The role of methanotrophy in the microbial carbon metabolism of temperate lakes

Previous stable isotope and biomarker evidence has indicated that methanotrophy is an important pathway in the microbial loop of freshwater ecosystems, despite the low cell abundance of methane-oxidizing bacteria (MOB) and the low methane concentrations relative to the more abundant dissolved organic carbon (DOC). However, quantitative estimations of the relative contribution of methanotrophy to the microbial carbon metabolism of lakes are scarce, and the mechanism allowing methanotrophy to be of comparable importance to DOC-consuming heterotrophy remained elusive. Using incubation experiments, microscopy, and multiple water column profiles in six temperate lakes, we show that MOB play a much larger role than their abundances alone suggest because of their larger cell size and higher specific activity. MOB activity is tightly constrained by the local methane:oxygen ratio, with DOC-rich lakes with large hypolimnetic volume fraction showing a higher carbon consumption through methanotrophy than heterotrophy at the whole water column level. Our findings suggest that methanotrophy could be a critical microbial carbon consumption pathway in many temperate lakes, challenging the prevailing view of a DOC-centric microbial metabolism in these ecosystems.

Bolstering fitness via CO2 fixation and organic carbon uptake: mixotrophs in modern groundwater

Current understanding of organic carbon inputs into ecosystems lacking photosynthetic primary production is predicated on data and inferences derived almost entirely from metagenomic analyses. The elevated abundances of putative chemolithoautotrophs in groundwaters suggest that dark CO2 fixation is an integral component of subsurface trophic webs. To understand the impact of autotrophically fixed carbon, the flux of CO2-derived carbon through various populations of subsurface microbiota must first be resolved, both quantitatively and temporally. Here we implement novel Stable Isotope Cluster Analysis to render a time-resolved and quantitative evaluation of 13CO2-derived carbon flow through a groundwater community in microcosms stimulated with reduced sulfur compounds. We demonstrate that mixotrophs, not strict autotrophs, were the most abundant active organisms in groundwater microcosms. Species of Hydrogenophaga, Polaromonas, Dechloromonas, and other metabolically versatile mixotrophs drove the production and remineralization of organic carbon. Their activity facilitated the replacement of 43% and 80% of total microbial carbon stores in the groundwater microcosms with 13C in just 21 and 70 days, respectively. The mixotrophs employed different strategies for satisfying their carbon requirements by balancing CO2 fixation and uptake of available organic compounds. These different strategies might provide fitness under nutrient-limited conditions, explaining the great abundances of mixotrophs in other oligotrophic habitats, such as the upper ocean and boreal lakes.

The optimization of table beet cultivation by using a boron-containing chelate

In the field experiment, the possibility of optimizing the cultivation of table beet Beta vulgaris L. using a borate complex based on ethylenediamine disuccinic acid (B-EDDS) was investigated. Before sowing, beet seeds were soaked for a day in a solution of this compound, as well as in solutions of boric acid and a borate complex based on ethylenediaminetetraacetic acid (B-EDTA), taken for comparison, with a concentration of solutes of 1.5·10-3 mol/L. During the growing season, the plants were sprayed twice with experimental solutions at a rate of 100 ml/m2. In the first decade of August, the content of photosynthetic pigments in plant leaves and microbial carbon in the soil was determined, after harvesting, sugars and betanin in root crops were determined by spectrophotometric methods. The boron content in beet root crops was analyzed by the fluorometric method. According to the results of the two-year experiment, all three boron-containing compounds, to varying degrees, had a positive effect on the experimental plant. In decreasing efficiency, they can be arranged in a row: B-EDDS > H3BO3> B-EDTA. Treatment with the solution of borate-ethylenediamine disuccinate complex increased the boron content in root crops by 65% in comparison with treatment with boric acid. In comparison with the control variant, the sugar content in root crops increased by 37%, the content of betanin increased by 25% and the yield of root crops increased by 39%. At the same time, the mass of microbial carbon in the soil increased by 20%, which serves as one of the arguments confirming the ecological safety of the compound under study. Judging by the results of the experiment, borate-ethylenediamine disuccinate has good prospects for use as boron micronutrient fertilizer.

Comparing an exponential respiration model to alternative models for soil respiration components in a Canadian wildfire chronosequence (FireResp v1.0)

Forest fires modify soil organic carbon and suppress soil respiration for many decades after the initial disturbance. The associated changes in soil autotrophic and heterotrophic respiration from the time of the forest fire, however, are less well characterized. The FireResp model predicts soil autotrophic and heterotrophic respiration parameterized with a novel dataset across a fire chronosequence in the Yukon and Northwest Territories of Canada. The dataset consisted of soil incubation experiments and field measurements of soil respiration and soil carbon stocks. The FireResp model contains submodels that consider a Q10 (exponential) model of respiration compared to models of heterotrophic respiration using Michaelis–Menten kinetics parameterized with soil microbial carbon. For model evaluation we applied the Akaike information criterion and compared predicted patterns in components of soil respiration across the chronosequence. Parameters estimated with data from the 5 cm soil depth had better model–data comparisons than parameters estimated with data from the 10 cm soil depth. The model–data fit was improved by including parameters estimated from soil incubation experiments. Models that incorporated microbial carbon with Michaelis–Menten kinetics reproduced patterns in autotrophic and heterotrophic soil respiration components across the chronosequence. Autotrophic respiration was associated with aboveground tree biomass at more recently burned sites, but this association was less robust at older sites in the chronosequence. Our results provide support for more structured soil respiration models than standard Q10 exponential models.