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.

氮的可用性通常会限制海洋系统的生物生产力，在海洋系统中，细菌和光合自养真核生物会将诸如铵之类的无机氮吸收到食物网中。最近，在有孔膜的有孔菌的原生生物中观察到铵的同化作用，可能是通过导入了有色塑料的谷氨酰胺合成酶/谷氨酸合酶（GS / GOGAT）同化途径引起的。但是，尚不知道普遍存在且多样化的异养原生生物是否具有天生的铵同化能力。使用稳定的同位素孵育（¹⁵ N-铵和¹³C-碳酸氢盐），并结合透射电子显微镜（TEM）和定量纳米级二次离子质谱（NanoSIMS）成像，我们研究了两个异养有孔虫对溶解性无机铵的吸收和同化作用；非变态底栖生物氨 以及一个浮游生物， Globigerina Bulloides。这些物种是异养的，不能进行光合作用。因此，他们没有吸收¹³ C-碳酸氢盐。然而，两个物种都吸收了¹⁵ N铵，并将其掺入对细胞的个体发育和发育具有直接重要性的细胞器中。这些观察结果表明，至少一些异养生物具有天然的无机铵同化细胞机制，突显了海洋微生物环中溶解无机氮（DIN）同化的新发现途径。