Maximize Energy Gain Research Articles

Timing mowing for maximal energy gain - Managing foraging habitat of wintering geese under extreme drought conditions

The availability of high-quality food resources is a critical determinant of wildlife fitness. Over the past two decades, phenological mismatch - the temporal misalignment between animals’ peak nutrient demand and optimal resource availability - has emerged as a significant conservation challenge. This issue is particularly worrisome for migratory birds, which must accumulate energy reserves to meet the elevated metabolic demands of migration between breeding and wintering grounds. In Poyang Lake, a crucial wintering ground along the East Asian-Australasian flyway, increasing asynchrony between vegetation growth and the migration of herbivorous waterbirds significantly impedes conservation efforts and presents a major management challenge for this Ramsar wetland. This study evaluates the efficacy of mowing, a grassland management measure, in regulating plant growth processes and restoring food resources for geese. In-situ mowing experiments were conducted with varying timings in Carex wet meadows, the primary foraging habitats of geese. Optimal mowing times were identified, and the maximum goose carrying capacity was assessed by comparing Carex growth and nutritional dynamics with goose dietary requirements. The results reveal that mowing effectively slows down the aging process of Carex, and protein content is identified as a critical limiting factor for geese foraging. Different mowing timings extend the suitable foraging period by 11–25 days. Estimates suggest varying carrying capacities with different mowing timings, supporting goose populations ranging from 133 to 2,046 in Changhuchi Lake during wintering. The optimal mowing window is early October, avoiding dates before late September and after late November. Moreover, multiple-stage mowing is recommended to accommodate different wintering stages. The study highlights mowing as a potential habitat restoration approach for goose conservation, effectively mitigating the challenges imposed by phenological mismatch directly and indirectly caused by anthropogenic activities.

Predators minimize energy costs, rather than maximize energy gains under warming: Evidence from a microcosm feeding experiment

Abstract Climate warming may alter predator–prey interactions and predator feeding behaviour due to increased metabolic demands. How predators meet these increased demands may depend on trade‐offs in prey energy content and body size, handling time and other functional constraints. We tested hypotheses associated with these trade‐offs with the predatory mite Stratiolaelaps scimitus, and three prey that differed in body size, energy content, and defenses (Folsomia candida, Oppia nitens, and Carpoglyphus lactis). We estimated metabolic rate, predation in choice and no choice feeding trials, movement rate, and lipid and protein content for all four species at 16°C and 24°C. We used these data to estimate the predator's energy demands and compared these to estimated energy intake in the choice feeding trials. Predators had greater metabolic demands at 24°C than at 16°C, but temperature did not affect predator or prey movement rates. Warming decreased lipid content, but not protein content, of all three prey species, leading to lower energy content for C. lactis and O. nitens, but not F. candida. In both feeding trials at 24°C, predators increased their feeding on the smaller, energy‐poor C. lactis, but not the larger, energy‐rich F. candida, resulting in lower estimated energy intake. S. scimitus did not feed on O. nitens at either temperature. Predators increasingly fed on small‐bodied prey under warming and not the large‐bodied prey despite the potential for greater energetic gains from larger prey. We posit that predators minimized energy lost during feeding through lower handling costs associated with C. lactis, rather than maximize energy gain. We conclude that selection of prey based on body size changes with temperature as a trade‐off for predators to balance increased metabolic demands. As predators provide top‐down control and regulate energy flow through the consumption of their prey, changes to predator feeding behaviour with climate warming may affect food web dynamics and ecosystem‐level processes. Read the free Plain Language Summary for this article on the Journal blog.

