Elemental content allometries and silicon uptake rates of planktonic Rhizaria: Insights into their ecology and role in biogeochemical cycles

Abstract

The last two decades have shown the importance of Rhizaria in the biogeochemical cycles of carbon and silicon in modern oceans. This eukaryotic supergroup, which includes Radiolaria and Phaeodaria, represents an important part of zooplanktonic carbon biomass and contributes to carbon and silica export. Still, accurate estimations of their carbon biomass are hindered by poor knowledge of their elemental composition, contrasting with well-established allometric carbon-to-volume relationships for smaller protists such as phytoplankton. Here, we directly measured carbon, nitrogen, and biogenic silica content as well as silicon uptake rates of planktonic Rhizaria. We highlight that size can be used as a predictor of elemental content for a broad variety of planktonic Rhizaria ranging from 200 μm to several mm, whereas size is weakly correlated with silicon uptake rates. Our results indicate that the scaling exponent of the carbon-to-volume allometry is significantly lower than those for smaller protists, underlining the low carbon strategy of these organisms. Still, we show that carbon and nitrogen densities span over four orders of magnitude, possibly accounting for the differences in depth ranges, nutritional modes and colonial or solitary forms. We estimate Rhizaria sinking speeds by combining carbon, nitrogen, and silica content data and show that great variability exists among the different taxa. Besides giving a better understanding of rhizarian ecology and biogeochemistry, these analyses, at the individual scale, are a first step to subsequent biomass and flux estimations at larger scales.

Global census of the significance of giant mesopelagic protists to the marine carbon and silicon cycles

“…2 and Supplementary Table 2 ). We apply the most recent allometric volume-to-elemental content relationships 16 , 18 to obtain carbon content of all Rhizaria groups, as well as the silica content for Phaeodaria only. Indeed, all Phaeodaria are known as silicifying, while silicified Collodaria cannot be distinguished from naked ones in UVP5 images.…”

“…This is about one to two orders of magnitude lower than prior estimates (0.204 Pg C and 0.061 Pg C in the top 500 m) 20 , 21 . In contrast to these studies, our estimates rely on dedicated volume-to-elemental content allometric relationships established from carbon and volume measurements on living specimens 16 , which showed lower carbon densities of large Rhizaria and the inadequacy of previous conversion factors. Yet, large discrepancies can be observed in model assessment ( R 2 metrics; Supplementary Table 3 ) depending on the use of random or spatial CV.…”

“…Phaeodaria, on the other hand, are strictly heterotrophic and mostly found in the mesopelagic layer, where they feed upon sinking particles 14 , 15 . Many large planktonic Rhizaria have adapted their lifestyle to food-depleted environments through a low cellular carbon density 16 . In contrast, siliceous Rhizaria are the most silicified pelagic organisms known to date 17 , 18 .…”

Gelatinous matrix, an original strategy to cope with oligotrophy in Nassellaria (Radiolaria)

“…The success of Radiolaria to adapt to diverse environments has made them important contributors of contemporary biogeochemical cycles. They can participate in the transformation of sinking particles and have been identified as a critical source of organic carbon [10, 11]. Global models predict that up to 20% of the biomass of the euphotic layer (0-200m) is due to Rhizaria, including Collodaria among other protists [12].…”

“…The success of Radiolaria to adapt to diverse environments has made them important contributors of contemporary biogeochemical cycles. They can participate in the transformation of sinking particles and have been identified as a critical source of organic carbon [11, 12]. Global models indicate that Rhizaria, including Collodaria among other protists, may constitute up to 20% of the biomass of the euphotic layer (0-200m) [13], although recent estimates suggest their global biomass in the epipelagic oceans is relatively low [14].…”