Environmental Growth Rates Research Articles

Bacteria on the hunt.

This study analysed environmental growth rates of bacterial predators to study their role in microbial food webs.

NET works after all? Engineering robustness through diversity

Classical Thermodynamics restricts engineering. These restrictions are independent of mechanism and kinetics, and thereby inescapable. Forgetting these restrictions can lead to over-optimistic designs for making bio-plastics from waste, and to erroneous ideas on early or new Life on this or other planets. This can be rectified by putting the thermodynamics in place. Is every biochemical network design feasible, provided one puts classical thermodynamics in place? Or, are there other, ill-recognized, generic restrictions to bioengineering?For a while a Non-Equilibrium Thermodynamics (NET) has been trying to discover behaviors of dynamical systems away from equilibrium that are completely independent of kinetics and mechanism and thereby not engineerable. The principles discovered were of limited use to bioengineering however.We here show that processes away from equilibrium must indeed depend on kinetics and mechanism, but, importantly, not on all kinetic and mechanistic details: There are limitations to what the engineering of mechanisms and kinetics can achieve. It is of course better to recognize what is impossible before trying to engineer the impossible. Importantly, the new NET methodology also shows that system properties that are possible, can be engineered only in certain ways.The new NET methodology also enables to understand, and perhaps engineer towards, a performance that, by adjusting the network, remains optimal when conditions are changing. Using our in silico discovery tool, we show that this may indeed occur in the Archeon S. solfataricus. What we call ‘variomatic’ gear shifting is a way that some cells may use to self-engineer their ways to maximal growth rates in environments that lack robust resources, such as in environments with fluctuating oxygen levels.Population heterogeneity is another mechanism that can increase the robustness of a cell factory. We discuss a NET principle that suggests ways in which one can engineer the cells’ diversity. Transcription burst size rather than kinetics should be modulated for making a diverse population perform much better than its average.

How fast-growing bacteria robustly tune their ribosome concentration to approximate growth-rate maximization.

Maximization of growth rate is an important fitness strategy for bacteria. Bacteria can achieve this by expressing proteins at optimal concentrations, such that resources are not wasted. This is exemplified for Escherichia coli by the increase of its ribosomal protein-fraction with growth rate, which precisely matches the increased protein synthesis demand. These findings and others have led to the hypothesis that E. coli aims to maximize its growth rate in environments that support growth. However, what kind of regulatory strategy is required for a robust, optimal adjustment of the ribosome concentration to the prevailing condition is still an open question. In the present study, we analyze the ppGpp-controlled mechanism of ribosome expression used by E. coli and show that this mechanism maintains the ribosomes saturated with its substrates. In this manner, overexpression of the highly abundant ribosomal proteins is prevented, and limited resources can be redirected to the synthesis of other growth-promoting enzymes. It turns out that the kinetic conditions for robust, optimal protein-partitioning, which are required for growth rate maximization across conditions, can be achieved with basic biochemical interactions. We show that inactive ribosomes are the most suitable ‘signal’ for tracking the intracellular nutritional state and for adjusting gene expression accordingly, as small deviations from optimal ribosome concentration cause a huge fractional change in ribosome inactivity. We expect to find this control logic implemented across fast-growing microbial species because growth rate maximization is a common selective pressure, ribosomes are typically highly abundant and thus costly, and the required control can be implemented by a small, simple network.

Flexible C : N ratio enhances metabolism of large phytoplankton when resource supply is intermittent

Abstract. Phytoplankton cell size influences particle sinking rate, food web interactions and biogeographical distributions. We present a model in which the uptake, storage and assimilation of nitrogen and carbon are explicitly resolved in different-sized phytoplankton cells. In the model, metabolism and cellular C : N ratio are influenced by the accumulation of carbon polymers such as carbohydrate and lipid, which is greatest when cells are nutrient starved, or exposed to high light. Allometric relations and empirical data sets are used to constrain the range of possible C : N, and indicate that larger cells can accumulate significantly more carbon storage compounds than smaller cells. When forced with extended periods of darkness combined with brief exposure to saturating irradiance, the model predicts organisms large enough to accumulate significant carbon reserves may on average synthesize protein and other functional apparatus up to five times faster than smaller organisms. The advantage of storage in terms of average daily protein synthesis rate is greatest when modeled organisms were previously nutrient starved, and carbon storage reservoirs saturated. Small organisms may therefore be at a disadvantage in terms of average daily growth rate in environments that involve prolonged periods of darkness and intermittent nutrient limitation. We suggest this mechanism is a significant constraint on phytoplankton C : N variability and cell size distribution in different oceanic regimes.

Selective consequences of catastrophes for growth rates in a stream-dwelling salmonid

Optimal life histories in a fluctuating environment are likely to differ from those that are optimal in a constant environment, but we have little understanding of the consequences of bounded fluctuations versus episodic massive mortality events. Catastrophic disturbances, such as floods, droughts, landslides and fires, substantially alter the population dynamics of affected populations, but little has been done to investigate how catastrophes may act as a selective agent for life-history traits. We use an individual-based model of population dynamics of the stream-dwelling salmonid marble trout (Salmo marmoratus) to investigate how trade-offs between the growth and mortality of individuals and density-dependent body growth can lead to the maintenance of a wide or narrow range of individual variation in body growth rates in environments that are constant (i.e., only demographic stochasticity), variable (i.e., environmental stochasticity), or variable with catastrophic events that cause massive mortalities (e.g., flash floods). We find that occasional episodes of massive mortality can substantially reduce persistent variability in individual growth rates. Lowering the population density reduces density dependence and allows for higher fitness of more opportunistic strategies (rapid growth and early maturation) during the recovery period.

