Sinks and Sources of Intracellular Nitrate in Gromiids.

Signe Høgslund,Tomas Cedhagen,Nils Risgaard-Petersen,Samuel S Bowser

doi:10.3389/fmicb.2017.00617

Abstract

A substantial nitrate pool is stored within living cells in various benthic marine environments. The fate of this bioavailable nitrogen differs according to the organisms managing the intracellular nitrate (ICN). While some light has been shed on the nitrate carried by diatoms and foraminiferans, no study has so far followed the nitrate kept by gromiids. Gromiids are large protists and their ICN concentration can exceed 1000x the ambient nitrate concentration. In the present study we investigated gromiids from diverse habitats and showed that they contained nitrate at concentrations ranging from 1 to 370 mM. We used 15N tracer techniques to investigate the source of this ICN, and found that it originated both from active nitrate uptake from the environment and from intracellular production, most likely through bacterial nitrification. Microsensor measurements showed that part of the ICN was denitrified to N2 when gromiids were exposed to anoxia. Denitrification seemed to be mediated by endobiotic bacteria because antibiotics inhibited denitrification activity. The active uptake of nitrate suggests that ICN plays a role in gromiid physiology and is not merely a consequence of the gromiid hosting a diverse bacterial community. Measurements of aerobic respiration rates and modeling of oxygen consumption by individual gromiid cells suggested that gromiids may occasionally turn anoxic by their own respiration activity and thus need strategies for coping with this self-inflicted anoxia.

Highlights

Bioavailable nitrogen, in form of nitrate, is present in the upper millimeters of the sediment pore water where it acts as a key nutrient and electron acceptor in many benthic marine ecosystems
All 107 individual gromiids tested in the present study contained nitrate at concentrations ranging from 0.7 to 80 mM, and the average intracellular nitrate (ICN) concentration for populations sampled in Arctic, temperate and Antarctic regions exceeded the ambient NO3− concentration by a factor of 102–104 (Table 1)
The source of the ICN was investigated in a series of experiments where the ICN pool in individual gromiids was followed by adding 15NO3− and 15NH4+ and by inhibiting nitrification with the addition of acetylene

Summary

Introduction

Bioavailable nitrogen, in form of nitrate, is present in the upper millimeters of the sediment pore water where it acts as a key nutrient and electron acceptor in many benthic marine ecosystems. A substantial nitrate pool is, maintained by eukaryotic microbes inhabiting the sediment environment. This intracellular nitrate (ICN) can comprise a nitrate pool that exceeds pore water pools by orders of magnitude (Mussmann et al, 2003; Høgslund et al, 2010; Kamp et al, 2015; Garcia-Robledo et al, 2016). Estimates of denitrification of ICN in foraminifers indicate that up to 90% of the general loss of nitrogen as nitrogen gas from some sediment is based on respiration of ICN (Piña-Ochoa et al, 2010; Dale et al, 2016) and reviewed by Kamp et al (2015)

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