Biogeochemistry of a large and deep tropical lake (Lake Kivu, East Africa: insights from a stable isotope study covering an annual cycle

Morana, Cédric; Darchambeau, François; Roland, Fleur; Borges, Alberto; Muvundja, Fabrice A.; Kelemen, Zita; Masilya, Pascal Mulungula; Descy, Jean‐Pierre; Bouillon, Steven

doi:10.5194/bg-12-4953-2015

Cited by 26 publications

(10 citation statements)

References 61 publications

(83 reference statements)

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“…). Stability of δ 13 C DOC throughout the year and uncoupling between the stable carbon isotopic compositions of DOC and POC has previously been observed and was attributed to rapid mineralization of phytoplankton‐derived DOC resulting in low contributions (< 1%) of locally produced DOC to the total DOC pool (Morana et al ). Similarly, in a productive system such as Lake Naarden, the stability of δ 13 C DOC likely also indicates that concentrations of locally produced phytoplankton‐derived DOC are much lower than allochthonous DOC and hence too low to affect the isotopic signal of the large total DOC pool.…”

Section: Discussionmentioning

confidence: 83%

Seasonal variability in phytoplankton stable carbon isotope ratios and bacterial carbon sources in a shallow Dutch lake

Lammers

Reichart

Middelburg

2017

Limnology & Oceanography

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Ecosystem metabolism of lakes strongly depends on the relative importance of local vs. allochthonous carbon sources and on microbial food-web functioning and structure. Over the year ecosystem metabolism varies as a result of seasonal changes in environmental parameters such as nutrient levels, light, temperature, and variability in the food web. This is reflected in isotopic compositions of phytoplankton and bacteria. Here, we present the results of a 17-month study on carbon dynamics in two basins of Lake Naarden, The Netherlands. One basin was restored after anthropogenic eutrophication, whereas the other basin remained eutrophic. We analyzed natural stable carbon isotope abundances (d 13 C) of dissolved inorganic carbon, dissolved organic carbon and macrophytes, and combined these data with compound-specific d 13 C analyses of phospholipid-derived fatty acids, produced by phytoplankton and bacteria. Isotopic fractionation (e) between phytoplankton biomass and CO 2(aq) was similar for diatoms and other eukaryotic phytoplankton, and differences between sampling sites were small. Highest e values were observed in winter with values of 23.5 6 0.6& for eukaryotic phytoplankton and 13.6 6 0.3& for cyanobacteria. Lowest e values were observed in summer: 10.5 6 0.3& for eukaryotic phytoplankton and 2.7 6 0.1& for cyanobacteria. The annual range in d 13 C bact was between 6.9& and 8.2& for the restored and eutrophic basin, respectively, while this range was between 11.6& and 13.1& for phytoplankton in the restored and eutrophic basin, respectively. Correlations between d 13 C phyto and d 13 C bact were strong at both sites. During summer and fall, bacterial biomass derives mainly from locally produced organic matter, with minor allochthonous contributions. Conversely, during winter, bacterial dependence on allochthonous carbon was 39-77% at the restored site, and 17-46% at the eutrophic site.

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Section: Discussionmentioning

confidence: 83%

Seasonal variability in phytoplankton stable carbon isotope ratios and bacterial carbon sources in a shallow Dutch lake

Lammers

Reichart

Middelburg

2017

Limnology & Oceanography

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“…Samples for δ 13 C-CH 4 determination were collected in 60 mL serum bottles following the same procedure as samples for CH 4 concentration determination. In the laboratory, δ 13 C-CH 4 was measured as described in Morana et al (2015b). Briefly, a 20 mL helium headspace was created in the serum bottles, then samples were vigorously shaken and left to equilibrate overnight.…”

