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Nitrous oxide (N 2 O) is a potent greenhouse gas and an ozone destroying substance. Yet, clear step-by-step protocols to measure N 2 O transformation rates in freshwater and marine environments are still lacking, challenging inter-comparability efforts. Here we present detailed protocols currently used by leading experts in the field to measure water-column N 2 O production and consumption rates in both marine and other aquatic environments. We present example 15 N-tracer incubation experiments in marine environments as well as templates to calculate both N 2 O production and consumption rates. We discuss important considerations and recommendations regarding (1) precautions to prevent oxygen (O 2 ) contamination during low-oxygen and anoxic incubations, (2) preferred bottles and stoppers, (3) procedures for 15 N-tracer addition, and (4) the choice of a fixative. We finally discuss data reporting and archiving. We expect these protocols will make 15 N-labeled N 2 O transformation rate measurements more accessible to the wider community and facilitate future inter-comparison between different laboratories. 

Yeast, molds and other fungi are found in most environments across the world. Many of the fungi that live on land today form relationships called symbioses with other microbes. Some of these relationships, like those formed with green algae, are beneficial and involve the exchange carbon, nitrogen and other important nutrients. Algae first evolved in the sea and it has been suggested that symbioses with fungi may have helped some algae to leave the water and to colonize the land more than 500 million years ago. A fungus called Mortierella elongata grows as a network of filaments in soils and produces large quantities of oils that have various industrial uses. While the details of Mortierella’s life in the wild are still not certain, the fungus is thought to survive by gaining nutrients from decaying matter and it is not known to form any symbioses with algae. In 2018, however, a team of researchers reported that, when M. elongata was grown in the laboratory with a marine alga known as Nannochloropsis oceanica, the two organisms appeared to form a symbiosis. Both the alga and fungus produce oil, and when grown together the two organisms produced more oil than when the fungus or algal cells were grown alone. However, it was not clear whether the fungus and alga actually benefit from the symbiosis, for example by exchanging nutrients and helping each other to resist stress. Du et al. – including many of the researchers involved in the earlier work – have now used biochemical techniques to study this relationship in more detail. The experiments found that there was a net flow of carbon from algal cells to the fungus, and a net flow of nitrogen in the opposite direction. When nutrients were scarce, algae and fungi grown in the same containers grew better than algae and fungi grown separately. Further, Mortierella only obtained carbon from living algae that attached to the fungal filaments and not from dead algae. Unexpectedly, further experiments found that when grown together over a period of several weeks or more some of the algal cells entered and lived within the filaments of the fungus. Previously, no algae had ever been seen to inhabit the living filaments of a fungus. These findings may help researchers to develop improved methods to produce oil from M. elongata and N. oceanica. Furthermore, this partnership provides a convenient new system to study how one organism can live within another and to understand how symbioses between algae and fungi may have first evolved. 

The potent greenhouse gas nitrous oxide (N₂O) may have been an important constituent of Earth's atmosphere during Proterozoic (~2.5–0.5 Ga). Here, we tested the hypothesis that chemodenitrification, the rapid reduction of nitric oxide by ferrous iron, would have enhanced the flux of N₂O from ferruginous Proterozoic seas. We empirically derived a rate law,, and measured an isotopic site preference of +16‰ for the reaction. Using this empirical rate law, and integrating across an oceanwide oxycline, we found that lownM NOand μM‐lowmMFe²⁺concentrations could have sustained a sea‐air flux of 100–200 Tg N₂O–N year⁻¹, if N₂fixation rates were near‐modern and all fixed N₂was emitted as N₂O. A 1D photochemical model was used to obtain steady‐state atmospheric N₂O concentrations as a function of sea‐air N₂O flux across the wide range of possiblepO₂values (0.001–1PAL). At 100–200 Tg N₂O–N year⁻¹and >0.1PALO₂, this model yielded low‐ppmv N₂O, which would produce several degrees of greenhouse warming at 1.6 ppmvCH₄and 320 ppmvCO₂. These results suggest that enhanced N₂O production in ferruginous seawater via a previously unconsidered chemodenitrification pathway may have helped to fill a Proterozoic “greenhouse gap,” reconciling an ice‐free Mesoproterozoic Earth with a less luminous early Sun. A particularly notable result was that high N₂O fluxes at intermediate O₂concentrations (0.01–0.1PAL) would have enhanced ozone screening of solarUVradiation. Due to rapid photolysis in the absence of an ozone shield, N₂O is unlikely to have been an important greenhouse gas if Mesoproterozoic O₂was 0.001PAL. At low O₂, N₂O might have played a more important role as life's primary terminal electron acceptor during the transition from an anoxic to oxic surface Earth, and correspondingly, from anaerobic to aerobic metabolisms.