Net Oxygen Research Articles

Oxygen and pH fluxes in shallow bay habitats: Evaluating the effectiveness of a macroalgal forest restoration.

Marine macroalgae are important primary producers in coastal ecosystems. Within sheltered and shallow bays in the Mediterranean, various Fucalean macroalgae and seagrasses coexist, creating habitats of high ecological importance. These habitats have historically suffered from various disturbances, and on this basis, active restoration actions have been proposed as potential solutions for their recovery. Here, we assessed the restoration success of a 10-year restored macroalgal forest by evaluating the recovery in terms of oxygen and pH fluxes and comparing those data with those of a healthy marine forest and a degraded habitat counterpart. We estimated the overall changes in dissolved oxygen and pH using light and dark community insitu incubations. We also determined the biomass and composition of macroalgal and macroinvertebrate compartments of each assemblage. During light incubations, the healthy and restored forest assemblages showed similar average net oxygen production, 5.7 times higher than in the degraded one, and a greater increase in pH. More than 95% of the incubated biomass corresponded to macroalgal and seagrass species. The restored forest showed a six-fold increase in biomass, most likely being responsible for the recovery of primary production. This work provides empirical evidence that the restoration of a single structural species, once successful in the early stages, can yield positive results by recovering processes such as primary production and dark respiration. Moreover, these results showcase differences in ecosystem functions between healthy (either mature or restored) and degraded habitats, highlighting the importance of protecting and preserving coastal marine forests.

Concurrent Measurement of O2 Production and Isoprene Emission During Photosynthesis: Pros, Cons and Metabolic Implications of Responses to Light, CO2 and Temperature.

Traditional leaf gas exchange experiments have focused on net CO2 exchange (Anet). Here, using California poplar (Populus trichocarpa), we coupled measurements of net oxygen production (NOP), isoprene emissions and δ18O in O2 to traditional CO2/H2O gas exchange with chlorophyll fluorescence, and measured light, CO2 and temperature response curves. This allowed us to obtain a comprehensive picture of the photosynthetic redox budget including electron transport rate (ETR) and estimates of the mean assimilatory quotient (AQ = Anet/NOP). We found that Anet and NOP were linearly correlated across environmental gradients with similar observed AQ values during light (1.25 ± 0.05) and CO2 responses (1.23 ± 0.07). In contrast, AQ was suppressed during leaf temperature responses in the light (0.87 ± 0.28), potentially due to the acceleration of alternative ETR sinks like lipid synthesis. Anet and NOP had an optimum temperature (Topt) of 31°C, while ETR and δ18O in O2 (35°C) and isoprene emissions (39°C) had distinctly higher Topt. The results confirm a tight connection between water oxidation and ETR and support a view of light-dependent lipid synthesis primarily driven by photosynthetic ATP/NADPH not consumed by the Calvin-Benson cycle, as an important thermotolerance mechanism linked with high rates of (photo)respiration and CO2/O2 recycling.

Beyond biomass: Resource effects on primary production and consumer nutrient assimilation in streams

Abstract Primary production in aquatic ecosystems is strongly controlled by resource availability (bottom‐up). At the same time, grazers exert top‐down pressure on algae. The effects of both resources and grazers on algal communities have been investigated extensively for both benthic and planktonic systems. However, most studies focus exclusively on net effects on algal standing stock, while the underlying ecological functions and processes often remain unknown. Here, we tested the effects of the two resources, light (L) and the limiting nutrient phosphorus (P), on both primary production (net oxygen production) by benthic algae (periphyton) and matter assimilation by the next trophic guild (particularly nitrogen assimilation by grazers). For this, we grew natural periphyton communities in stream mesocosm flumes together with two native grazer taxa and 15N as a tracer. Both resources resulted in an initial increase in net primary production, but this effect equalised after 36 days in the more mature communities. While both resources still showed strong positive effects on algal biovolume, the biovolume‐specific net primary production decreased with resource level and was lowest in the double elevated (+L, +P) treatment. The response of nitrogen assimilation by grazers was more closely related to the response of periphyton quality (stoichiometric and taxonomic composition) than to its biomass. In addition, light affected the nitrogen assimilation of both grazers negatively. These findings demonstrate that the responses of ecological functions, here primary production and assimilation by grazers, are not necessarily proportional to algal biomass and are thus not easily predictable from the common use of biomass as a proxy for functions.

