The Trichodesmium genus comprises some of the most abundant N2-fixing organisms in oligotrophic marine ecosystems. Since nitrogenase, the key enzyme for N2 fixation, is irreversibly inhibited upon O2 exposure, these organisms have to coordinate their N2-fixing ability with simultaneous photosynthetic O2 production. Although being the principal object of many laboratory and field studies, the overall process of how Trichodesmium reconciles these two mutually exclusive processes remains unresolved. This is in part due to contradictory results that fuel the Trichodesmium enigma. In this review, we sift through methodological details that could potentially explain the discrepancy between findings related to Trichodesmium's physiology. In doing so, we exhaustively contrast studies concerning both spatial and temporal nitrogenase protective strategies, with particular attention to more recent insights. Finally, we suggest new experimental approaches for solving the complex orchestration of N2 fixation and photosynthesis in Trichodesmium.

• Klíčová slova
• O2-scavenging mechanisms, cyanobacteria, diazocyte, immunolabelling, method comparison, microscopy,
• Publikační typ
• časopisecké články MeSH
• přehledy MeSH

Stable isotope probing (SIP) combined with nano-scale secondary ion mass spectrometry (nanoSIMS) is a powerful approach to quantify assimilation rates of elements such as C and N into individual microbial cells. Here, we use mathematical modeling to investigate how the derived rate estimates depend on the model used to describe substrate assimilation by a cell during a SIP incubation. We show that the most commonly used model, which is based on the simplifying assumptions of linearly increasing biomass of individual cells over time and no cell division, can yield underestimated assimilation rates when compared to rates derived from a model that accounts for cell division. This difference occurs because the isotopic labeling of a dividing cell increases more rapidly over time compared to a non-dividing cell and becomes more pronounced as the labeling increases above a threshold value that depends on the cell cycle stage of the measured cell. Based on the modeling results, we present formulae for estimating assimilation rates in cells and discuss their underlying assumptions, conditions of applicability, and implications for the interpretation of intercellular variability in assimilation rates derived from nanoSIMS data, including the impacts of storage inclusion metabolism. We offer the formulae as a Matlab script to facilitate rapid data evaluation by nanoSIMS users.

• Klíčová slova
• assimilation rates, cell growth model, nanoSIMS, stable isotope probing, storage inclusions,
• Publikační typ
• časopisecké články MeSH

Nitrogen-fixing organisms are of importance to the environment, providing bioavailable nitrogen to the biosphere. Quantitative models have been used to complement the laboratory experiments and in situ measurements, where such evaluations are difficult or costly. Here, we review the current state of the quantitative modeling of nitrogen-fixing organisms and ways to enhance the bridge between theoretical and empirical studies.

• Klíčová slova
• Mathematical model, Nitrogen fixation, Nitrogen fixers, Oxygen, Photosynthesis, Quantitative model,
• Publikační typ
• časopisecké články MeSH
• přehledy MeSH

Nitrogen fixing plankton provide nitrogen to fuel marine ecosystems and biogeochemical cycles but the factors that constrain their growth and habitat remain poorly understood. Here we investigate the importance of metabolic specialization in unicellular diazotroph populations, using laboratory experiments and model simulations. In clonal cultures of Crocosphaera watsonii and Cyanothece sp. spiked with 15N2, cellular 15N enrichment developed a bimodal distribution within colonies, indicating that N2 fixation was confined to a subpopulation. In a model of population metabolism, heterogeneous nitrogen (N2) fixation rates substantially reduce the respiration rate required to protect nitrogenase from O2. The energy savings from metabolic specialization is highest at slow growth rates, allowing populations to survive in deeper waters where light is low but nutrients are high. Our results suggest that heterogeneous N2 fixation in colonies of unicellular diazotrophs confers an energetic advantage that expands the ecological niche and may have facilitated the evolution of multicellular diazotrophs.

