Our understanding of the players and pathways of the global nitrogen cycle has advanced substantially over recent years with discoveries of several new groups of organisms and new types of metabolism. This review focuses on recently discovered processes that add new functionality to the nitrogen cycle and on the organisms that perform these functions. The processes include denitrification and other dissimilatory nitrogen transformations in eukaryotes, anaerobic ammonium oxidation, and anaerobic methane oxidation with nitrite. Of these, anaerobic ammonium oxidation coupled to nitrite reduction by anammox bacteria has been well documented in natural environments and constitutes an important sink for fixed nitrogen. Benthic foraminifera also contribute substantially to denitrification in some sediments, in what potentially represents an ancestral eukaryotic metabolism. The ecophysiology of the novel organisms and their interactions with classical types of nitrogen metabolism are important for understanding t...

New Pathways and Processes in the Global Nitrogen Cycle

Production and consumption of methane in freshwater lake ecosystems.

1. Many freshwater systems receive substantial inputs of terrestrial organic matter. Terrestrially derived dissolved organic carbon (t-DOC) inputs can modify light availability, the spatial distrib ...

https://onlinelibrary.wiley.com/doi/pdf/10.1111/fwb.12909

How important are terrestrial organic carbon inputs for secondary production in freshwater ecosystems

Tropical reservoirs have been identified as important methane (CH4) sources to the atmosphere, primarily through turbine and downstream degassing. However, the importance of ebullition (gas bubbling) remains unclear. We hypothesized that ebullition is a disproportionately large CH4 source from reservoirs with dendritic littoral zones because of ebullition hot spots occurring where rivers supply allochthonous organic material. We explored this hypothesis in Lake Kariba (Zambia/Zimbabwe; surface area >5000 km2) by surveying ebullition in bays with and without river inputs using an echosounder and traditional surface chambers. The two techniques yielded similar results, and revealed substantially higher fluxes in river deltas (∼103 mg CH4 m–2 d–1) compared to nonriver bays (<100 mg CH4 m–2 d–1). Hydroacoustic measurements resolved at 5 m intervals showed that flux events varied over several orders of magnitude (up to 105 mg CH4 m–2 d–1), and also identified strong differences in ebullition frequency. Both fa...

Spatial Heterogeneity of Methane Ebullition in a Large Tropical Reservoir

In this paper we provide an overview of new
knowledge on oxygen depletion (hypoxia) and related phenomena
in aquatic systems resulting from the EU-FP7
project HYPOX (“In situ monitoring of oxygen depletion in
hypoxic ecosystems of coastal and open seas, and landlocked
water bodies”, www.hypox.net). In view of the anticipated
oxygen loss in aquatic systems due to eutrophication and climate
change, HYPOX was set up to improve capacities to
monitor hypoxia as well as to understand its causes and consequences.
Temporal dynamics and spatial patterns of hypoxia were
analyzed in field studies in various aquatic environments, including
the Baltic Sea, the Black Sea, Scottish and Scandinavian
fjords, Ionian Sea lagoons and embayments, and Swiss
lakes. Examples of episodic and rapid (hours) occurrences of
hypoxia, as well as seasonal changes in bottom-water oxygenation
in stratified systems, are discussed. Geologically
driven hypoxia caused by gas seepage is demonstrated. Using
novel technologies, temporal and spatial patterns of watercolumn
oxygenation, from basin-scale seasonal patterns to
meter-scale sub-micromolar oxygen distributions, were resolved.
Existing multidecadal monitoring data were used to
demonstrate the imprint of climate change and eutrophication
on long-term oxygen distributions. Organic and inorganic
proxies were used to extend investigations on past oxygen
conditions to centennial and even longer timescales that
cannot be resolved by monitoring. The effects of hypoxia on
faunal communities and biogeochemical processes were also
addressed in the project. An investigation of benthic fauna is
presented as an example of hypoxia-devastated benthic communities
that slowly recover upon a reduction in eutrophication
in a system where naturally occurring hypoxia overlaps
with anthropogenic hypoxia. Biogeochemical investigations
reveal that oxygen intrusions have a strong effect on the microbially
mediated redox cycling of elements. Observations
and modeling studies of the sediments demonstrate the effect
of seasonally changing oxygen conditions on benthic mineralization
pathways and fluxes. Data quality and access are
crucial in hypoxia research. Technical issues are therefore
also addressed, including the availability of suitable sensor
technology to resolve the gradual changes in bottom-water
oxygen in marine systems that can be expected as a result of
climate change. Using cabled observatories as examples, we
show how the benefit of continuous oxygen monitoring can
be maximized by adopting proper quality control. Finally,
we discuss strategies for state-of-the-art data archiving and
dissemination in compliance with global standards, and how
ocean observations can contribute to global earth observation
attempts.

/pdf/investigating-hypoxia-in-aquatic-environments-diverse-25aqd6h78r.pdf

Investigating hypoxia in aquatic environments: diverse approaches to addressing a complex phenomenon

The build-up of methane in the hypolimnion of the eutrophic Lake Rotsee (Lucerne, Switzerland) was monitored over a full year. Sources and sinks of methane in the water column were characterized by measuring concentrations and carbon isotopic composition. In fall, high methane concentrations (up to 1 mM) were measured in the anoxic water layer. In the oxic layer, methane concentrations were much lower and the isotopic composition shifted towards heavy carbon isotopes. Methane oxidation rates peaked at the interface between oxic and anoxic water layers at around 8–10 m depth. The electron balance between the oxidants oxygen, sulphate, and nitrate, and the reductants methane, sulphide and ammonium, matched very well in the chemocline during the stratified season. The profile of carbon isotopic composition of methane showed strong indications for methane oxidation at the chemocline (including the oxycline). Aerobic methane oxidizing bacteria were detected at the interface using fluorescence in situ hybridization. Sequencing the responsible organisms from DGGE bands revealed that aerobic methanotrophs type I closely related to Methylomonas were present. Sulphate consumption occurred at the sediment surface and, only towards the end of the stagnation period, matched with a zone of methane consumption. In any case, the flux of sulphate below the chemocline was not sufficient to oxidize all the methane and other oxidants like nitrate, iron or manganese are necessary for the observed methane oxidation. Although most of the methane was oxidized either aerobically or anaerobically, Lake Rotsee was still a source of methane to the atmosphere with emission rates between 0.2 mg CH4 m−2 day−1 in February and 7 mg CH4 m−2 day−1 in November.

/pdf/oxidation-and-emission-of-methane-in-a-monomictic-lake-1inptsgtu4.pdf

E. Durisch-Kaiser

Papers

Oxidation and emission of methane in a monomictic lake (Rotsee, Switzerland)