Stack-level analysis of the performance variation in air-cooled PEMFC with Z-type anode manifold

Air-cooled proton exchange membrane fuel cell (PEMFC) stack holds significant promise due to its potential to reduce system complexity. The anode manifold type leads to gas maldistribution in an air-cooled stack, and affects the efficiency and life span of the stack. In this study, 3D multi-physics model of stacks ranging from 10 to 40 cells are established to investigate the gas distribution characteristic of Z-type anode manifold. Considering the heat dissipation aspects only, it is advisable to keep a thin endplate while meeting the required stress tolerances. The core temperature of the stack with 20 mm endplates measures 43.8 °C, while the stack with 6 mm endplates reaches a core temperature of 45.5 °C. Under a cathode air flow velocity condition of 4 m‧s−1, the maximum net oxygen consumption reaches 1.69 mol‧s−1 in Cell-4. Furthermore, the electrochemical reaction intensity decreases sequentially from Cell-4 and Cell-5 in the middle to Cell-1 and Cell-10 in the edges of the stack. Additionally, cells near the anode inlet and outlet are prone to have the insufficient gas supply in the Z-type anode manifold stack.

AI optimization framework using digital layouts of array structures: A case study for fuel cells

It is challenging for common artificial intelligence (AI) frameworks to optimize layouts of array structures, because AI is unable to recognize and adjust layouts. To address this challenge, a novel AI optimization framework has been proposed, where layouts of array structures are represented in digital codes to enable AI in layout optimizations. The optimizations of baffle layout and modular channel layout in proton exchange membrane fuel cells are used to validate the methodology. For the multi-objective optimization of baffle layout, although the channel with optimal baffle layout (OC) exhibits a little higher pressure drop than the conventional channel, it increases the net power density and oxygen uniformity by 2.4% and 1.5%, respectively. Consistently, for the multi-objective optimization of channel layout, the optimal flow field with an optimal channel layout (OFF-OC) shows a small increase of pressure drop that can be compensated by the much-enhanced output. So, the net power density of OFF-OC has been elevated by 7.0% in comparison with that of the conventional parallel flow field. Meanwhile, the oxygen uniformity of OFF-OC has also been improved by 4.0%. The simulation results have been verified experimentally. These examples have demonstrated the effectiveness of the AI optimization framework using digital layouts.

Unveiling gas–liquid metal reactions in metal additive manufacturing: High-fidelity modeling validated with experiments

Gases in the atmosphere inevitably react with the melt pool during metal additive manufacturing (AM). Oxygen is particularly reactive, and excessive uncontrolled oxidation is detrimental, so most machines purge the chamber with inert gases, which can minimize but not eliminate such reactions. Alternatively, some users exploit the gas–liquid metal reactivity as an opportunity to introduce beneficial precipitates into the melt pool (“reactive AM”). However, the gas–liquid metal reaction and mechanisms in both scenarios remain unclear. Experimental works hitherto provide different explanations to the same phenomena. Therefore, this work seeks to clarify the mass transfer process of oxygen during metal AM through high-fidelity modeling by considering the competition between diffusion and chemical reaction, suboxide evaporation, and the influence of the vapor plume. The simulation results, validated with experiments, provide consolidated insights into the oxygen evolution behavior during metal AM. Counterintuitively, higher melt pool temperatures do not necessarily lead to greater oxidation rates during processing. The melt pool has regions of high and low oxygen gains due to temperature-dependent reaction regimes, with concurrent oxygen loss from evaporation of metal suboxides. Thus, the net oxygen flux varies for different materials, and the oxygen content cumulatively changes as multiple tracks are scanned. Overall, this work provides useful guidance to the AM community that seeks to ameliorate or exploit the inevitable gas–liquid interaction in metal AM. Cost-saving measures may be possible for determining the purity of shielding gas used in AM, and physics-guided measures can be taken to limit or control gas–liquid metal reactions.

Critical thermal maxima and oxygen uptake in Elysia viridis, a sea slug that steals chloroplasts to photosynthesize.