• MeSH
• biologická evoluce * MeSH
• biologické modely MeSH
• Cyanothece růst a vývoj   metabolismus   MeSH
• dusík metabolismus   MeSH
• ekosystém MeSH
• energetický metabolismus * MeSH
• fixace dusíku * MeSH
• fyziologická adaptace MeSH
• počítačová simulace MeSH
• sinice růst a vývoj   metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• dusík MeSH

Trichodesmium is an important dinitrogen (N2)-fixing cyanobacterium in marine ecosystems. Recent nucleic acid analyses indicate that Trichodesmium colonies with their diverse epibionts support various nitrogen (N) transformations beyond N2 fixation. However, rates of these transformations and concentration gradients of N compounds in Trichodesmium colonies remain largely unresolved. We combined isotope-tracer incubations, micro-profiling and numeric modelling to explore carbon fixation, N cycling processes as well as oxygen, ammonium and nitrate concentration gradients in individual field-sampled Trichodesmium colonies. Colonies were net-autotrophic, with carbon and N2 fixation occurring mostly during the day. Ten percent of the fixed N was released as ammonium after 12-h incubations. Nitrification was not detectable but nitrate consumption was high when nitrate was added. The consumed nitrate was partly reduced to ammonium, while denitrification was insignificant. Thus, the potential N transformation network was characterised by fixed N gain and recycling processes rather than denitrification. Oxygen concentrations within colonies were ~60-200% air-saturation. Moreover, our modelling predicted steep concentration gradients, with up to 6-fold higher ammonium concentrations, and nitrate depletion in the colony centre compared to the ambient seawater. These gradients created a chemically heterogeneous microenvironment, presumably facilitating diverse microbial metabolisms in millimetre-sized Trichodesmium colonies.

• MeSH
• amoniové sloučeniny metabolismus   MeSH
• autotrofní procesy MeSH
• denitrifikace MeSH
• dusičnany metabolismus   MeSH
• dusík metabolismus   MeSH
• fixace dusíku MeSH
• koloběh dusíku MeSH
• koloběh uhlíku MeSH
• kyslík metabolismus   MeSH
• mořská voda mikrobiologie   MeSH
• nitrifikace MeSH
• oxid uhličitý metabolismus   MeSH
• Trichodesmium metabolismus   MeSH
• uhlík metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Research Support, U.S. Gov't, Non-P.H.S. MeSH
• Názvy látek
• amoniové sloučeniny MeSH
• dusičnany MeSH
• dusík MeSH
• kyslík MeSH
• oxid uhličitý MeSH
• uhlík MeSH

Microbial biomass is a key parameter needed for the quantification of microbial turnover rates and their contribution to the biogeochemical element cycles. However, estimates of microbial biomass rely on empirically derived mass-to-volume relationships, and large discrepancies exist between the available empirical conversion factors. Here we report a significant nonlinear relationship between carbon mass and cell volume ([Formula: see text]; [Formula: see text]) based on direct cell mass, volume, and elemental composition measurements of 12 prokaryotic species with average volumes between 0.011 and 0.705 μm3 The carbon mass density of our measured cells ranged from 250 to 1,800 fg of C μm-3 for the measured cell volumes. Compared to other currently used models, our relationship yielded up to 300% higher carbon mass values. A compilation of our and previously published data showed that cells with larger volumes (>0.5 μm3) display a constant (carbon) mass-to-volume ratio, whereas cells with volumes below 0.5 μm3 exhibit a nonlinear increase in (carbon) mass density with decreasing volume. Small microorganisms dominate marine and freshwater bacterioplankton as well as soils and marine and terrestrial subsurface. The application of our experimentally determined conversion factors will help to quantify the true contribution of these microorganisms to ecosystem functions and global microbial biomass.IMPORTANCE Microorganisms are a major component of Earth's biosphere, and their activity significantly affects the biogeochemical cycling of bioavailable elements. To correctly determine the flux of carbon and energy in the environment, reliable estimates of microbial abundances and cellular carbon content are necessary. However, accurate assessments of cellular carbon content and dry weight are not trivial to obtain. Here we report direct measurements of cell dry and carbon mass of environmentally relevant prokaryotic microorganisms using a microfluidic mass sensor. We show a significant nonlinear relationship between carbon mass and cell volume and discuss this relationship in the light of currently used cellular mass models.

• Klíčová slova
• bacterioplankton, carbon content, microbial biomass, microorganisms, subsurface,
• MeSH
• Bacteria chemie   MeSH
• biomasa MeSH
• fyziologie bakterií * MeSH
• mořská voda mikrobiologie   MeSH
• půdní mikrobiologie * MeSH
• sladká voda mikrobiologie   MeSH
• uhlík analýza   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• uhlík MeSH