Nutrient fluxes and grazing by photosymbiotic Acantharia - subcellular and bulk approach
Creators: name: Mansour, Joost Samir
name: LeKieffre, Charlotte
name: Decelle, Johan
name: Hansen, Per Juel
name: Le Panse, Sophie
name: Leroux, Cedric
name: Gallet, Benoit
name: Not, Fabrice
Publisher: name: Zenodo
Year: 2022
Subjects: Acantharia
Radiolaria
Stable isotopes
NanoSims
Carbon uptake
plankton
mixotrophy
grazing
nitrogen uptake
photosymbiosis

description: Acantharia (Radiolaria) are ubiquitous, heterotrophic single-celled plankton in oceanic waters. Their contribution and roles in ecosystems have previously been underestimated, being elusive due to their broad size range and fragility. Yet, recent studies show that they are major components of the planktic community contributing greatly to, among others, the carbon flux. Many acantharian taxa are known to form a photosymbiosis with the microalgae <em>Phaeocystis </em>sp., which is heavily modified inside the acantharian host cell. These modifications seem to aim to exploit photosynthetic capabilities. Whereas observed pseudopodial extensions can suggest an active predation mechanism. The mixotrophic mode of nutrient acquisition of the Acantharia allows energy and biomass to go through the food web at varying trophic levels. Thereby, potentially enhancing biomass transfer efficiency to higher trophic levels, and when taken into account would increase carbon export estimates of biogeochemical models by up to 30%. Nutrient uptake rates, indicating dependence on either trophic mode, are therefore crucial in the parametrization of carbon budgets in planktonic ecosystem models.<br> <br> Yet, the difficulties inherent to the study of live Radiolaria make it that very little is known about their physiology, and thus the metabolic interactions of host and symbiont, or how much Acantharia rely on either photosynthesis or feeding. Here we aimed to elucidate the metabolic dialogue between these symbiotic partners. Therefore, we used single-cell isolations of Acantharia incubated with stable isotopes of carbon and nitrogen. This allowed bulk rate measurements of photosynthetic carbon uptake under different conditions of nitrogen availability (nitrate or ammonium) as well as, single-cell chemical imaging to spatially visualize carbon/nitrogen uptake, incorporation, and photosynthate translocation between symbionts and host over time.<br> <br> Results obtained in this study suggest that the uptake of inorganic nutrients in this symbiotic association depends on light, and thus photosynthesis of the symbionts. Carbon uptake rates were unaffected by the nitrogen source (<em>i.e.</em>, nitrate or ammonium). The total inorganic carbon uptake rate was 1112±82 pgC h<sup>-1</sup> Acantharia<sup>‑1</sup>, 22.3±1.6 pgC h<sup>-1</sup> symbiont cell<sup>-1</sup>assuming 50 symbionts per Acantharia, at ~155-μmol photons m<sup>−2 </sup>s<sup>−1 </sup>irradiance. The Acantharia studied could use both inorganic ammonium and nitrate, but ammonium was taken up at a ~5 times higher rate. <br> Prey ingestion of the haptophyte, <em>Isochrysis galbana</em>, was detected using labeled algae. Significant grazing by Acantharia could only be established on the dinoflagellate <em>Effrenium voratum</em>, with a grazing rate of 728 prey Acantharia<sup>‑1</sup> hour<sup>-1</sup> (<em>i.e.</em>, ~56.3 ngC h<sup>-1</sup>, 46% of total holobiont carbon content) at a ratio of 1.06x10<sup>4 </sup>prey predator<sup>-1</sup>. Daily photosynthetic carbon uptake rates made up ~14.5% of the total holobiont carbon content (0.9% hourly).<br> <br> Survival of Spumellaria (Radiolaria) has been shown to be photosynthesis dependent (Swanberg and Anderson, 1985). Food in absence of light could not extend the survival of the cells. Symbiont photosynthesis thus fulfills an important role in these radiolarians metabolism. Contrastingly, it has also been hypothesized that photosynthesis in other mixotrophic organisms predominately provides energy.<br> Whereas, a daily consumption of 14.5% of the holobiont’s carbon content might be sufficient for substance and growth, chemical imaging showed no measurable fixation in the host, and thus no carbon transfer from symbionts. The usage of recently assimilated photosynthates is implied by a decreasing δ<sup>13</sup>C over time. However, the specifics of photosynthate usages cannot yet be ascertained and might be used either directly at the symbionts or at the host cell. We might hypothesize that the synthesis of carbon storages molecules in <em>Phaeocystis, </em>the acantharian symbiont, proceeds in a matter more similar to plants—in their plastids, and the nutrient localization could thus indicate that photosynthates are in fact primarily used for symbiont maintenance or for energy or catabolic processes of the host (with minimal intermediate storage). Our results would currently suggest that if the Acantharia cell obtains photosynthetically acquired carbon by translocation it is not assimilated in the host cell, but might still be used for catabolic processes to obtain energy.