Ontogeny of movement patterns and habitat selection in juvenile albatrosses

Optimal selection of foraging habitats is key to survival, but it remains unclear how naïve individuals are able to locate patchily‐distributed resources and maximize energy gain in completely new environments. In most animals, juveniles disperse unaccompanied by their parents, and hence their movements are likely guided, at least at fine scales, by external cues. However, the extent to which environmental processes and individual learning shape habitat selection and movement strategies of juveniles remains unclear, especially in species with cryptic life‐stages. Here, we use a mechanistic modelling framework – integrated step selection analysis – to examine the development of habitat preferences in a pelagic seabird with a prolonged period of immaturity, the grey‐headed albatross Thalassarche chrysostoma. Juveniles were tracked from Bird Island, South Georgia, in two years (n = 9 in 2018 and n = 12 in 2019), using satellite transmitters (platform terminal transmitters), and we investigated ontogenetic changes in individual movement characteristics (step lengths and turning angles) in response to two environmental variables; tailwind support (which enables low‐cost movement) and chlorophyll a concentration (a proxy for resources) during their first four months at sea. Naïve juveniles dispersed rapidly away from South Georgia towards the same general region (subantarctic and subtropical waters in the east Atlantic Ocean) by increasing their travel speeds and directional persistence in response to favourable wind conditions. In the first month post‐fledging, juveniles also responded to local resource availability (chlorophyll a concentration) by reducing travel speeds in more productive regions, but thereafter engaged in comparatively slower and more sinuous movements, apparently focusing foraging effort on frontal zones. While complex movement strategies such as long‐distance migrations may take several years to develop, our results indicate that dispersing juveniles are able to respond rapidly both to changes in wind and local resource availability, maximising flight and foraging efficiency.

Reduced diurnal activity and increased stopover duration by molting Swainson’s Thrushes

AbstractMigration consists of a sequence of small- to large-scale flights often separated by stopovers for refueling. Tradeoffs between minimizing migration time (more flights, shorter stopovers) and maximizing energy gain (fewer flights, longer stopovers) will affect overall migration timing. For example, some individuals make long-term stopovers in high-quality habitat that maximize energy gain (e.g., molt-migration), but movement to those habitats likely costs time. We used radio telemetry and blood plasma metabolite levels to examine physiological and behavioral tradeoffs between molt-migrant (birds molting at the molt stopover; n = 59) and post-molt (birds that presumably completed their molt elsewhere; n = 19) migrant Swainson’s Thrushes (Catharus ustulatus) near Montreal, Canada. Molt-migration was a large time investment as the average stopover duration for molt-migrants was of 47 ± 9 days (~13% of the entire annual cycle), almost twice as long as previously assumed from banding records, and far longer than stopovers of post-molting individuals (7 ± 2 days). Daily mortality rate during the molt stopover was similar to the average annual daily mortality rate. Molt-migrants’ circadian rhythms closely matched light levels, whereas post-molting birds had irregular rhythms and averaged 1 hr greater activity per day than molt-migrants. Despite being less active, molt-migrants had similar refueling rates based on metabolite profiles. As compared with migrants that completed molt earlier, molt-migrants at this stopover site had slower subsequent migration rates. Thus, birds using long-term stopovers appeared to tradeoff energy (efficient refueling) for time (slower subsequent migration).

Experimental evaluation of used engine oil based thermal energy storage coupled with novel evacuated tube solar air collector (NETAC)

The significant challenge in vacuum tube solar air collector is worse performance after sunset which prompts the thermal energy storage. In present manuscript, the used engine oil based thermal energy storage coupled with novel evacuated tube solar air collector (NETAC) is developed. The NETAC investigation is evaluated during winter season for hot air production for 13 h at diverse air flow rates and different directions of air flow (parallel air flow & counter air flow). The maximal temperature difference (24.8 oC) of air is obtained at low air flow rate (79.5 Kg/h) and maximal mean efficiency (28.8%) of NETAC at high air flow rate (159 Kg/h) with circular fin and counter flow arrangement. The maximal mean energy efficiency (27.15%) and exergy efficiency (24.8%) of the oil thermal storage tank is attained at high air flow rate (159 Kg/h) with circular fin and parallel flow arrangement. The maximal mean energy gain (538.7 W) by air is obtained at high air flow rate (159 Kg/h) with circular fin and parallel flow arrangement.

