Heterotrophic Foraminifera Capable of Inorganic Nitrogen Assimilation

Abstract

Nitrogen availability often limits biological productivity in marine systems, where inorganic nitrogen, such as ammonium is assimilated into the food web by bacteria and photoautotrophic eukaryotes. Recently, ammonium assimilation was observed in kleptoplast-containing protists of the phylum foraminifera, possibly via the glutamine synthetase/glutamate synthase (GS/GOGAT) assimilation pathway imported with the kleptoplasts. However, it is not known if the ubiquitous and diverse heterotrophic protists have an innate ability for ammonium assimilation. Using stable isotope incubations (¹⁵N-ammonium and ¹³C-bicarbonate) and combining transmission electron microscopy (TEM) with quantitative nanoscale secondary ion mass spectrometry (NanoSIMS) imaging, we investigated the uptake and assimilation of dissolved inorganic ammonium by two heterotrophic foraminifera; a non-kleptoplastic benthic species, Ammonia sp., and a planktonic species, Globigerina bulloides. These species are heterotrophic and not capable of photosynthesis. Accordingly, they did not assimilate ¹³C-bicarbonate. However, both species assimilated dissolved ¹⁵N-ammonium and incorporated it into organelles of direct importance for ontogenetic growth and development of the cell. These observations demonstrate that at least some heterotrophic protists have an innate cellular mechanism for inorganic ammonium assimilation, highlighting a newly discovered pathway for dissolved inorganic nitrogen (DIN) assimilation within the marine microbial loop.

Keywords: ammonium assimilation; foraminifera; heterotrophic protists; heterotrophy; marine; nitrogen cycle.

FIGURE 1

TEM micrographs of the cytoplasm of Ammonia sp. (A) Global view of the cytoplasm with numerous lipid droplets, vacuoles and a few residual bodies. (B) Higher magnification image of mitochondria, electron-opaque bodies, and fibrillar vesicles. (C) Detailed structure of a fibrillar vesicle, fibrils are organized in parallel. (D) Mitochondria with visible intact cristae and double membrane. eo, electron-opaque bodies; fv, fibrillar vesicles; li, lipid droplets; m, mitochondria; ol, organic lining; re, residual bodies; v, vacuoles.

FIGURE 2

TEM micrographs of the cytoplasm of G. bulloides. Views of the cytoplasm in specimen incubated 6 h (A) and 18 h (B). (C) Detailed structure of fibrillar vesicle, electron-opaque bodies, and mitochondria. Black arrows: electron-opaque bodies (eo). dv, degradation vacuole; f, fibrillar bodies (form B, see text); f*, later stage of fibrillar bodies (form C); fv, fibrillar vesicles; m, mitochondria; v, vacuoles.

FIGURE 3

¹³C and ¹⁵N cellular localization in the cytoplasm of Ammonia sp. after 20 h of incubation in light with H¹³CO₃^– and ¹⁵NH₄⁺. Left column: TEM micrographs; central column: corresponding NanoSIMS δ¹⁵N map; right column: corresponding NanoSIMS δ¹³C map. Arrows: electron-opaque bodies; circles: fibrillar vesicles. dv, degradation vacuoles; li, lipid droplets; n, nucleoplasm; nu, nucleus.

FIGURE 4

¹³C and ¹⁵N cellular localization in the cytoplasm of G. bulloides after 6 and 18 h of incubation in light with H¹³CO₃^– and ¹⁵NH₄⁺. Left column: TEM micrographs; central column: corresponding NanoSIMS δ¹⁵N map; right column: corresponding NanoSIMS δ¹³C map. Arrows: electron-opaque bodies; arrowheads: prokaryote-like vesicles; circles: fibrillar vesicles. ch, condensed chromatin; dv, degradation vacuoles; f, fibrillar bodies (form B, see text); f*, later stage of fibrillar bodies (form C); li, lipid droplets; n, nucleoplasm; nu, nucleus; v, vacuoles.