Photosynthetic animals produce oxygen, providing an ideal lens for studying how oxygen dynamics influence thermal sensitivity. The algivorous sea slug Elysia viridis can steal and retain chloroplasts from the marine alga Bryopsis sp. for months when starved, but chloroplast retention is mere weeks when they are fed another green alga, Chaetomorpha sp. To examine plasticity in thermal tolerance and changes in net oxygen exchange when fed and starving, slugs fed each alga were acclimated to 17°C (the current maximum temperature to which they are exposed in nature) and 22°C (the increase predicted for 2100) and measured at different points during starvation. We also examined increased illumination to evaluate a potential tradeoff between increased oxygen production but faster chloroplast degradation. Following acclimation, we subjected slugs to acute thermal stress to determine their thermal tolerance. We also measured net oxygen exchange before and after acute thermal stress. Thermal tolerance improved in slugs acclimated to 22°C, indicating they can acclimate to temperatures higher than they naturally experience. All slugs exhibited net oxygen uptake, and rates were highest in recently fed slugs before exposure to acute thermal stress. Oxygen uptake was suppressed following acute thermal stress. Under brighter light, slugs exhibited improved thermal tolerance, possibly because photosynthetic oxygen production alleviated oxygen limitation. Accordingly, this advantage disappeared later in starvation when photosynthesis ceased. Thus, E. viridis can cope with heatwaves by suppressing metabolism and plastically adjusting heat tolerance; however, starvation influences a slug's thermal tolerance and oxygen uptake such that continuous access to algal food for its potential nutritive and oxygenic benefits is critical when facing thermal stress.

Net Heart Rate for Estimating Oxygen Consumption in Active Adults.

The aim of this study was to verify the accuracy of predicting oxygen consumption (O2) in predominantly aerobic activities based on net heart rate (netHR), sex, and body mass index (BMI) in active adults. NetHR is the value of the difference between the resting HR (HRrest) and the average HR value obtained during a given session or period of physical activity. These activities must be continuous, submaximal, and of a stabilized intensity. The magnitude of the netHR depends mainly on the intensity of the exercise. The HR is measured in beats per minute (bpm). A total of 156 participants, 52 women and 104 men, between the ages of 18 and 81, had their netHR and net oxygen intake (netVO2) assessed. There were 79 participants in group 1 (prediction sample) (52 males and 27 females). There were 77 people in group 2 (validation sample) (52 males and 25 females). The results of the multiple linear regression showed that netVO2 (R2 = 85.2%, SEE = 3.38) could be significantly predicted by sex (p < 0.001), netHR (p < 0.001), and BMI (p < 0.001). The Bland-Altman plots satisfied the agreement requirements, and the comparison of the measured and estimated netVO2 revealed non-significant differences with a trivial effect size. We calculated the formula NetVO2 (mL/(kg·min)) = 16 + 3.67 (sex) + 0.27 (netHR) - 0.57 (BMI) to predict netVO2, where netVO2 is the amount of oxygen uptake (mL/(kg·min)) above the resting value, netHR is the heart rate (beats per minute) above the resting value measured during exercise, sex is equal to zero for women and one for men, and BMI is the body mass index. In addition, based on the knowledge of VO2, it was possible to estimate the energy expenditure from a particular training session, and to determine or prescribe the exercise intensity in MET (metabolic equivalent of task).

Stability of Near-Surface Nitrogen Vacancy Centers Using Dielectric Surface Passivation.

We study the photophysical stability of ensemble near-surface nitrogen vacancy (NV) centers in diamond under vacuum and air. The optically detected magnetic resonance contrast of the NV centers was measured following exposure to laser illumination, showing opposing trends in air compared to vacuum (increasing by up to 9% and dropping by up to 25%, respectively). Characterization using X-ray photoelectron spectroscopy (XPS) suggests a surface reconstruction: In air, atmospheric oxygen adsorption on a surface leads to an increase in NV- fraction, whereas in vacuum, net oxygen desorption increases the NV0 fraction. NV charge state switching is confirmed by photoluminescence spectroscopy. Deposition of ∼2 nm alumina (Al2O3) over the diamond surface was shown to stabilize the NV charge state under illumination in either environment, attributed to a more stable surface electronegativity. The use of an alumina coating on diamond is therefore a promising approach to improve the resilience of NV sensors.