Maximizing the energy potential of urban sludge treatment: An experimental study and a scenario-based energy analysis focusing on anaerobic digestion with ultrasound pretreatment and sludge combustion

With stricter regulations and concerns over sustainability, combustion can be a beneficial handling method as dried sludge can be used as an energy source. Anaerobic digestion of sludge also provides energy in the form of biogas without the need for sludge drying. Yet, anaerobic digestion reduces the calorific value (CV) of sludge. In this study, the effect of anaerobic digestion (with or without ultrasound pretreatment) on the fuel properties of sludge were examined through laboratory experiments. Additionally, a number of sludge management strategies were evaluated to maximize energy gain. Temporal changes in fuel properties of sludge were tracked during digestion. Results showed that sonication provided a 3.4-fold increase in soluble COD, 15% increase in biogas production and 32% increase in methane production, while the CV of sludge reduced from 14.7 kJ/g to 10 kJ/g. Energy balances for a number of sludge management process trains showed that the optimal method was the combination of thickening, dewatering, thermal drying and combustion. Addition of digestion and ultrasound pretreatment to this scenario reduced the energy gained from sludge. Yet, high ambient temperatures impacted results such that the net energy gain by the scenario including digestion and combustion was close to that of combustion only.

Dynamics of Laser-Charged UAVs: A Battery Perspective

In this article, we aim to sustain unmanned aerial vehicle (UAV)-based missions for longer periods of times through different techniques. First, we consider on-the-mission UAV recharging by a low-power laser source (below 1 kW). In order to achieve the maximal energy gain from the low-power laser source, we propose an operational compromise, which consists of the UAV resting over buildings with cleared line of sight to the laser source. Second, to provide a precise energy consumption/harvesting estimation at the UAV, we investigate the latter's dynamics in a mission environment. Indeed, we study the UAV's battery dynamics by leveraging the electrical models for motors and battery. Subsequently, using these models, the path planning problem in a particular Internet-of-Things-based usecase is revisited from the battery perspective. The objective is to extend the UAV's operation time using both laser-charging and accurate battery level estimation. Through a graph theory approach, the problem is solved optimally, and compared to benchmark trajectory approaches. Numerical results demonstrate the efficiency of this novel battery perspective for all path planning approaches. In contrast, we found that the energy perspective is very conservative and does not exploit optimally the available energy resources. Nevertheless, we propose a simple adjustment method to correct the energy perspective, by carefully evaluating the energy as a function of the UAV motion regimes. Finally, the impact of several parameters, such as turbulence and distance to charging source, is studied.

Adaptive model tuning studies for non-invasive diagnostics and feedback control of plasma wakefield acceleration at FACET-II

Beam-driven plasma wakefield acceleration (PWFA) achieves the same energy gain in a single meter, for which conventional accelerators require several kilometers, however much work is still required to match the beam quality of conventional accelerators. The PWFA processes to be studied at the FACET-II facility will utilize extremely short (down to a few fs) high energy (10 GeV), high charge (few nC) electron and eventually positron bunches. The PWFA process is extremely sensitive to the detailed longitudinal current profiles of these bunches and it would be of great benefit to have precise measurement and control of these profiles. We present an adaptive model tuning technique for FACET-II to adaptively tune online models based on real time accelerator and beam data to continuously provide a non-invasive diagnostic which predicts the longitudinal phase space (LPS) of extremely short and highly compressed electron beams, which otherwise require destructive X-band transverse deflecting cavity (XTCAV)-based measurements. Based on simulation studies, our method has the potential for: (1). The development of a non-invasive longitudinal phase space diagnostic by adaptively tuning models based on non-invasive measurements such as energy spread spectra which could be recorded at all of the bunch compressors in the facility. (2). Utilize these diagnostic to perform model-independent feedback-based to achieve desired longitudinal phase space distributions. (3). Utilize the model-independent feedback approach to maximize energy gain while minimizing emittance growth and energy gain variance of the PWFA process by tuning accelerator parameters, while monitoring what longitudinal phase space the algorithm has found, information that will be useful for further analytical and simulation studies.

