Wiebke Mohr

Nitrogen fixation is crucial for sustaining productivity in most of the open ocean. Cyanobacteria are the most prominent N2-fixers, but based on the nifH gene, a marker gene of the enzyme that fixes N2 into ammonia, non-cyanobacterial N2-fixers often predominate the N2-fixing community. Yet, the vast majority of them remain poorly characterized. In the oligotrophic South Pacific gyre, we found that most nifH gene sequences belonged to non-cyanobacterial N2-fixers that dominated the waters with measurable N2 fixation rates. Approximately two thirds of the non-cyanobacterial sequences affiliated with the group “Marine 1” which also contains the recently identified diatom symbiont Ca. Tectiglobus diatomicola, a heterotrophic bacterium belonging to the order Rhizobiales, and its closest relative, Ca. Tectiglobus profundi. Using fluorescence in situ hybridization and electron microscopy, we found that Ca. Tectiglobus-diatom symbioses were present throughout the gyre. These diatom symbioses were also present in samples devoid of nifH from Ca. T. diatomicola and Ca. T. profundi indicating that other members of the “Marine 1” group are also diatom symbionts. At least two morphologically distinct diatoms harbored Ca. Tectiglobus symbionts, revealing a so far unknown diversity in hosts for these rhizobial N2-fixers. Single-cell activity measurements showed that Ca. Tectiglobus-diatom symbioses actively fixed nitrogen and could account for up to 40% of the N2 fixation in the South Pacific gyre. Given the size of the largest oceanic biome and the abundance of Ca. Tectiglobus-related nifH genes in other ocean regions, these heterotrophic N2-fixers likely play a major role in marine nitrogen cycling.

Nitrogen (N₂) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean¹ and has a key role in fuelling the biological carbon pump². Oceanic N₂ fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N₂ into ammonia, are widespread among marine bacteria and archaea^3–5. Little is known about these non-cyanobacterial N₂ fixers, and direct proof that they can fix nitrogen in the ocean has so far been lacking. Here we report the discovery of a non-cyanobacterial N₂-fixing symbiont, ‘Candidatus Tectiglobus diatomicola’, which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N₂-fixing symbiont belongs to the order Rhizobiales and its association with a unicellular diatom expands the known hosts for this order beyond the well-known N₂-fixing rhizobia–legume symbioses on land⁶. Our results show that the rhizobia–diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N₂ fixers in the tropical North Atlantic, and that they might be responsible for N₂ fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.

The regulation of O₂ production by cyanobacteria is critical to understanding the co-evolution of oxygenic photosynthesis (OP) and Earth’s redox landscape. This includes their response to electron donors for competitive anoxygenic photosynthesis, like arsenic. In this work, we assessed the effect of arsenic cycling on photosynthetic activity in a modern cyanobacterial mat thriving beneath an arsenate-rich (∼9-10 µM) water column in the high-altitude central Andes, using biogeochemical and omics approaches. Microsensor measurements and hyperspectral imaging revealed two O₂-producing cyanobacterial layers. During the afternoon, OP ceased in the lower, Chl f-dominated layer. Ex-situ measurements revealed that the combination of high light and arsenic induced a prominent rise in local H₂O₂ concentration, which then coincided with the interruption of OP. Mat incubations suggested that after OP ceased, cyanobacteria transitioned to As(III)-driven anoxygenic photosynthesis using far-red light. Additional incubations and metatranscriptomics on in-situ samples reveal simultaneous As(V) reduction during the day, at sufficiently high rates to supply electron donors for anoxygenic photosynthesis. This study proposes that As(III) can serve as an alternative electron donor for cyanobacterial photosynthesis. It also reveals the crucial role of arsenic in moderating OP, a consequence of the interaction between arsenic and reactive oxygen species under high irradiance. Given the widespread abundance of arsenic in the Precambrian, we further discuss how this regulatory mechanism could have played an important role in the early evolution of OP.