Implications of increased intertidal inundation on seagrass net primary production

Seagrass meadows are productive and functionally important seafloor habitats that are declining in many parts of the world under pressure from human activities. Seabed light availability is inversely related to water depth, thus sea level rise associated with global warming may impinge upon seagrass productivity and its contributions to coastal ecosystems. Across natural variation in seagrass bed depth driven by tidal oscillations and distance from shore, we quantified seafloor oxygen exchange every 15 min for three days and nights at sites in New Zealand using aquatic eddy covariance to better understand the implications of sea level rise on shallow seagrass. Four sites of differing mean depth (1.5–4.0 m deep at high tide) were net producers of oxygen when photosynthetically active radiation at the seabed was >87.5 μmol photons m−2 s−1, with net oxygen production >4 mmol m−2 h−1 at all sites during peak sunlight hours. The effect of light attenuation on dissolved oxygen production was evident when comparing fluxes in the morning (low tide) and evening (high tide), with relatively similar incident light levels arriving at water surface at these times of day. The shallowest site was expected to receive the greatest amount of light at the seabed and be most productive. However, the second shallowest site was the most productive, due to higher seagrass cover and clearer water (less light attenuation in the water column). Higher cover of seagrass likely reduced sediment resuspension while increasing photosynthetic capacity (i.e., more photosynthetic biomass). The existence of turbid fringes of water near the upper shore and climate-related shifts in suspended sediment concentrations may interfere with predictions of shoreward migration of seagrass meadows, which are already complicated by interactions with upper intertidal vegetation (e.g., mangroves) and human built structures (e.g., seawalls).

Ameliorating Effects of Graphene Oxide on Cadmium Accumulation and Eco-Physiological Characteristics in a Greening Hyperaccumulator (Lonicera japonica Thunb.).

Graphene oxide (GO), as a novel carbon-based nanomaterial (CBN), has been widely applied to every respect of social life due to its unique composite properties. The widespread use of GO inevitably promotes its interaction with heavy metal cadmium (Cd), and influences its functional behavior. However, little information is available on the effects of GO on greening hyperaccumulators under co-occurring Cd. In this study, we chose a typical greening hyperaccumulator (Lonicera japonica Thunb.) to show the effect of GO on Cd accumulation, growth, net photosynthesis rate (Pn), carbon sequestration and oxygen release functions of the plant under Cd stress. The different GO-Cd treatments were set up by (0, 10, 50 and 100 mg L-1) GO and (0, 5 and 25 mg L-1) Cd in solution culture. The maximum rate of Cd accumulation in the roots and shoots of the plant were increased by 10 mg L-1 GO (exposed to 5 mg L-1 Cd), indicating that low-concentration GO (10 mg L-1) combined with low-concentration Cd (5 mg L-1) might stimulate the absorption of Cd by L. japonica. Under GO treatments without Cd, the dry weight of root and shoot biomass, Pn value, carbon sequestration per unit leaf area and oxygen release per unit leaf area all increased in various degrees, especially under 10 mg L-1 GO, were 20.67%, 12.04%, 35% and 28.73% higher than the control. Under GO-Cd treatments, it is observed that the cooperation of low-concentration GO (10 mg L-1) and low-concentration Cd (5 mg L-1) could significantly stimulate Cd accumulation, growth, photosynthesis, carbon sequestration and oxygen release functions of the plant. These results indicated that suitable concentrations of GO could significantly alleviate the effects of Cd on L. japonica, which is helpful for expanding the phytoremediation application of greening hyperaccumulators faced with coexistence with environment of nanomaterials and heavy metals.

Effect of ocean outfall discharge volume and dissolved inorganic nitrogen load on urban eutrophication outcomes in the Southern California Bight

Climate change is increasing drought severity worldwide. Ocean discharges of municipal wastewater are a target for potable water recycling. Potable water recycling would reduce wastewater volume; however, the effect on mass nitrogen loading is dependent on treatment. In cases where nitrogen mass loading is not altered or altered minimally, this practice has the potential to influence spatial patterns in coastal eutrophication. We apply a physical-biogeochemical numerical ocean model to understand the influence of nitrogen management and potable wastewater recycling on net primary productivity (NPP), pH, and oxygen. We model several theoretical management scenarios by combining dissolved inorganic nitrogen (DIN) reductions from 50 to 85% and recycling from 0 to 90%, applied to 19 generalized wastewater outfalls in the Southern California Bight. Under no recycling, NPP, acidification, and oxygen loss decline with DIN reductions, which simulated habitat volume expansion for pelagic calcifiers and aerobic taxa. Recycling scenarios under intermediate DIN reduction show patchier areas of pH and oxygen loss with steeper vertical declines relative to a “no recycling” scenario. These patches are diminished under 85% DIN reduction across all recycling levels, suggesting nitrogen management lowers eutrophication risk even with concentrated discharges. These findings represent a novel application of ocean numerical models to investigate the regional effects of idealized outfall management on eutrophication. Additional work is needed to investigate more realistic outfall-specific water recycling and nutrient management scenarios and to contextualize the benefit of these management actions, given accelerating acidification and hypoxia from climate change.