Further laboratory investigations into the Relative Diffusional Growth Rate theory of thunderstorm electrification

Predictions from the Relative Diffusional Growth Rate theory have allowed us to further examine controls over the sign of graupel charge in laboratory experiments involving collisions between ice crystals and riming graupel. This has been achieved primarily through the use of two-cloud experiments in which a supercooled droplet cloud and an ice crystal cloud are mixed together. A range of crystal-cloud nucleation techniques has enabled substantial control over the preconditioning of ice crystal surface growth rates such as to control transient rapid crystal growth on mixing with a droplet cloud prior to interaction with a riming target. We have qualitatively identified the effects that crystal surface growth rate and size have on the rate of transient rapid growth after mixing. Crystals with higher surface growth rates in environments nearer to water saturation reduce the effect of transient rapid growth on mixing leading to positive graupel charging. Crystals with lower surface growth rates in environments nearer to ice saturation enhance transient rapid growth on mixing – particularly for smaller crystal sizes – to promote negative graupel charging. This is consistent with the Relative Diffusional Growth Rate theory which has been developed from many laboratory studies and shows that in collisions between ice particles, the surface growing faster by diffusion charges positively.Using both the shortest and longest nucleation techniques developed, it was possible to extend negative charging to approximately −20°C and −14°C respectively. Being able to control cloud microphysics and determine its effect on charge sign has led to the conclusion that numerical models of thunderstorm charge development need to take account of the wide range of specific microphysical conditions and their effects on cloud particle charging throughout many regions and the lifetime of a thunderstorm.We also provide evidence in support of new interpretations of the results of other researchers by using predictions of the Relative Diffusional Growth Rate theory to enable the development of experimental techniques to promote negative graupel charging to record high temperatures, and consequently allow negative charging at any sub-zero temperature applicable to thunderstorms.

LOSS OF EVOLUTIONARY RESISTANCE BY THE OLIGOCHAETELIMNODRILUS HOFFMEISTERITO A TOXIC SUBSTANCE-COST OR GENE FLOW?

The oligochaete Limnodrilus hoffmeisteri at Foundry Cove (FC), New York evolved genetic resistance to cadmium (Cd) and lost resistance after contaminated sediments were removed by dredging. Selection (on survival time in dissolved Cd) was used to generate tolerance to evaluate fitness cost, the commonplace expectation for evolutionary reversal. The hypothesis that gene flow from neighboring populations could "swamp" resistance was addressed by 16S rDNA sequences. In disagreement with the cost hypothesis, selected-Cd tolerant worms and controls showed no difference in total fecundity or growth rate in environments. Highly-Cd-tolerant worms of the FC-selected population grew rapidly at different temperatures and showed no growth impairment in the presence of Cd, indicating metabolically efficient resistance. Genetic structure at FC was consistent with invasion of genotypes from an adjacent population in the time since dredging. Applying selection to lines from FC and a reference site, demonstrated a more rapid increase in Cd tolerance in FC-origin lines, indicating standing allelic variation for resistance at FC (despite phenotypic erosion). The selection experiment supports the view that resistance is simply controlled--probably by one allele of large effect. Whether such rapid "readaptation" could occur naturally is an important question for understanding broad effects of pollutants.

Contribution of anodic dissolution and hydrogen embrittlement to corrosion-fatigue crack growth

1. The corrosion fatigue crack growth rate depends not only on the stress intensity factor but also on the electrochemical conditions at the crack tip that can be characterized by the pH values of the environment and the electrode potential of the metal. 2. Any value of the corrosion fatigue crack growth rate in steels (under variable electrochemical conditions at the crack tip) corresponds to a certain fixed pH value and a fixed value of the electrode potential at the crack tip; moreover, any section of the crack growth rate diagrams corresponds to certain fixed ranges of the indicated parameters. 3. If the stress intensity factor is fixed, then the crack growth rate varies as a function of the pH value of the environment and the electrode potential of the metal at the crack tip. 4. We proposed a method for the evaluation of the contributions of the mechanisms of local anodic dissolution and hydrogen embrittlement to the increment of the crack growth rate in aqueous environments. The method is based on the analysis of the pH value of the environment and the electrode potential of the metal characterizing the intensity of the processes of local anodic dissolution and hydrogen depolarization at the crack tip as well as on the application of the results of fatigue crack growth tests in air and environments taking into account the electrochemical conditions at the crack tip. 5. As an example, the increments of the fatigue crack growth rate in environments caused by local anodic dissolution and hydrogen embrittlement were determined for two different metal-environment systems (40Kh steel in water and 20Kh13 steel in a 3% NaCl solution). The maps of action of these mechanisms were constructed depending on the stress intensity factor and the loading frequency. Hydrogen embrittlement was proved to be the predominant mechanism of changes in the crack growth rate for 40Kh steel in water. At the same time, local anodic dissolution was predominant for 20Kh13 steel in a 3% NaCl solution. 6. The proposed method for the quantitative evaluation of local anodic dissolution and hydrogen embrittlement is fairly universal and allows one not only to determine the predominant mechanism but also to study the properties of both known and newly created types of corrosion-resistant steels.