Methylphosphonate is an organic phosphorus compound used by microorganisms when phosphate, a key nutrient limiting growth in most marine surface waters, becomes unavailable. Microbial methylphosphonate use can result in the formation of methane, a potent greenhouse gas, in oxic waters where methane production is traditionally unexpected. The extent and controlling factors of such aerobic methane formation remain underexplored. Here, we show high potential net rates of methylphosphonate-driven methane formation (median 0.4 nmol methane L⁻¹ d⁻¹) in the upper water column of the western tropical North Atlantic. The rates are repressed but still quantifiable in the presence of in-situ or added phosphate, suggesting that some methylphosphonate-driven methane formation persists in phosphate-replete waters. The genetic potential for methylphosphonate utilisation is present in and transcribed by key photo- and heterotrophic microbial taxa, such as Pelagibacterales, SAR116, and Trichodesmium. While the large cyanobacterial nitrogen-fixers dominate in the surface layer, phosphonate utilisation by Alphaproteobacteria appears to become more important in deeper depths. We estimate that at our study site, a substantial part (median 11%) of the measured surface carbon fixation can be sustained by phosphorus liberated from phosphonate utilisation, highlighting the ecological importance of phosphonates in the carbon cycle of the oligotrophic ocean.

In modern ecosystems, the carbon stable isotope (delta C-13) ratios of consumers generally conform to the principle "you are what you eat, +1 parts per thousand." However, this metric may not apply to microbial mat systems where diverse communities, using a variety of carbon substrates via multiple assimilation pathways, live in close physical association and phagocytosis is minimal or absent. To interpret the delta C-13 record of the Proterozoic and early Paleozoic, when mat-based productivity likely was widespread, it is necessary to understand how a microbially driven producer-consumer structure affects the delta C-13 compositions of biomass and preservable lipids. Protein Stable Isotope Fingerprinting (P-SIF) is a recently developed method that allows measurement of the delta C-13 values of whole proteins, separated from environmental samples and identified taxonomically via proteomics. Here, we use P-SIF to determine the trophic relationships in a microbial mat sample from Chocolate Pots Hot Springs, Yellowstone National Park (YNP), USA. In this mat, proteins from heterotrophic bacteria are indistinguishable from cyanobacterial proteins, indicating that "you are what you eat, +1 parts per thousand" is not applicable. To explain this finding, we hypothesize that sugar production and consumption dominate the net ecosystem metabolism, yielding a community in which producers and consumers share primary photosynthate as a common resource. This idea was validated by confirming that glucose moieties in exopolysaccharide were equal in delta C-13 composition to both cyanobacterial and heterotrophic proteins, and by confirming that highly C-13-depleted fatty acids (FAs) of Cyanobacteria dominate the lipid pool, consistent with flux-balance expectations for systems that overproduce primary photosynthate. Overall, the results confirm that the delta C-13 composition of microbial biomass and lipids is tied to specific metabolites, rather than to autotrophy versus heterotrophy or to individual trophic levels. Therefore, we suggest that aerobic microbial heterotrophy is simply a case of "you are what you eat."

The surface waters of the open ocean are mostly oversaturated with methane and thus act as a net source to the atmosphere. In situ production from methylated compounds or inorganic carbon has been proposed to act as a potential source of methane in the oxygenated surface waters. However, the distribution and importance of aerobic methane production in different marine regions remains poorly constrained. We investigated the processes and microorganisms involved in aerobic methane production in the surface waters of the western Tropical North Atlantic off Barbados. Using stable isotope incubation experiments, we showed that within 24 hours methane was readily produced from methylphosphonate (MPn) but not from dissolved inorganic carbon. MPn-derived methane production reached up to ca. 8 nmol l-1 d-1, with the highest rates measured in surface waters above the deep chlorophyll maximum (DCM). Additions of inorganic phosphate resulted in the suppression of methane production in the surface waters but not below the DCM. Additional controlling factors of MPn-derived methane production, both physicochemical (depth, nutrients) as well as biological (primary production, nitrogen fixation), were also investigated. Our metagenomic and metatranscriptomic analyses revealed that various microbial groups, including Trichodesmium and Alphaproteobacteria, had the capacity to utilise MPn in situ, making them potential contributors to methane production in this region. Overall, our results highlight the importance of MPn-derived methane production in the phosphate-limited western Tropical North Atlantic and identify the controlling factors that may regulate aerobic methane production and thereby exert control over marine methane emissions.