Population and functional changes in a multispecies co-culture of marine microalgae and cyanobacteria under a combination of different salinity and temperature levels

Changes in the temperature or salinity of ocean waters can affect marine organisms at multiple trophic levels. Both environmental variables could have an impact on marine microalgae populations. Therefore, the effect of the combination of three levels of temperature (20, 24 and 28 °C), and three levels of salinity (33, 36, and 39 PSU) were evaluated on the growth of a multispecies community of five common species of phytoplankton: (one cyanobacteria, Synechococcus sp., and four microalgae, Chaetoceros gracilis, Amphidinium carterae, Pleurochrysis roscoffensis and Rhodomonas baltica). The co-culture was monitored by flow cytometry under controlled conditions in a 96 h study. The effect of both variables on dissolved oxygen concentrations was measured using the SDR SensorDish Reader system. The results demonstrated that Synechococcus sp., C. gracilis, and A. carterae displayed a high growth at the temperature of 28 °C combined with the lowest salinity assayed. However, salinity increases negatively affected the growth of P. roscoffensis and R. baltica. Decreased salinity combined with decreased temperature exhibited a higher net O2 production. The interaction of two environmental factors related to global change such as temperature and salinity can cause structural (community growth) and functional (net oxygen production) changes in a phytoplanktonic community.

The mechanisms underpinning the slow component of [Formula: see text] in humans.

When exercising above the lactic threshold (LT), the slow component of oxygen uptake ([Formula: see text]) appears, mainly ascribed to the progressive recruitment of Type II fibers. However, also the progressive decay of the economy of contraction may contribute to it. We investigated oxygen uptake ([Formula: see text]) during isometric contractions clamping torque (T) or muscular activation to quantify the contributions of the two mechanisms. We assessed for 7minT of the leg extensors, net oxygen uptake ([Formula: see text]) and root mean square (RMS) from vastus lateralis (VL) in 11 volunteers (21 ± 2 yy; 1.73 ± 0.11m; 67 ± 14kg) during cyclic isometric contractions (contraction/relaxation 5s/5s): (i) at 65% of maximal voluntary contraction (MVC) (FB-Torque) and; (ii) keeping the level of RMS equal to that at 65% of MVC (FB-EMG). [Formula: see text] after the third minute in FB-Torque increased with time ([Formula: see text] = 94 × t + 564; R2 = 0.99; P = 0.001), but not during FB-EMG. [Formula: see text]/T increased only during FB-Torque ([Formula: see text]/T = 1.10 × t + 0.57; R2 = 0.99; P = 0.001). RMS was larger in FB-Torque than in FB-EMG and significantly increased in the first three minutes of exercise to stabilize till the end of the trial, indicating that the pool of recruited MUs remained constant despite [Formula: see text]. The analysis of the RMS, [Formula: see text] and T during FB-Torque suggests that the intrinsic mechanism attributable to the decay of contraction efficiency was responsible for an increase of [Formula: see text] equal to 18% of the total [Formula: see text].

High-resolution study of the air-sea CO2 flux and net community oxygen production in the Ligurian Sea by a fleet of gliders

Intense glider monitoring was conducted in the Ligurian Sea for five months to capture the Net Community Production (NCP) variability in one of the most dynamic and productive regions of the Mediterranean Sea. Using the SeaExplorer glider technology, we were able to observe continuously from January to the end of May 2018 the physical and biogeochemical variables during the last period of intense convection observed in this region. High-frequency measurements from these gliders provided valuable information for determining dissolved O2 (DO) concentrations between coastal and open sea waters. Our DO balance approach provided an estimate of NCP fluxes complemented by the prediction of air-sea CO2 fluxes based on a neural network adapted to the Mediterranean Sea (CANYON-MED). Based on our NCP calculation method, our results show that the air-sea O2 flux and DO inventory have contributed largely to the NCP variability. The NCP values also suggest that heterotrophic conditions were predominant in winter and became autotrophic in spring, with strong variability in coastal waters due to the occurrence of sub-mesoscale structures. Finally, CO2 fluxes at the air-sea interface reveal that during the convection period, the central zone of the Ligurian Sea acted as a CO2 sink from January to March with little impact on NCP fluxes counterbalanced by a thermal effect of seawater.

