Carbon isotope evidence for large methane emissions to the Proterozoic atmosphere

The Proterozoic Era records two periods of abundant positive carbon isotope excursions (CIEs), conventionally interpreted as resulting from increased organic carbon burial and leading to Earth’s surface oxygenation. As strong spatial variations in the amplitude and duration of these excursions are uncovered, this interpretation is challenged. Here, by studying the carbon cycle in the Dziani Dzaha Lake, we propose that they could be due to regionally variable methane emissions to the atmosphere. This lake presents carbon isotope signatures deviated by ~  + 12‰ compared to the modern ocean and shares a unique combination of analogies with putative Proterozoic lakes, interior seas or restricted epireic seas. A simple box model of its Carbon cycle demonstrates that its current isotopic signatures are due to high primary productivity, efficiently mineralized by methanogenesis, and to subsequent methane emissions to the atmosphere. By analogy, these results might allow the reinterpretation of some positive CIEs as at least partly due to regionally large methane emissions. This supports the view that methane may have been a major greenhouse gas during the Proterozoic Era, keeping the Earth from major glaciations, especially during periods of positive CIEs, when increased organic carbon burial would have drowned down atmospheric CO2

Microbial food web structural and functional responses to oyster and fish as top predators

International audienceThe impact of fish and oysters on components of the pelagic microbial food web (MFW) was studied in a 10 d mesocosm experiment using Mediterranean coastal waters. Two mesocosms contained natural water only (‘Controls’), 2 contained natural water with Crassostrea gigas (‘Oyster’), and 2 contained natural water with Atherina spp. (‘Fish’). Abundances and biomasses of microorganisms (viruses, bacteria, phytoplankton, heterotrophic flagellates, and ciliates) were measured to estimate their contribution to the total microbial carbon biomass. Two MFW indices, the microbial autotroph:heterotroph C biomass ratio (A:H) structural index and the gross primary production:respiration ratio (GPP:R) functional index, were defined. In the Fish mesocosms, selective predation on zooplankton led to a trophic cascade with 51% higher phyto- plankton C biomass and consequently higher A:H and GPP:R than in the Controls. By the end of the experiment, the Oyster mesocosms had a bacterial C biomass 87% higher and phytoplankton C biomass 93% lower than the Controls, giving significantly lower A:H and GPP:R (1), whereas oyster activities made the MFW more hetero- trophic (both indices <1). These MFW indices can therefore be used to assess the impact of multi- ple local and global forcing factors on the MFW. The results presented here also have implications for sustainable management of coastal environments, suggesting that intense cultivation of filter feeders can be coupled with management to encourage wild local zooplanktivorous fishes to maintain a more resilient system and preserve the equilibrium of the MFW

Results of Sobol’s sensitivity analysis.

<p>Ranking of first index sensitivities for the most relevant 5 parameters affecting dinoflagellate abundance (<i>H</i>; red bars), infected dinoflagellate cells (<i>I</i>; grey bars) and dinospore abundance (<i>P</i>; black bars) under oligotrophic and eutrophic conditions (1 and 36 μM nitrate, respectively). Parameters are numbered from 1 (most influencing) to 5. Meanings of parameter symbols are the same as indicated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127623#pone.0127623.t001" target="_blank">Table 1</a>.</p

Values of parameters and state variables considered in the numerical simulations.

<p>^aOnly <i>Prorocentrum triestinum</i> was considered because this species contributed to 99% of the total dinoflagellate abundance in Thau Lagoon (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127623#pone.0127623.s001" target="_blank">S1 Appendix</a>, section S1-2).</p><p>^bMaximal growth rate of <i>P</i>. <i>triestinum</i>.</p><p>^cAverage value of half saturation constants for nitrate uptake of dinoflagellate species presented in Table 3.7 of this author.</p><p>^dNitrogen cell quota of <i>Prorocentrum micans</i>.</p><p>^eParameters of <i>Amoebophrya</i> sp. infecting <i>Karlodinium micrum</i>.</p><p>^fAverage value of the mean doubling rates (d^–1) of <i>Leptocylindrus minimus</i>, <i>Leptocylindrus danicus</i>, <i>Cylindrotheca closterium</i> and <i>Thalassionema nitzschioides</i> (the dominant diatoms species in Thau Lagoon; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127623#pone.0127623.s001" target="_blank">S1 Appendix</a>, section S1-2).</p><p>^gHalf saturation constant for nitrate uptake of <i>Pseudo-nitzschia</i> sp. shown in Table 3.7 of this author.</p><p>^hAverage values of nitrogen cell quota of diatom species presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127623#pone.0127623.t001" target="_blank">Table 1</a> of these authors.</p><p>ⁱAverage value of nanophytoplankton growth rates (d^–1) presented by these authors in their <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127623#pone.0127623.g004" target="_blank">Fig 4</a> (only control experiments, with no nutrient addition, were considered).</p><p>^jValue for nanophytoplankton natural assemblages in Thau lagoon presented by these authors.</p><p>^kAverage values of nitrogen cell quota of nanoplankton species presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127623#pone.0127623.t001" target="_blank">Table 1</a> of these authors.</p><p>^lAverage values of <i>Tiarina fusum</i> feeding on <i>Lingulodinium polyedrum</i> and <i>Scrippsiella trochoidea</i> (values in ng C^–1 were transformed to cells L^–1 considering carbon content per cell) given by the authors.</p><p>^mBased on average values estimated from growth rates of <i>Brachionus plicatilis</i>, <i>Brachionus rotundiformis</i> and <i>Brachionus</i> sp. feeding on <i>Tetraselmis suecica</i> and <i>Nannochloris atomus</i> (prey concentrations presented in ng C^–1 were converted to cells L^–1 by considering the cellular carbon content of a cell with equivalent spherical diameter of 10 μm and the equation for carbon conversion given by [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127623#pone.0127623.ref029" target="_blank">29</a>]).</p><p>Values of parameters and state variables considered in the numerical simulations.</p

Consensus phylogenetic tree (maximum likelihood) constructed using 16S rRNA gene sequences (1310 bp).

<p>Clustering of Dziani Dzaha <i>Arthrospira</i> sp. (in bold) with related Oscillatoriales species is provided with accession number and collection codes. <i>Gloeobacter violaceus</i> PCC7421 was used as outgroup. Number near nodes indicates bootstrap values >50% for neighbor joining, maximum parsimony and maximum likelihood analyses.</p

Chlorophyll <i>a</i> dynamics in the Dziani Dzaha during 2010 and 2011 surveys.

<p>(A) Sum of all chl <i>a</i> measures depending on sampling depth for 2010 (closed circles, n = 27) and 2011 (open circles, n = 37) surveys. (B) Box-plot indicating median, 10th and 90th percentiles (whiskers) and outliers (closed circles) for chl <i>a</i> concentrations including single samples collected in 2007 (n = 3) and 2009 (n = 3), and 2010 (n = 33) and 2011 (n = 47) surveys.</p