Section: Environmental Setting Of the Study Sitesmentioning

confidence: 99%

Methane paradox in tropical lakes? Sedimentary fluxes rather than pelagic production in oxic conditions sustain methanotrophy and emissions to the atmosphere

et al. 2020

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Abstract. Despite growing evidence that methane (CH4) formation could also occur in well-oxygenated surface fresh waters, its significance at the ecosystem scale is uncertain. Empirical models based on data gathered at high latitude predict that the contribution of oxic CH4 increases with lake size and should represent the majority of CH4 emissions in large lakes. However, such predictive models could not directly apply to tropical lakes, which differ from their temperate counterparts in some fundamental characteristics, such as year-round elevated water temperature. We conducted stable-isotope tracer experiments, which revealed that oxic CH4 production is closely related to phytoplankton metabolism and is a common feature in five contrasting African lakes. Nevertheless, methanotrophic activity in surface waters and CH4 emissions to the atmosphere were predominantly fuelled by CH4 generated in sediments and physically transported to the surface. Indeed, CH4 bubble dissolution flux and diffusive benthic CH4 flux were several orders of magnitude higher than CH4 production in surface waters. Microbial CH4 consumption dramatically decreased with increasing sunlight intensity, suggesting that the freshwater “CH4 paradox” might be also partly explained by photo-inhibition of CH4 oxidizers in the illuminated zone. Sunlight appeared as an overlooked but important factor determining the CH4 dynamics in surface waters, directly affecting its production by photoautotrophs and consumption by methanotrophs.

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“…Large amounts of methane and CO 2 accumulate in the deep monimolimnion, with the corresponding risk of limnic gas eruptions (Zhang and Kling, 2006; Hirslund, 2012). However, methanotrophs, mainly close relatives of type X CH 4 -oxidizing bacteria (Pasche et al, 2011), very actively oxidize methane (Morana et al, 2015b), mostly aerobically in the oxycline, driving low methane release to the atmosphere (Morana et al, 2015a; Zigah et al, 2015). Nutrient availability in upper layers is low, with average mixolimnetic chlorophyll-a concentrations of 2.2 mg m -3 .…”

Section: Photoferrotrophy In Modern Water Columns: Occurrence and Sigmentioning

confidence: 99%

Photoferrotrophy: Remains of an Ancient Photosynthesis in Modern Environments

et al. 2017

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Photoferrotrophy, the process by which inorganic carbon is fixed into organic matter using light as an energy source and reduced iron [Fe(II)] as an electron donor, has been proposed as one of the oldest photoautotrophic metabolisms on Earth. Under the iron-rich (ferruginous) but sulfide poor conditions dominating the Archean ocean, this type of metabolism could have accounted for most of the primary production in the photic zone. Here we review the current knowledge of biogeochemical, microbial and phylogenetic aspects of photoferrotrophy, and evaluate the ecological significance of this process in ancient and modern environments. From the ferruginous conditions that prevailed during most of the Archean, the ancient ocean evolved toward euxinic (anoxic and sulfide rich) conditions and, finally, much after the advent of oxygenic photosynthesis, to a predominantly oxic environment. Under these new conditions photoferrotrophs lost importance as primary producers, and now photoferrotrophy remains as a vestige of a formerly relevant photosynthetic process. Apart from the geological record and other biogeochemical markers, modern environments resembling the redox conditions of these ancient oceans can offer insights into the past significance of photoferrotrophy and help to explain how this metabolism operated as an important source of organic carbon for the early biosphere. Iron-rich meromictic (permanently stratified) lakes can be considered as modern analogs of the ancient Archean ocean, as they present anoxic ferruginous water columns where light can still be available at the chemocline, thus offering suitable niches for photoferrotrophs. A few bacterial strains of purple bacteria as well as of green sulfur bacteria have been shown to possess photoferrotrophic capacities, and hence, could thrive in these modern Archean ocean analogs. Studies addressing the occurrence and the biogeochemical significance of photoferrotrophy in ferruginous environments have been conducted so far in lakes Matano, Pavin, La Cruz, and the Kabuno Bay of Lake Kivu. To date, only in the latter two lakes a biogeochemical role of photoferrotrophs has been confirmed. In this review we critically summarize the current knowledge on iron-driven photosynthesis, as a remains of ancient Earth biogeochemistry.

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Biogeochemistry of a large and deep tropical lake (Lake Kivu, East Africa: insights from a stable isotope study covering an annual cycle

Cited by 26 publications

References 61 publications

Seasonal variability in phytoplankton stable carbon isotope ratios and bacterial carbon sources in a shallow Dutch lake

Seasonal variability in phytoplankton stable carbon isotope ratios and bacterial carbon sources in a shallow Dutch lake

Methane paradox in tropical lakes? Sedimentary fluxes rather than pelagic production in oxic conditions sustain methanotrophy and emissions to the atmosphere

Photoferrotrophy: Remains of an Ancient Photosynthesis in Modern Environments

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