highly variable. For example, in productive areas of the World Ocean, where Rhizaria are less abundant and diatoms concentrations are much higher, the impact of these protists on the biogenic silica biomass in surface waters is much lower, as in the case of the Ross Sea in Antarctica, where Rhizaria account for only 0.1% at most of the total biogenic silica (Llopis Monferrer et al, 2021).…”

New evaluation of species‐specific biogenic silica flux of radiolarians (Rhizaria) in the western Arctic Ocean using microfocus X‐ray computed tomography

“…There are a few ways to estimate the values of biogenic silica content of individual specimens. One way is by chemical analysis as performed by Llopis Monferrer et al (2020, 2021), another way is by mass measurement of only a few large (> 400 μ m) radiolarian species using an electronic precision balance and then take the average (Takahashi 1991) or using simple radiolarian skeleton models that combine geometrical shapes such as spheres, cones, cylinders, ellipsoids, and cuboids (Jacot Des Combes and Abelmann 2009). In fact, since the skeletons of radiolarians have a very complicated structure, they cannot be exactly reproduced with a simple geometric model.…”