Victor G. Gorshkov

Mean mass-specific metabolic rates are strikingly similar across life's major domains: Evidence for life's metabolic optimum

A fundamental but unanswered biological question asks how much energy, on average, Earths different life forms spend per unit mass per unit time to remain alive. Here, using the largest database to date, for 3,006 species that includes most of the range of biological diversity on the planet—from bacteria to elephants, and algae to sapling trees—we show that metabolism displays a striking degree of homeostasis across all of life. We demonstrate that, despite the enormous biochemical, physiological, and ecological differences between the surveyed species that vary over 1020-fold in body mass, mean metabolic rates of major taxonomic groups displayed at physiological rest converge on a narrow range from 0.3 to 9 W kg−1. This 30-fold variation among lifes disparate forms represents a remarkably small range compared with the 4,000- to 65,000-fold difference between the mean metabolic rates of the smallest and largest organisms that would be observed if life as a whole conformed to universal quarter-power or third-power allometric scaling laws. The observed broad convergence on a narrow range of basal metabolic rates suggests that organismal designs that fit in this physiological window have been favored by natural selection across all of lifes major kingdoms, and that this range might therefore be considered as optimal for living matter as a whole.

Energetics of the smallest: Do bacteria breathe at the same rate as whales?

Power laws describing the dependence of metabolic rate on body mass have been established for many taxa, but not for prokaryotes, despite the ecological dominance of the smallest living beings. Our analysis of 80 prokaryote species with cell volumes ranging more than 1 000 000-fold revealed no significant relationship between mass-specific metabolic rate q and cell mass. By absolute values, mean endogenous mass-specific metabolic rates of non-growing bacteria are similar to basal rates of eukaryote unicells, terrestrial arthropods and mammals. Maximum mass-specific metabolic rates displayed by growing bacteria are close to the record tissue-specific metabolic rates of insects, amphibia, birds and mammals. Minimum mass-specific metabolic rates of prokaryotes coincide with those of larger organisms in various energy-saving regimes: sit-and-wait strategists in arthropods, poikilotherms surviving anoxia, hibernating mammals. These observations suggest a size-independent value around which the mass-specific metabolic rates vary bounded by universal upper and lower limits in all body size intervals.

Gigantism, temperature and metabolic rate in terrestrial poikilotherms

The mechanisms dictating upper limits to animal body size are not well understood. We have analysed body length data for the largest representatives of 24 taxa of terrestrial poikilotherms from tropical, temperate and polar environments. We find that poikilothermic giants on land become two–three times shorter per each 10 degrees of decrease in ambient temperature. We quantify that this diminution of maximum body size accurately compensates the drop of metabolic rate dictated by lower temperature. This supports the idea that the upper limit to body size within each taxon can be set by a temperature-independent critical minimum value of mass-specific metabolic rate, a fall below which is not compatible with successful biological performance.

ENERGY PARTITIONING BETWEEN DIFFERENT-SIZED ORGANISMS AND ECOSYSTEM STABILITY

The metabolic approach to ecology presented by Brown et al. (2004) stems from the seminal work of West et al. (1997). They hypothesized that material transport within living beings is organized such as to minimize the scaling of total hydrodynamic resistance through vascular networks. Based on this assumption, the organismal metabolic power P was theoretically predicted to scale with body mass M as P } M 3/4. By additionally assuming that organismal metabolic processes accelerate with temperature in the same manner as individual biochemical reactions, a temperature correction factor was added to this scaling:

Why Does Air Passage over Forest Yield More Rain? Examining the Coupling between Rainfall, Pressure, and Atmospheric Moisture Content*

The influence of forest loss on rainfall remains poorly understood. Addressing this challenge, Spracklen et al. recently presented a pantropical study of rainfall and land cover that showed that satellite-derived rainfall measures were positively correlated with the degree to which model-derived air trajectories had been exposed to forest cover. This result confirms the influence of vegetation on regional rainfall patterns suggested in previous studies. However, the conclusion of Spracklen et al.—that differences in rainfall reflect air moisture content resulting from evapotranspiration while the circulation pattern remains unchanged—appears undermined by methodological inconsistencies. Here methodological problems are identified with the underlying analyses and the quantitative estimates for rainfall changepredicted ifforestcover islost in theAmazon. Alternative explanations are presented thatinclude the distinct role of forest evapotranspiration in creating low-pressure systems that draw moisture from the oceans to the continental hinterland. A wholly new analysis of meteorological data from three regions in Brazil, including the central Amazon forest, reveals a tendency for rainy days during the wet season with column water vapor (CWV) exceeding 50mm to have higher pressure than rainless days, while at lower CWV, rainy days tend to have lower pressure than rainless days. The coupling between atmospheric moisture content and circulation dynamics underlines that the danger posed by forest loss is greater than suggested by consideration of moisture recycling alone.

