The global geographic distribution of subseafloor sedimentary microbes and the cause(s) of that distribution are largely unexplored. Here, we show that total microbial cell abundance in subseafloor sediment varies between sites by ca. five orders of magnitude. This variation is strongly correlated with mean sedimentation rate and distance from land. Based on these correlations, we estimate global subseafloor sedimentary microbial abundance to be 2.9⋅1029 cells [corresponding to 4.1 petagram (Pg) C and ∼0.6% of Earth’s total living biomass]. This estimate of subseafloor sedimentary microbial abundance is roughly equal to previous estimates of total microbial abundance in seawater and total microbial abundance in soil. It is much lower than previous estimates of subseafloor sedimentary microbial abundance. In consequence, we estimate Earth’s total number of microbes and total living biomass to be, respectively, 50–78% and 10–45% lower than previous estimates.

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Global distribution of microbial abundance and biomass in subseafloor sediment

SAR11 is a group of small, carbon-oxidizing bacteria that reach a global estimated population size of 2.4×1028 cells—approximately 25% of all plankton. They are found throughout the oceans but reach their largest numbers in stratified, oligotrophic gyres, which are an expanding habitat in the warming oceans. SAR11 likely had a Precambrian origin and, over geological time, evolved into the niche of harvesting labile, low-molecular-weight dissolved organic matter (DOM). SAR11 cells are minimal in size and complexity, a phenomenon known as streamlining that is thought to benefit them by lowering the material costs of replication and maximizing transport functions that are essential to competition at ultralow nutrient concentrations. One of the surprises in SAR11 metabolism is their ability to both oxidize and produce a variety of volatile organic compounds that can diffuse into the atmosphere. SAR11 cells divide slowly and lack many forms of regulation commonly used by bacterial cells to adjust to changing e...

SAR11 Bacteria: The Most Abundant Plankton in the Oceans.

Under the current paradigm, organic matter decomposition and nutrient cycling rates are a function of the imbalance between substrate and microbial biomass stoichiometry. Challenging this view, we demonstrate that in an individual-based model, microbial community dynamics alter relative C and N limitation during litter decomposition, leading to a system behaviour not predictable from stoichiometric theory alone. Rather, the dynamics of interacting functional groups lead to an adaptation at the community level, which accelerates nitrogen recycling in litter with high initial C : N ratios and thus alleviates microbial N limitation. This mechanism allows microbial decomposers to overcome large imbalances between resource and biomass stoichiometry without the need to decrease carbon use efficiency (CUE), which is in contrast to predictions of traditional stoichiometric mass balance equations. We conclude that identifying and implementing microbial community-driven mechanisms in biogeochemical models are necessary for accurately predicting terrestrial C fluxes in response to changing environmental conditions.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/ele.12269

Microbial community dynamics alleviate stoichiometric constraints during litter decay

The ability of microorganisms to withstand long periods with extremely low energy input has gained increasing scientific attention in recent years. Starvation experiments in the laboratory have shown that a phylogenetically wide range of microorganisms evolve fitness-enhancing genetic traits within weeks of incubation under low-energy stress. Studies on natural environments that are cut off from new energy supplies over geologic time scales, such as deeply buried sediments, suggest that similar adaptations might mediate survival under energy limitation in the environment. Yet, the extent to which laboratory-based evidence of starvation survival in pure or mixed cultures can be extrapolated to sustained microbial ecosystems in nature remains unclear. In this review, we discuss past investigations on microbial energy requirements and adaptations to energy limitation, identify gaps in our current knowledge, and outline possible future foci of research on life under extreme energy limitation.

Life under extreme energy limitation: a synthesis of laboratory- and field-based investigations.

Soil microorganisms control carbon losses from soils to the atmosphere1-3, yet their responses to climate warming are often short-lived and unpredictable4-7. Two mechanisms, microbial acclimation and substrate depletion, have been proposed to explain temporary warming effects on soil microbial activity8-10. However, empirical support for either mechanism is unconvincing. Here we used geothermal temperature gradients (> 50 years of field warming)11 and a short-term experiment to show that microbial activity (gross rates of growth, turnover, respiration and carbon uptake) is intrinsically temperature sensitive and does not acclimate to warming (+ 6 oC) over weeks or decades. Permanently accelerated microbial activity caused carbon loss from soil. However, soil carbon loss was temporary because substrate depletion reduced microbial biomass and constrained the influence of microbes over the ecosystem. A microbial biogeochemical model12-14 showed that these observations are reproducible through a modest, but permanent, acceleration in microbial physiology. These findings reveal a mechanism by which intrinsic microbial temperature sensitivity and substrate depletion together dictate warming effects on soil carbon loss via their control over microbial biomass. We thus provide a framework for interpreting the links between temperature, microbial activity and soil carbon loss on timescales relevant to Earth's climate system.

/pdf/microbial-temperature-sensitivity-and-biomass-change-explain-1b9l37x671.pdf

Microbial temperature sensitivity and biomass change explain soil carbon loss with warming.