Microplastic pollution associated with reduced respiration in seagrass (Zostera marina L.) and associated epiphytes

Seagrasses provide crucial ecosystem services of relevance for the marine environment. However, anthropogenic activities are causing global seagrass decline. Increasing microplastic (MP) concentrations have been recognized as a novel threat to many marine organisms, but their effects on marine plants remain underexplored. Here, we investigate the effects of microplastic (polyethylene (PE) and polypropylene (PP)) exposure on the photosynthesis and respiration of the seagrass Zostera marina L. and its associated epiphytes. Measurements were conducted on seagrass leaves with and without epiphyte cover, as well as on epiphytes scraped off the leaf surface. Net gas exchange and pH drift measurements were used to determine rates of photosynthesis and respiration, as well as the ability of leaves and epiphytes to utilize bicarbonate. In addition, variable chlorophyll fluorescence imaging was employed to quantify the photosynthetic capacity of seagrass leaves. Our results show a limited effect of short-term (14 days) microplastic exposure on seagrass leaves and their associated epiphytes, although the photosynthetic activity and respiration rates were gradually reduced for bare seagrass leaves with increasing microplastic concentrations (25-1000 mg MP L-1). A &amp;gt;50% reduction in dark respiration of bare leaves was found at the highest MP exposure, while respiration rates of leaves with epiphytes and separated epiphytes were reduced by maximally ~45 and 30% upon MP exposure, respectively. Short-term microplastic exposure did not alter i) the ability to utilize bicarbonate, ii) the maximum quantum yield of PSII (FV/FM), nor iii) the light utilization efficiency of Z. marina leaves and associated epiphytes. The compensation irradiance decreased for all investigated specimens, and seagrass leaves (with and without epiphytes) were able to retain a positive net oxygen balance throughout all treatments. We speculate that the observed decrease in photosynthetic activity and respiration was caused by leachates from microplastics. Our findings thus indicate that seagrass Z. marina largely possess resilience toward microplastic pollution at its current level.

Weak acids produced during anaerobic respiration suppress both photosynthesis and aerobic respiration

While photosynthesis transforms sunlight energy into sugar, aerobic and anaerobic respiration (fermentation) catabolizes sugars to fuel cellular activities. These processes take place within one cell across several compartments, however it remains largely unexplored how they interact with one another. Here we report that the weak acids produced during fermentation down-regulate both photosynthesis and aerobic respiration. This effect is mechanistically explained with an “ion trapping” model, in which the lipid bilayer selectively traps protons that effectively acidify subcellular compartments with smaller buffer capacities – such as the thylakoid lumen. Physiologically, we propose that under certain conditions, e.g., dim light at dawn, tuning down the photosynthetic light reaction could mitigate the pressure on its electron transport chains, while suppression of respiration could accelerate the net oxygen evolution, thus speeding up the recovery from hypoxia. Since we show that this effect is conserved across photosynthetic phyla, these results indicate that fermentation metabolites exert widespread feedback control over photosynthesis and aerobic respiration. This likely allows algae to better cope with changing environmental conditions.

Can osmotrophy in Gonyostomum semen explain why lake browning drives an expansion of the alga in parts of Europe?

Gonyostomum semen experiences a remarkable expansion in parts of Europe, partially driven by rising levels of dissolved organic matter (DOM), also called lake browning. One way to explain the stimulating effect of browning is to assume that G. semen combines photosynthesis with osmotrophy, the ability to utilize DOM as source of nutrients or energy. The present work tested this idea on a nearly unialgal G. semen bloom. Continuous vertical profiling found that each night, G. semen moved into the oxic-anoxic interface or the anoxic hypolimnion, i.e., into places where redox processes can give access to DOM via dissolution of ferric oxide-organic matter complexes. In the hypolimnion, conditions were right for a build-up of DOM, which, however, never happened. Optical analyses of hypolimnetic DOM indicated that the build-up was prevented by the removal of terrestrially derived DOM, which was correlated with G. semen. During the daytime, G. semen migrated into the epilimnion. Here, net oxygen production was near zero, indicating that G. semen combined photosynthesis with heterotrophic nutrition. Optical analyses of epilimnetic DOM hinted at the utilization of terrestrially derived DOM. Taken together, the above findings provide circumstantial evidence for G. semen combining photosynthesis with osmotrophy in nature. The proposed utilization of terrestrially derived DOM suggests that lake browning may favor G. semen by improving the access to DOM that the alga can utilize. Browning may also benefit G. semen by promoting summer anoxia that causes a reductive release of DOM from complexes with ferric oxides, improving the alga’s access to DOM.

