Low Aeration Conditions Research Articles

A novel process based on powder carriers demonstrates robustness in nitrogen and phosphorus removal from real municipal wastewater

The development of efficient and low-consumption wastewater upgrading process is currently at the forefront of the wastewater treatment field. In this study, a novel wastewater treatment process based on powder carriers was proposed. Three systems, namely the activated sludge (AS) system, powder carrier (PC) system, and moving bed biofilm reactor (MBBR) system, were established and operated for over 140 days to treat real municipal wastewater. The characteristics and differences between the three systems were comprehensively investigated. The results suggested that the PC system exhibited notable advantages in nitrogen and phosphorus removal, especially under high influent load and low aeration conditions. The PC system, characterized by a higher nitrification rate compared to the MBBR system and a higher denitrification rate compared to the AS system, contributed to the stable nitrogen removal performance. The particle size of the zoogloea increased under the linkage of the powder carriers, and the mean size of micro-granules reached 170.88 μm. Large number of hydrophobic functional groups on sludge surface, coupled with increased protein content in EPS, further promoted sludge aggregation. Micro-granules formation improved settling performance and enhanced the abundance and activity of functional microbes. A significant enrichment in denitrifying bacteria and denitrifying phosphorus accumulating bacteria was observed in PC system. Up-regulation of the napA, narG, and nosZ genes was responsible for efficient nitrogen removal of the PC system. Moreover, a higher abundance in polyphosphate phosphotransferase (2.11 %) was found in PC system compared with AS and MBBR systems. The increase in the enzymes associated with poly-β-hydroxybutyrate (PHB) synthesis metabolism in PC system provided the energy for denitrification and phosphorus removal processes.

Optimized recombinant production of the bacteriocin garvicin Q by Corynebacterium glutamicum.

Bacteriocins are antimicrobial peptides applied in food preservation and are interesting candidates as alternatives to conventional antibiotics or as microbiome modulators. Recently, we established Corynebacterium glutamicum as a suitable production host for various bacteriocins including garvicin Q (GarQ). Here, we establish secretion of GarQ by C. glutamicum via the Sec translocon achieving GarQ titers of about 7 mg L-1 in initial fermentations. At neutral pH, the cationic peptide is efficiently adsorbed to the negatively charged envelope of producer bacteria limiting availability of the bacteriocin in culture supernatants. A combination of CaCl2 and Tween 80 efficiently reduces GarQ adsorption to C. glutamicum. Moreover, cultivation in minimal medium supplemented with CaCl2 and Tween 80 improves GarQ production by C. glutamicum to about 15 mg L-1 but Tween 80 resulted in reduced GarQ activity at later timepoints. Using a reporter strain and proteomic analyses, we identified HtrA, a protease associated with secretion stress, as another potential factor limiting GarQ production. Transferring production to HtrA-deficient C. glutamicum K9 improves GarQ titers to close to 40 mg L-1. Applying conditions of low aeration prevented loss in activity at later timepoints and improved GarQ titers to about 100 mg L-1. This is about 50-fold higher than previously shown with a C. glutamicum strain employing the native GarQ transporter GarCD for secretion and in the range of levels observed with the native producer Lactococcus petauri B1726. Additionally, we tested several synthetic variants of GarQ and were able to show that exchange of the methionine in position 5 to a phenylalanine (GarQM5F) results in markedly increased activity against Lactococcus lactis and Listeria monocytogenes. In summary, our findings shed light on several aspects of recombinant GarQ production that may also be of relevance for production with natural producers and other bacteriocins.

Effect of different inocula on the granulation process, reactor performance and biodiesel production of algal-bacterial granular sludge (ABGS) under low aeration conditions

The algal-bacterial granular sludge (ABGS) system is a prospective wastewater treatment technology, but few studies focused on the effects of different inoculum types on the establishment of the ABGS system under low aeration conditions (step-decrease superficial gas velocity from 1.4 to 0.5 cm/s). Results from this study indicated that compared with other inocula, the ABGS formed by co-inoculating aerobic granular sludge (AGS) and targeted algae (Chlorella) exhibited a shorter granulation period (shortened by 15 days), higher total nitrogen (89.4%) and PO43--P (95.0%) removal efficiencies, and a greater yield of fatty acid methyl esters (FAMEs) (9.04 mg/g MLSS). This was possibly attributed to that the functional bacteria (e.g. Thauera, Gemmobacter and Rhodobacter) in the inoculated AGS facilitated the ABGS granulation. The inoculated algae promoted their effective enrichment under illumination conditions and enhanced the production of extracellular polymeric substances, thus improving the stability of ABGS. The enriched algae were attached to the outer layer of the granules, which could provide sufficient oxygen for bacterial metabolism, revealing the inherent mechanisms for the good stability of ABGS under low aeration intensity. Overall, the rapid granulation of ABGS can be achieved by inoculating optimal inocula under low aeration conditions, which is convenient and economically feasible, and motivates the application of algal-bacterial consortia.

Biooxidation of hydrogen sulfide to sulfur by moderate thermophilic acidophilic bacteria.