Effects of anaerobic digestion enhanced by ultrasound pretreatment on the fuel properties of municipal sludge.

In this study, effects of ultrasound pretreatment on combustion characteristics and elemental composition of municipal sludge were examined for energy-based evaluation of sludge pretreatment. Waste activated sludge (WAS) from a municipal wastewater treatment plant was pretreated with ultrasound at varying durations and was subjected to anaerobic digestion in a biochemical methane potential (BMP) assay. Changes in gas production rates, calorific value (CV), elemental compositions, and ash contents of sludge samples were examined to assess the effects of pretreatment and digestion. Sonication at 0.73W/mL enhanced gas production by 28%. Moreover, volatile solids (VS) and chemical oxygen demand (COD) removals increased from 41 to 45% and 33 to 37%, respectively. Following anaerobic digestion, CVs of samples decreased by about 18%. Sonicated samples exhibited a higher decrease. In order to quantify the change in overall energy content, total solids (TS) reduction was also taken into account. Loss was magnified as both CV and the amount of TS that would provide the overall energy were reduced. This loss was 38% for the control group and 41% for the 15min sonicated sludge. Digestion decreased the C content of sludge by about 20% and H content by 50% due to biogas production. Ash content increased relatively as some of the combustible solids were lost due to digestion. Experimental results indicate that if sludge is to be combusted, digestion with or without ultrasound pretreatment may be disadvantageous if the aim is to maximize energy gain from sludge.

Assessment of two different methods of calculating a heating demand

Abstract Phenomenon of today′s era is lowering overall energy performance and greenhouse gas emissions. European Union and its members came from an “uncontrollable” fossil fuel usage to an era of dependence reduction from fossil fuel usage. EU industry is now in era of utilizing renewable energy sources. Construction industry is responsible for 40% of total energy performance of EU; hence the great emphasis on energy effective design on new buildings as well as importance on energy effective renovation of already constructed buildings. Effort on lowering total energy performance in construction industry has signed on research on new project solutions, new insulation materials and new building services technologies which are maximizing energy gain from renewable energy sources. Simulation software and different engines are more than helpful tool when predicting building thermal behaviour. That is why simulation software is used more often in the process of designing energy efficient buildings or in the process of energy performance optimization of existing buildings. simulation and analytics itself, is dealing with simulation of chosen buildings (family houses) on heating demand based on the newest knowledge of constructions and materials. Results of heating demands of different family houses were confronted against heating demands calculated according national standards – using non-variable conditions. Next, the differences between two calculation approaches were characterized in percentages.

Life-cycle energy and climate benefits of energy recovery from wastes and biomass residues in the United States

Agricultural and forestry residues, animal manure and municipal solid waste are replenishable and widely available. However, harnessing these heterogeneous and diffuse resources for energy requires a holistic assessment of alternative conversion pathways, taking into account spatial factors. Here, we analyse, from a life-cycle assessment perspective, the potential renewable energy production, net energy gain and greenhouse gas (GHG) emission reduction for each distinct type of waste feedstock under different conversion technology pathways. The utilization of all available wastes and residues in the contiguous United States can generate 3.1–3.8 exajoules (EJ) of renewable energy, but only deliver 2.4–3.2 EJ of net energy gain, and displace 103–178 million tonnes of CO2-equivalent GHG emissions. For any given waste feedstock, looking across all US counties where it is available, except in rare instances, no single conversion pathway simultaneously maximizes renewable energy production, net energy gain and GHG mitigation. Maximizing energy gain and minimizing life-cycle emissions from wastes and biomass residues requires a holistic assessment of alternative energy recovery pathways. Liu and Rajagopal estimate that the United States can generate up to 3.2 EJ of net energy and reduce up to 178 million tonnes of CO2-equivalent GHG emissions from 29 different waste streams combined.

