Anoxic chlorophyll maximum enhances local organic matter remineralization and nitrogen loss in Lake Tanganyika

Callbeck, Cameron M.; Ehrenfels, Benedikt; Baumann, Karl; Wehrli, Bernhard; Schubert, Carsten J.

doi:10.1038/s41467-021-21115-5

Cited by 27 publications

(31 citation statements)

References 81 publications

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“…Upon return to the laboratory, the filters were stored at −80°C. Filters were then thawed and washed with TE buffer (1x) prior to DNA extraction using the AllPrep DNA/RNA extraction kit from Qiagen (Callbeck et al, 2021). The metagenomes were sequenced using the Illumina NextSeq platform to generate 150 bp paired end reads (averaging ∼350 bp in length; Novogene Hong Kong).…”

Section: Dna Sampling and Metagenomicsmentioning

confidence: 99%

Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

Ehrenfels¹,

Bartosiewicz²,

Mbonde³

et al. 2021

Front. Environ. Sci.

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In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium- to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extent the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted, but phosphate remained available near the surface, high densities of diazotrophic cyanobacteria were associated with a low nitrate supply to surface waters. Our data provide first support for two conceptual scenarios, where the relative position of the thermocline and the euphotic depth may create a functional niche for diazotrophic cyanobacteria: when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline, diazotrophic cyanobacteria, comprising Dolichospermum and Anabaenopsis, are key players in the medium-to large-sized phytoplankton community. By contrast, a thermocline located within the euphotic zone allows for a rapid vertical transport of nitrate for a thriving nitrate-assimilating phytoplankton community that evidently outcompetes diazotrophic cyanobacteria. This study highlights that, under nitrogen-depleted conditions, diazotrophic cyanobacteria can also grow in response to a reduced nutrient resupply to the productive surface waters.

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Section: Dna Sampling and Metagenomicsmentioning

confidence: 99%

Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

Ehrenfels¹,

Bartosiewicz²,

Mbonde³

et al. 2021

Front. Environ. Sci.

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“…Carbon fixation incubations and rate calculations were done as described in Schunck et al (2013) and Callbeck et al (2021). Briefly, samples were carefully filled from the Niskin into 4.5 L polycarbonate bottles capped with polypropylene membranes.…”

Section: Co2 Fixation Ratesmentioning

confidence: 99%

Isotopic signatures induced by upwelling reveal regional fish populations in Lake Tanganyika

Ehrenfels

Junker

Namutebi

et al. 2021

Preprint

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Lake Tanganyika's pelagic fish sustain the second largest inland fishery in Africa and are under pressure from heavy fishing and global warming related increases in stratification. Only little is known about whether basin-scale hydrodynamics - including a more stratified north and an upwelling-driven south - lead to regional fish populations with varying ecological adaptations. Here, we examine whether the basin-scale dynamics leave distinct isotopic imprints in the pelagic fish of Lake Tanganyika, which may reveal differences in habitat, diet, or lipid content. We conducted two lake-wide campaigns during different seasons and collected physical, nutrient, chlorophyll, phytoplankton and zooplankton data. Additionally, we analyzed the pelagic fish - the clupeids Stolothrissa tanganicae, Limnothrissa miodon and four Lates species - for their isotopic and elemental carbon (C) and nitrogen (N) compositions. The δ13C values were significantly higher in the productive south after the upwelling/mixing period across all trophic levels, implying that the fish have regional foraging grounds, and thus record these latitudinal isotope gradients. However, the degree of regional isolation is insufficient to suppress lake-wide gene flow, suggesting that the fish form regional populations only on a basin-wide and seasonal scale. Based on δ15N and C:N ratios, we found no strong evidence for varying diets or lipid contents between those populations. Additional analyses revealed that isotopic variations between specimens from the same location are not linked to genetic differences. Our findings provide fundamental insight on the connectivity and ecology of Lake Tanganyika's pelagic fish and imply that sustainable management strategies may adopt basin-scale fishing quotas.

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“…Suitable growth conditions and N substrates are key factors for the removal process. Accordingly, parameters that influence N removal rates are NO 3 concentration, organic matter (OM) input and remineralization, or redox conditions, indicated e.g., by O 2 concentrations, among others 1,4,10,11,16,[18][19][20][21][22][23] . Most of these factors are intricately linked to the overall trophic status of lakes.…”

Section: Importance ¶mentioning

confidence: 99%

“…Microbial ecology has made considerable progress over recent decades on detecting, characterizing, and quantifying the key gene families encoding for central enzymatic systems of the N transformation pathways (N transformation genes) in environmental systems using a variety of molecular tools 15,17,[26][27][28][29] . This has improved our understanding of the microbial ecology of nitrification, complete ammonia oxidation (comammox), assimilatory and dissimilatory nitrate reduction to ammonia (ANRA and DNRA), denitrification, anammox, and N 2 fixation 1,14,22,27,28,30,31 . Metagenomic sequencing technology allows for a more holistic view on microbial N cycling by characterization of multiple N gene families in parallel 32 .…”

Section: Importance ¶mentioning

confidence: 99%

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Trophic status and local conditions affect microbial potential for denitrification versus internal nitrogen cycling in lake sediments

Thoma

et al. 2021

Preprint

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The nitrogen (N) cycle is of global importance as N is an essential element and a limiting nutrient in terrestrial and aquatic ecosystems. Excessive anthropogenic N fertilizer usage threatens sensitive downstream aquatic ecosystems. Although freshwater lake sediments remove N through various microbial transformation processes, few studies have investigated the microbial communities involved. In an integrated biogeochemical and microbiological study on a eutrophic and oligotrophic lake, we estimated N removal rates in the sediments from porewater concentration gradients. Simultaneously, the abundance of different microbial N transformation genes was investigated using metagenomics on a seasonal and spatial scale. We observed that contrasting nutrient concentrations in the sediments were reflected in distinct microbial community compositions and significant differences in the abundance of various N transformation genes. Within each lake, we observed a more pronounced spatial than seasonal variability. The eutrophic Lake Baldegg showed a higher denitrification potential with higher nosZ gene (N2O reductase) abundance and higher nirS:nirK (nitrite reductase) ratio, indicating a greater capacity for complete denitrification. Correspondingly, this lake had a higher N removal efficiency. The oligotrophic Lake Sarnen, in contrast, had a higher potential for DNRA and nitrification, and specifically a high abundance of Nitrospirae, including some capable of comammox. In general, the oligotrophic lake ecosystems had a higher microbial diversity, thus acting as an important habitat for oligotrophic microbes. Our results demonstrate that knowledge of the genomic N transformation potential is important for interpreting N process rates and understanding the limitations of the N cycle response to environmental drivers.

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Anoxic chlorophyll maximum enhances local organic matter remineralization and nitrogen loss in Lake Tanganyika

Cited by 27 publications

References 81 publications

Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

Isotopic signatures induced by upwelling reveal regional fish populations in Lake Tanganyika

Trophic status and local conditions affect microbial potential for denitrification versus internal nitrogen cycling in lake sediments

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