Symbiotic N₂-fixing microorganisms have a crucial role in the assimilation of nitrogen by eukaryotes in nitrogen-limited environments^1–3. Particularly among land plants, N₂-fixing symbionts occur in a variety of distantly related plant lineages and often involve an intimate association between host and symbiont^2,4. Descriptions of such intimate symbioses are lacking for seagrasses, which evolved around 100 million years ago from terrestrial flowering plants that migrated back to the sea⁵. Here we describe an N₂-fixing symbiont, ‘Candidatus Celerinatantimonas neptuna’, that lives inside seagrass root tissue, where it provides ammonia and amino acids to its host in exchange for sugars. As such, this symbiosis is reminiscent of terrestrial N₂-fixing plant symbioses. The symbiosis between Ca. C. neptuna and its host Posidonia oceanica enables highly productive seagrass meadows to thrive in the nitrogen-limited Mediterranean Sea. Relatives of Ca. C. neptuna occur worldwide in coastal ecosystems, in which they may form similar symbioses with other seagrasses and saltmarsh plants. Just like N₂-fixing microorganisms might have aided the colonization of nitrogen-poor soils by early land plants⁶, the ancestors of Ca. C. neptuna and its relatives probably enabled flowering plants to invade nitrogen-poor marine habitats, where they formed extremely efficient blue carbon ecosystems⁷.

Oligotrophic ocean gyre ecosystems may be expanding due to rising global temperatures [1,2,3,4,5]. Models predicting carbon flow through these changing ecosystems require accurate descriptions of phytoplankton communities and their metabolic activities [6]. We therefore measured distributions and activities of cyanobacteria and small photosynthetic eukaryotes throughout the euphotic zone on a zonal transect through the South Pacific Ocean, focusing on the ultraoligotrophic waters of the South Pacific Gyre (SPG). Bulk rates of CO₂ fixation were low (0.1 µmol C l⁻¹ d⁻¹) but pervasive throughout both the surface mixed-layer (upper 150 m), as well as the deep chlorophyll a maximum of the core SPG. Chloroplast 16S rRNA metabarcoding, and single-cell ¹³CO₂ uptake experiments demonstrated niche differentiation among the small eukaryotes and picocyanobacteria. Prochlorococcus abundances, activity, and growth were more closely associated with the rims of the gyre. Small, fast-growing, photosynthetic eukaryotes, likely related to the Pelagophyceae, characterized the deep chlorophyll a maximum. In contrast, a slower growing population of photosynthetic eukaryotes, likely comprised of Dictyochophyceae and Chrysophyceae, dominated the mixed layer that contributed 65–88% of the areal CO₂ fixation within the core SPG. Small photosynthetic eukaryotes may thus play an underappreciated role in CO₂ fixation in the surface mixed-layer waters of ultraoligotrophic ecosystems.

Biological N₂fixation was key to the expansion of life on early Earth. The N₂-fixing microorganisms and the nitrogenase type used in the Proterozoic are unknown, although it has been proposed that the canonical molybdenum-nitrogenase was not used due to low molybdenum availability. We investigate N₂fixation in Lake Cadagno, an analogue system to the sulfidic Proterozoic continental margins, using a combination of biogeochemical, molecular and single cell techniques. In Lake Cadagno, purple sulfur bacteria (PSB) are responsible for high N₂fixation rates, to our knowledge providing the first direct evidence for PSB in situ N₂fixation. Surprisingly, no alternative nitrogenases are detectable, and N₂fixation is exclusively catalyzed by molybdenum-nitrogenase. Our results show that molybdenum-nitrogenase is functional at low molybdenum conditions in situ and that in contrast to previous beliefs, PSB may have driven N₂fixation in the Proterozoic ocean.

The science guiding the \EURECA campaign and its measurements are presented. \EURECA comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic — eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, \EURECA marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that \EURECA explored — from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation — are presented along with an overview \EURECA’s outreach activities, environmental impact, and guidelines for scientific practice.