Co-digestion of Wastewater-Grown Filamentous Algae With Sewage Sludge Improves Biomethane Production and Energy Balance Compared to Thermal, Chemical, or Thermochemical Pretreatments

Wastewater algal treatment systems show improved economic viability and enhanced energy return on investment if integrated with biofuel production. One option is to anaerobically digest the algae to generate bio-methane. This method is appropriate for relatively low lipid filamentous algae typical of turf scrubbers®. But unbalanced carbon-to-nitrogen (C/N) ratio and the resistance of algae to biodegradation can limit biomass conversion into bio-methane. To evaluate options to enhance bio-methane production, an indigenous assembly of macro-algae was established and cultivated in CO2-infused secondary wastewater effluent, then harvested and either anaerobically digested using pretreatments or co-digested with sewage sludge. Results were used to develop methane production kinetic models and perform an AD system energy balance analysis to assess the feasibility of pretreatment and co-digestion for a scaled process. Floways were dominated by Ulothrix and Oedogonium algae and had periphyton biomass production rates that averaged 3.7±0.4 g VS m-2 d-1 (±1SD) during the initial 7-day colonization period. Biomass increased by 62% in the second half of the 14-day experiment. Ultimate methane yield from harvested biomass was improved relative to controls (30613 mL gVS-1) through thermal pretreatment by 15%, dilute acid by 5%, dilute alkali by 17%, acid- and alkali-assisted thermochemical pretreatments by 23 and 27%, respectively. However, all pretreatment methods undermined the energy balance parameters including Net Energy Ratio (NER) and Net Energy Efficiency (NEE) due to the heat required for thermal pretreatments and electricity needed to produce chemical reagents. In contrast, co-digestion of algal biomass with sewage sludge enhanced methane generation yielding up to 4013 mL gVS-1 at algae to sludge ratio of 20% to 80%. Co-digestion with sludge also strongly improved AD system energy balance. NER and NEE increased from 2.8 and 73% for algae alone to 4.3 and 81% for a 20%:80% algae to sludge mix respectively. Moreover, the Net Energy Recovery during co-digestion reached 39% compared to 26% and 33% when algae or sewage sludge were processed as single-substrates. Thus, co-digestion of algae with sewage sludge serves as an attractive option for maximizing energy gain from AD of biomass harvested from filamentous algal treatment systems.

Reproductive class influences risk tolerance during denning and spring for American black bears (Ursus americanus)

AbstractDen sites are important for many species’ ecology, and individuals should choose den sites that minimize risk and maximize energy gain. We examined den site selection and spring space use for black bears (Ursus americanus) at the individual level, and analyzed support for the anthropogenic risk avoidance and infanticide avoidance hypotheses. We obtained telemetry data for 94 individual bears (71 F and 23 M) and collected 162 den locations from Michigan (2009–2011 and 2013–2015), Missouri (2010–2016), and Mississippi (2008–2017), USA. Of 71 females, 37 were monitored during multiple reproductive states. We used generalized linear mixed models to evaluate the influence of reproductive state (female with cubs, female with yearlings, female alone, or male) on den distance to the nearest road, two road indexes (den emergence and spring), spring home range size, mean spring road distance, and a NDVI selection index. All black bears, except females with cubs, chose den locations farther from roads than available to them, supporting the anthropogenic risk avoidance hypothesis. Additionally, females with cubs displayed the smallest spring home ranges among females, as well as denned closer to roads and used areas closer to roads during spring than females with yearlings, supporting the infanticide avoidance hypothesis. The differential responses by the same females when having cubs or yearlings point to strategies to maximize reproductive success. By investigating behavior considering individual availability, we revealed patterns that might not be detected by population‐level assessments.

Electron acceleration driven by sub-cycle and single-cycle focused optical pulse with radially polarized electromagnetic field.