The copper industry utilizes significant amounts of sulfuric acid in its processes, generating sulfate as waste. While sulfate-reducing bacteria can remove sulfate, it produces hydrogen sulfide (H2S) as a byproduct. This study examined the capability of a consortium consisting of Sulfobacillus thermosulfidooxidans and Sulfobacillus acidophilus to partially oxidize H2S to S° at a temperature of 45°C. A fixed-bed bioreactor, with glass rings as support material and sodium thiosulfate as a model electron donor, was inoculated with the consortium. Formation of biofilms was crucial to maintain the bioreactor's steady state, despite high flow rates. Afterward, the electron donor was changed to H2S. When the bioreactor was operated continuously and with high aeration, H2S was fully oxidized to SO42-. However, under conditions of low aeration and at a concentration of 0.26g/L of H2S, the consortium was able to oxidize H2S to S° with a 13% yield. S° was discovered attached to the glass rings and jarosite. The results indicate that the consortium could oxidize H2S to S° with a 13% yield under low aeration and at a concentration of 0.26g/L of H2S. The findings highlight the capability of a Sulfobacillus consortium to convert H2S into S°, providing a potential solution for addressing environmental and safety issues associated with sulfate waste generated by the mining industry.

Evaluation of ammonia emission reducing effect by adding waste cooking oil in pilot-scale composting of dairy cattle manure.

In our previous study, we observed that the addition of waste cooking oil (WCO) reduced ammonia (NH3) emissions during laboratory-scale composting of dairy cattle manure under low-aeration condition. Therefore, this study aimed to evaluate the effect of addition of WCO on NH3 emissions reduction during pilot-scale composting of dairy cattle manure, which is close to the conditions of practical composting treatment. Composting tests were conducted using pilot-scale composting facilities (1.8 m3 of capacity). The composting mixtures were prepared from manure, sawdust, and WCO. Two treatments were set: without WCO (Control) and with WCO added to 3 wt% of manure (WCO3). Composting was conducted under continuous aeration at 40 L/min, corresponding to 22.2 L/(min‧m3) of the mixture at the start of composting. The changes in temperatures, NH3 concentrations in the exhaust gases, and contents of the composted mixtures were analyzed. Based on these analysis results, the effect of WCO addition on NH3 emissions and nitrogen loss during composting was evaluated. During composting, the temperature increase of the composting mixture became higher, and the decreases of weight and water content of the mixture became larger in WCO3 than in Control. In the decrease of weight, and the residual weight and water content of the mixture, significant differences (p<0.05) were detected between the two treatments at the end of composting. The NH3 concentrations in the exhaust gases tended to be lower in WCO3 than in Control. Nitrogen loss was 21.5% lower in WCO3 than in Control. Reduction of NH3 emissions by the addition of WCO under low aeration condition was observed in pilot-scale composting, as well as in laboratory-scale composting. This result suggests that this method is effective in reducing NH3 emissions in practicalscale composting.

Rapid establishment of algal-bacterial granular sludge system by applying mycelial pellets in a lab-scale photo-reactor under low aeration conditions: Performance and mechanism analysis

Light-driven algal-bacterial granular sludge (ABGS) is an innovative low-carbon technology with significant merits in treating municipal wastewater, but how to shorten the photogranulation process, especially under low aeration conditions, is largely unknown. Herein, two strategies were proposed to accelerate the start-up of the ABGS system in photo-sequencing batch reactors (PSBRs) with a low superficial gas velocity of 0.5 cm/s. Compared to directly dosing mycelial pellets (MPs), applying MPs to flocculate algae and using the formed algal-mycelial pellets (AMPs) as carriers enhanced the establishment of the algal-bacterial symbiosis. The ABGS system developed rapidly within 20 days, with a large particle diameter (mean diameter of 321 μm) and excellent settleability (SVI30 of 55.4 mL/g). More importantly, this system could be stably operated for at least 100 days, mainly attributed to the reinforced secretion of protein with unique secondary structure and elevated hydrophobic functional groups. As for the reactor performance, the average removal efficiencies of the ABGS system were 97.8% for organic matter, 80.0% for total nitrogen, and 84.4% for phosphorus. The enrichment of functional bacteria and algae, and the up-regulation of functional genes and enzymes involved in electron production and transport processes likely drove the transformation of the pollutants, underlining the inherent mechanism for the excellent nutrient removal performance. This study provides a promising approach to solve the problem of a long ABGS start-up period and unstable granular structure under low aeration conditions, which is significant for achieving effective wastewater treatment without energy intensive aeration.

Differences in Bioenergetic Metabolism of Obligately Alkaliphilic Bacillaceae Under High pH Depend on the Aeration Conditions.