The bacterial biomass is an essential point in microbial ecology. The dry weight and carbon con� tent of microorganisms are traditionally used for the calculation of the fluxes of carbon and energy in aquatic ecosystems. Since direct measurements of these parameters in natural conditions are rather difficult, a certain biomass-carbon converting factor is used, which is determined experimentally or by empirical calculations. However, the relationship between the cell volume and its dry weight (in particular, carbon) does not depend only on the cell size and the conditions of its growth but also on the fixation and staining methods used. We made an attempt to summarize all the present data on the relationships between the cell volume, its dry weight, and the carbon content. Thus, the principal goal of the present study was searching for a generally applicable or methodologydependent converting factor for the bacterial biomass calculation. Thereto, all the data available were grouped according to the dye used, as well as to the methods of fixation and the dry weight determination. The data on the most frequent combinations of dies and fixatives are insufficient for precise calculations; therefore, we suggest a relationship generalized for different methods of carbon and cell volume recalculations and applicable for bacterial cells larger than 0.025 μm 3 , as fgC cell -1 = 133.754 × V 0.438 .

Relationships between the cell volume and the carbon content of bacteria

Data on the bacterial and primary production, which were obtained simultaneously for the same water samples, are presented for three regions of the Kara Sea. The samples were collected for the transect westwards of the Yamal Peninsula, along the St. Anna Trough, and the transect in Ob Bay. Direct counts of the DAPI-stained bacterial cells were performed. The bacterial production and grazing rates were determined using a direct method when metabolic inhibitors vancomycin and penicillin were added. The primary production rates were estimated using the 14C method.

Bacterial and primary production in the pelagic zone of the Kara Sea

The state of the shelf pelagic ecosystem has been assessed based on multidisciplinary monitoring performed in the northeastern Black Sea in 2005–2014. Seasonal and interannual variations in sea surface temperature (SST) and chlorophyll a (Chl-a) concentration have been analyzed along with the concentration of nutrients (silicate, nitrogen, and phosphate), biomass, and taxonomic compositions of phytoplankton, zooplankton, and gelatinous macroplankton. The linear trend shows an increase in the annual average SST by 0.9°C over the last decade. An increase in the winter SST is accompanied by a decrease in the concentration of silicates in spring (p < 0.05) and an increase in summer SST, by a decrease in Chl-a concentration and biomass of diatoms in the period of summer to fall (p < 0.05). A decrease in the phosphate concentration also has a negative effect on the development of diatoms (p < 0.01). The decrease in diatom biomass caused a decrease in herbivorous zooplankton biomass in the second half of the year (p = 0.05). Correlation analysis shows no significant dependence between the biomass of gelatinous top predators and mesozooplankton biomass. The assessed current state of the shelf pelagic ecosystem is regarded as stable; however, trends of a decrease in biomass and a change in the taxonomic composition of phytoplankton and zooplankton are observed during the last 2 years; the latter is likely to result from both direct and indirect effects of temperature increase in the upper sea layer.

Marine environmental monitoring in the shelf zone of the Black Sea: Assessment of the current state of the pelagic ecosystem

Studies were conducted in shallow and deepwater areas of the Kara Sea. The abundance of bacteria (NB) and the abundance of viruses (NV) ranged within (19.4–2215.1) × 103 cells/ml and (97.6–5796.8) × 103 particles/ml, respectively. The virus to bacteria ratio varied from 1.4 to 29.1. A positive correlation was found between NB and NV (R = 0.87, n = 45, p = 0.05. Using electron transmission microscopy it was detected that the frequency of visibly infected cells of bacteria (FVIC) varied from 0.2 to 1.9% of NB. The maximum values of FVIC were recorded in the estuary of the Yenisei River. The infected cells of bacteria contained from 4 to 127 (an average of 12) phages/cell of mature viruses. Virus-mediated mortality of bacteria was 0.5% and varied from 1.4 to 16.1% of the total mortality of bacterioplankton. This indicates a minor role of viruses in the control of overabundance and production of bacterioplankton in the Kara Sea during the surveyed period.

Virioplankton in the Kara Sea: The impact of viruses on mortality of heterotrophic bacteria

The spatial distribution of the primary production (PP) and the chlorophyll a concentration (Chl) were investigated during two research cruises in the Drake Passage area in October–November of 2007 and 2008. The algorithm evaluating the integral PP (PPint) for the water column in this area was developed based on the data on the surface chlorophyll (Chls) and the incident solar irradiance obtained in 2004–2008 in the Atlantic Sector of the Southern Ocean. The results obtained both by the experimental and model approaches suggested that the Polar Front (PF) region of the Drake Passage was characterized by low values of both the PPint (<100 mg C/m2 per day) and Chls (0.08–0.20 mg/m3) in October–November. Low values of the Chls and relatively high phaeophytine a concentrations indicated the winter succession state of the phytoplankton community in the Antarctic Ocean and the southern Polar Frontal Zone (PFZ). The seasonal warming of the surface water layers and the developing pycnocline resulted in a phytoplankton bloom and a Chls concentration of more than 1 mg/m3 in mid-November in this area and the Subantarctic waters.

N. D. Romanova

Papers

Relationships between the cell volume and the carbon content of bacteria

Bacterial and primary production in the pelagic zone of the Kara Sea

Marine environmental monitoring in the shelf zone of the Black Sea: Assessment of the current state of the pelagic ecosystem

Virioplankton in the Kara Sea: The impact of viruses on mortality of heterotrophic bacteria

Spatial Variability of the Primary Production and Chlorophyll a Concentration in the Drake Passage in the Austral Spring