The space-time properties of the expressions of sub-cycle and single-cycle focused optical pulses with radially polarized electromagnetic field based on the Sink-Source model are studied. The self-induced blue shift of the center frequency of spectrum in the center of the pulse field is found to have an important impact on the electrons acceleration. When the electrons approach to the center of pulse, the electrons will obtain a large kinetic energy gain in a short time. The effect of radiation-reaction force can't be ignored if the net kinetic energy gain of electrons is more than GeVs. The electrons will deviate from the original acceleration channel and the gain of kinetic energy that electrons may gain will be greatly reduced if the radiation-reaction effect is considered. In contrast to the few-cycle laser pulse accelerating electrons, the gain of kinetic energy obtained by electrons is a few times higher and the corresponding peak optical power is one order of magnitude lower in the case of the sub-cycle laser pulses accelerating electrons. The maximal kinetic energy gain of electrons is robust against the variation of the incident angles.

Optimum Orientation Angles for Photovoltaic Arrays to Maximize Energy Gain at Near Equator Location in Indonesia

Photovoltaic (PV) performance is affected by its orientation angle. Estimating optimum orientation angles for PV arrays is necessary in order to obtain maximum solar radiation and PV output. Meteorological data during the period of 2008-2014 are used to develop empirical correlation of the diffuse fraction on a horizontal surface in Pontianak, Indonesia (latitude 0.0263( S). MATLAB software is used to simulate the total solar radiation incident on a surface facing due north with an optimum tilt angle. The maximum annual solar energy is 1893 kWh/m2/year by adjusting PV arrays monthly. The loss of energy is about 0.27 % and 1.34 % when a tilt angle is adjusted seasonally and yearly in comparison to the monthly optimum tilt angle. It is also found that the value of capacity utilization factor (CUF) of the PV system for Pontianak is 19.57 % - 19.83 %, while the performance ratio (PR) is 79.44 % - 79.47 % at the optimum orientation angles.

Dual Axis and Single Axis Sun Tracking Maximal Energy Gain

Dual Axis and Single Axis Sun Tracking Maximal Energy Gain

Flight modeling and simulation for dynamic soaring with small unmanned air vehicles

The design of guidance and control strategies is a promising study trend of dynamic soaring for small unmanned aerial vehicles, for which the flight modeling and simulation specifically for soaring-capable unmanned aerial vehicles is significant and necessary. The aim of this paper is to propose a flight simulation platform for dynamic soaring. In order to do so, firstly, two different sets of equations of motion of small unmanned aerial vehicles have been derived and their characteristics are compared: one is expressed in body-fixed frame and the other in air-relative flight path frame. Secondly, the latter set is used for energy analysis to maximize energy gain while climbing and diving and minimize energy cost during turning in dynamic soaring. While the former serves to build the dynamic soaring simulation platform, in which a piecewise trajectory-based guidance and control strategy according to the energy analysis is proposed tracking the optimum climb and bank angles and traveling toward desired directions. Simulation results indicate that the unmanned aerial vehicles can perform dynamic soaring toward various directions in different wind fields, follow asymptotically the typical straight-line and circular-orbit paths by repeating soaring cycles.

The cost of reproduction: differential resource specialization in female and male California sea otters.

Intraspecific variation in behavior and diet can have important consequences for population and ecosystem dynamics. Here, we examine how differences in reproductive investment and spatial ecology influence individual diet specialization in male and female southern sea otters (Enhydra lutris nereis). We hypothesize that greater reproductive constraints and smaller home ranges of females lead to more pronounced intraspecific competition and increased specialization. We integrate stable carbon (δ(13)C) and nitrogen (δ(15)N)isotope analysis of sea otter vibrissae with long-term observational studies of five subpopulations in California. We define individual diet specialization as low ratios of within-individual variation (WIC) to total population niche width (TNW). We compare isotopic and observational based metrics of WIC/TNW for males and females to data on population densities, and movement patterns using both general linear and linear mixed-effects models. Consistent with our hypothesis, increasing population density is associated with increased individual diet specialization by females but not by males. Additionally, we find the amount of coastline in a sea otter's home range positively related with individual dietary variability, with increased range span resulting in weaker specialization for both males and females. We attribute our results to sex-based differences in movement, with females needing to specialize in their small ranges to maximize energy gain, and posit that the paradigm of individual prey specialization in sea otters with increased intraspecific competition may be a pattern driven largely by females. Our work highlights a potentially broader role of sex in the mechanistic pressures promoting and maintaining dietspecialization.