Analysis of marine heatwaves and biogeochemistry in the Northern Arabian Sea

Marine heatwaves (MHWs) are abnormal Sea Surface Temperature (SST) conditions in which temperature exceeds the 90th percentile threshold limit. It has significant impacts on marine biodiversity causing large-scale coral bleaching, fish deaths or migration, and local weather changes. Analyzing MHWs and their causative mechanism can help to understand the notable influence on biodiversity. In this paper, we have investigated the MHW events in the Arabian Sea (AS) for the period 1982 to 2021. Our study reveals that an average of two MHW events occur every year with an increasing linear trend of 1.22 MHW events per decade in the northern AS region. In addition, we found a category-II MHW event that lasted about 134 days in the year 2020. This northern AS MHW event has a maximum, mean, and cumulative intensity of about 2.76 °C, 1.94 °C, and 259.35 °C-days (above the climatological mean), respectively, from 9thJune 2020 to 20thOctober 2020. According to our analysis, the western AS showed relatively higher intensity than the eastern part, especially near the coasts. The entire AS showed an increasing trend of MHW. We found that the decrease in wind velocity, increase of net downward heat flux, and lowering of surface ocean current is the main cause of the 2020 northern AS MHW event. Many of the biogeochemistry parameters, namely, chlorophyll (-16.2255%), net primary production (-25.5948%), dissolved oxygen (-0.7323%), nitrate (-23.9284%), and phosphate (-49.2542%) show the lowest values during the 2020 MHW event period except for silicate (13.9649%) for their corresponding climatological mean. Decreasing trends are also observed for chlorophyll (-0.0020 mg/m3-year), net primary production (-0.1747 mg/m3-year), oxygen (-0.1079 mmol/m3-year), nitrate (-0.0080 mmol/m3-year) and phosphate (-0.0076 mmol/m3-year) with increasing MHW days except for silicate that shows an increasing trend of 0.0018 mmol/m3-year with increasing MHW days with time.Plain Language SummaryForty years of SST data is used to find and analyze the most intense MHW events in the Arabian Sea (AS). We calculated its characteristics such as maximum, mean, cumulative intensity, and duration to understand and further analyze these MHW events. We have analyzed the time series of the number of MHW events per year from 1982 to 2021 alongside the trend to understand and predict the future MHW events in the northern AS region. We generated spatial plots of different MHW parameters over the entire AS and tried to understand their behavior. We compared changes in six marine biogeochemistry parameters spatially and temporally with the yearly MHW days and observed their relationship.

Artificial intelligence based structural optimization of solid oxide fuel cell with three-dimensional reticulated trapezoidal flow field

Three-dimensional Reticulated trapezoidal flow field (RTFF) is promising in improving the performance and durability of the solid oxide fuel cell (SOFC). However, the structural complexity makes it challenging for the geometry configuration of the splitter and mixer. To this end, an intelligent optimization framework is proposed by coupling artificial neural network (ANN) and non-dominated sorting genetic algorithm-II (NSGA-II), in order to maximize the net power density and oxygen uniformity simultaneously. The ANN prediction model is trained to obtain the computationally efficient surrogate model of the computational fluid dynamics (CFD) numerical simulation. NSGA-II is used for the multi-objective optimization of the RTFF structural parameters. The results illustrate that the prediction model is of high prediction precision and generalization capability. In comparison to SOFC with conventional parallel flow fields (CPFF), the degree of the performance improvement of SOFC with optimized RTFF depends on the working condition, i.e., fuel and air flow rates and operating temperatures. The SOFC with the optimal RTFF achieves a higher molar concentration of oxygen and a more uniform distribution of oxygen and current density than the CPFF SOFC. The proposed optimization framework provides an efficient design method for the development of the next-generation SOFC flow field.