The Biotic Pump: Condensation, atmospheric dynamics and climate

Intense condensation associated with high evaporation from natural forest cover maintains regions of low atmospheric pressure on land. This causes moist air to flow from ocean to land, which compensates the river runoff. Deforestation induces large-scale desiccation by disrupting this flow. Here we overview this theory and quantify the horizontal pressure gradients that govern the continental moisture supply. High evaporation and extensive natural forests guarantee both a stable and high throughput hydrological cycle. Forests protect a continent against devastating floods, droughts, hurricanes, and tornadoes. Sustaining natural forests is a sound strategy for water security and climate stabilisation.

A critique of some modern applications of the Carnot heat engine concept: the dissipative heat engine cannot exist

In several recent studies, a heat engine operating on the basis of the Carnot cycle is considered, where the mechanical work performed by the engine is dissipated within the engine at the temperature of the warmer isotherm and the resulting heat is added to the engine together with an external heat input. This internal dissipation is supposed to increase the total heat input to the engine and elevate the amount of mechanical work produced by the engine per cycle. Here it is argued that such a dissipative heat engine violates the laws of thermodynamics. The existing physical models employing the dissipative heat engine concept, in particular the heat engine model of hurricane development, need to be revised.A heat engine operating on the basis of the Carnot cycle is considered, where the mechanical work performed is dissipated within the engine at the temperature of the warmer isotherm and the resulting heat is added to the engine together with an external heat input. The resulting work performed by the engine per cycle is increased at the expense of dissipated work produced in the previous cycle. It is shown that such a dissipative heat engine is thermodynamically inconsistent violating the first and second laws of thermodynamics. The existing physical models employing the dissipative heat engine concept, in particular, the heat engine model of hurricane development, are physically invalid.

The Key Physical Parameters Governing Frictional Dissipation in a Precipitating Atmosphere

Precipitation generates small-scale turbulent air flows—the energy of which ultimately dissipates to heat. The power of this process has previously been estimated to be around 2‐4Wm 22 in the tropics: a value comparable in magnitude to the dynamic power of global atmospheric circulation. Here it is suggested that the true value is approximately half the value of this previous estimate. The result reflects a revised evaluation of the mean precipitation pathlength HP. The dependence of HP on surface temperature, relative humidity, temperature lapse rate, and degree of condensation in the ascending air were investigated. These analyses indicate that the degree of condensation, defined as the relative change of the saturated water vapor mixing ratio in the region of condensation, is a major factor determining HP. From this theory the authors develop an estimate indicating that the mean large-scale rate of frictional dissipation associated with total precipitation in the tropics lies between 1 and 2Wm 22 and show empirical evidence in support of this estimate. Under terrestrial conditions frictional dissipation is found to constitute a minor fraction of the dynamic power of condensation-induced atmospheric circulation, which is estimated to be at least 2.5 times larger. However, because HP increases with increasing surface temperature Ts, the rate of frictional dissipation wouldexceedthepowerofcondensation-induceddynamics,andthusblockmajorcirculation,atTs*320Kin a moist adiabatic atmosphere.

On the dependence of speciation rates on species abundance and characteristic population size

The question of the potential importance for speciation of large/small population sizes remains open. We compare speciation rates in twelve major taxonomic groups that differ by twenty orders of magnitude in characteristic species abundance (global population number). It is observed that the twenty orders of magnitude’s difference in species abundances scales to less than two orders of magnitude’s difference in speciation rates. As far as species abundance largely determines the rate of generation of intraspecific endogenous genetic variation, the result obtained suggests that the latter rate is not a limiting factor for speciation. Furthermore, the observed approximate constancy of speciation rates in different taxa cannot be accounted for by assuming a neutral or nearly neutral molecular clock in subdivided populations. Neutral fixation is only relevant in sufficiently small populations with 4Nev < 1, which appears an unrealistic condition for many taxa of the smaller organisms. Further research is clearly needed to reveal the mechanisms that could equate the evolutionary pace in taxa with dramatically different population sizes

Comment on "Energy Uptake and Allocation During Ontogeny"

We demonstrate that the model of energy allocation during ontogeny of Hou et al. (Reports, 31 October 2008, p. 736) fails to account for the observed elevation of metabolic rate in growing organisms compared with similarly sized adults of different species. The basic model assumptions of the three-quarter power scaling for resting metabolism and constancy of the mass-specific maintenance metabolism need to be reassessed.