Free-swimming northern elephant seals have low field metabolic rates that are sensitive to an increased cost of transport.

Widely ranging marine predators often adopt stereotyped, energy-saving behaviours to minimize the energetic cost of transport while maximizing energy gain. Environmental and anthropogenic disturbances can disrupt energy balance by prompting avoidance behaviours that increase transport costs, thereby decreasing foraging efficiency. We examined the ability of 12 free-ranging, juvenile northern elephant seals (Mirounga angustirostris) to mitigate the effects of experimentally increased transport costs by modifying their behaviour and/or energy use in a compensatory manner. Under normal locomotion, elephant seals had low energy requirements (106.5±28.2 kJ kg(-1) day(-1)), approaching or even falling below predictions of basal requirements. Seals responded to a small increase in locomotion costs by spending more time resting between dives (149±44 s) compared with matched control treatments (102±11 s; P<0.01). Despite incurred costs, most other dive and transit behaviours were conserved across treatments, including fixed, rhythmic swimming gaits. Because of this, and because each flipper stroke had a predictable effect on total costs (P<0.001), total energy expenditure was strongly correlated with time spent at sea under both treatments (P<0.0001). These results suggest that transiting elephant seals have a limited capacity to modify their locomotory behaviour without increasing their transport costs. Based on this, we conclude that elephant seals and other ocean predators occupying similar niches may be particularly sensitive to increased transport costs incurred when avoiding unanticipated disturbances.

Evaluating microbial chemical choices: The ocean chemistry basis for the competition between use of O2 or NO3− as an electron acceptor

The traditional ocean chemical explanation for the emergence of suboxia is that once O2 levels decline to about 10µmolkg−1 then onset of NO3− reduction occurs. This piece of ocean chemical lore is well founded in observations and is typically phrased as a microbial choice and not as an obligate requirement. The argument based on O2 levels alone could also be phrased as being dependent on an equivalent amount of NO3− that would yield the same energy gain. This description is based on the availability of the electron acceptor: but the oxidation reactions are usually written out as free energy yield per mole of organic matter, thus not addressing the oxidant availability constraint invoked by ocean scientists. Here we show that the argument can be phrased simply as competing rate processes dependent on the free energy yield ratio per amount of electron acceptor obtained, and thus the [NO3−]:[O2] molar ratio is the critical variable. The rate at which a microbe can acquire either O2 or NO3− to carry out the oxidation reactions is dependent on both the concentration in the bulk ocean, and on the diffusivity within the microbial external molecular boundary layer. From the free energy yield calculations combined with the ~25% greater diffusivity of the O2 molecule we find that the equivalent energy yield occurs at a ratio of about 3.8 NO3−:O2 for a typical Redfield ratio reaction, consistent with an ocean where NO3− reduction onset occurs at about 10μmol O2:40μmol NO3−, and the reactions then proceed in parallel along a line of this slope until the next energy barrier is approached. Within highly localized microbial consortia intensely reducing pockets may occur in a bulk ocean containing finite low O2 levels; and the local flux of reduced species from strongly reducing shelf sediments will perturb the large scale water column relationship. But all localized reactions drive towards maximal energy gain from their immediate diffusive surroundings, thus the ocean macroscopic chemical fields quite well approximate the net efficiency and operational mode of the ensemble microbial engine.