Alkaliphilic Bacillaceae appear to produce ATP based on the H+-based chemiosmotic theory. However, the bulk-based chemiosmotic theory cannot explain the ATP production in alkaliphilic bacteria because the H+ concentration required for driving ATP synthesis through the ATPase does not occur under the alkaline conditions. Alkaliphilic bacteria produce ATP in an H+-diluted environment by retaining scarce H+ extruded by the respiratory chain on the outer surface of the membrane and increasing the potential of the H+ for ATP production on the outer surface of the membrane using specific mechanisms of ATP production. Under high-aeration conditions, the high ΔΨ (ca. -170 mV) of the obligate alkaliphilic Evansella clarkii retains H+ at the outer surface of the membrane and increases the intensity of the protonmotive force (Δp) per H+ across the membrane. One of the reasons for the production of high ΔΨ is the Donnan potential, which arises owing to the induction of impermeable negative charges in the cytoplasm. The intensity of the potential is further enhanced in the alkaliphiles compared with neutralophiles because of the higher intracellular pH (ca. pH 8.1). However, the high ΔΨ observed under high-aeration conditions decreased (∼ -140 mV) under low-aeration conditions. E. clarkii produced 2.5–6.3-fold higher membrane bound cytochrome c in the content of the cell extract under low-aeration conditions than under high-aeration conditions. The predominant membrane-bound cytochrome c in the outer surface of the membrane possesses an extra Asn-rich segment between the membrane anchor and the main body of protein. This structure may influence the formation of an H+-bond network that accumulates H+ on the outer surface of the membrane. Following accumulation of the H+-bond network producing cytochrome c, E. clarkii constructs an H+ capacitor to overcome the energy limitation of low aeration at high pH conditions. E. clarkii produces more ATP than other neutralophilic bacteria by enhancing the efficacy per H+ in ATP synthesis. In low H+ environments, E. clarkii utilizes H+ efficiently by taking advantage of its high ΔΨ under high-aeration conditions, whereas under low-aeration conditions E. clarkii uses cytochrome c bound on its outer surface of the membrane as an H+ capacitor.

Rapid start-up of photo-granule process in a photo-sequencing batch reactor under low aeration conditions: Effect of inoculum AGS size

The photo-granule process is an effective and economically feasible alternative for wastewater treatment, but little information is available regarding how to speed up the photo-granulation process. In this study, the effect of inoculum aerobic granular sludge (AGS) size on the start-up of the photo-granule process was investigated under low aeration conditions (superficial gas velocity of 0.5 cm/s). For this purpose, the inoculum AGS was sorted into various size-categories (0.4–0.8 mm, 0.8–1.4 mm, 1.4–2.2 mm, and > 2.2 mm) to serve as individual inoculum sludge. The excellent settling properties (SVI5 of 39.3 mL/g), strong mechanical strength, efficient nutrient removal (COD: 94.2–97.1%; TN: 80.1–84.8%; TP: 60.4–91.5%), and high biodiesel yields (12.11 mg/g MLSS) were rapidly achieved in the system inoculated with 0.8–1.4 mm AGS. The granulation process was facilitated by filamentous algae as the nucleus, extracellular polymeric substances as the backbone, and the enrichment of functional bacteria (such as Thauera and Sphingorhabdus). Furthermore, the inherent influencing mechanisms of inoculum AGS size on the photo-granulation were revealed from cellular hydrophobicity, surface thermodynamics, and sludge aggregation behavior. This study provides a novel start-up approach of the photo-granule process by inoculating with the optimal AGS size, which is convenient, practically feasible and significantly reduced the aeration consumption.

Aerobic biocathodes with potential regulation for ammonia oxidation with concomitant cathodic oxygen reduction and their microbial communities

Aerobic biocathodes are effective construct for the simultaneous nitrification and denitrification, but the disturbance of cathodic oxygen reduction on ammonia oxidation and denitrification remains unclear. In this study, we revealed the oxygen reduction peak at −0.4 V (versus silver/silver chloride) by cyclic voltammetry analysis at a cathode without a biofilm. The reduction peak, however, showed a right shift from −0.4 to −0.3 V for the biocathode, indicating that the aerobic biocathode could simultaneously perform traditional nitrification and cathode oxygen reduction. Therefore, different electrode potentials ranging from −0.5 to −0.1 V were designed for regulating the ammonia oxidation rate, and the results showed that the highest oxidation rate reached 3.08 mg/h/L at a potential of −0.2 V under a low-aeration rate of 5 mL/min. High-throughput sequencing showed that Nitrosomonas and Rhodococcus were the dominant nitrogen removal genera in the biocathode, and the abundance of Devosia was related to the interactions between the aeration rate and the electrode potential. Furthermore, the amoC and hao genes responded to aeration and electrode potential regulation, and −0.2 V was more suitable for promoting the denitrification process under low-aeration conditions. Therefore, these findings provided new insights on cathodic potential control for ammonia oxidation and nitrogen removal as well as for the regulation of microbial communities.

Rapid Start-Up of Photo-Granule Process in a Photo-Sequencing Batch Reactor Under Low Aeration Condition: Impact of Inoculum Ags Size

Rapid Start-Up of Photo-Granule Process in a Photo-Sequencing Batch Reactor Under Low Aeration Condition: Impact of Inoculum Ags Size

Formation and characteristics of filamentous granular sludge.

Aerobic granular sludge process as a promising biotechnology has been one of the research hotspots in the area of wastewater treatment during the last two decades. In our study, after around 60 days' operation, filamentous granular sludge (FGS) was formed under low aeration (SAV = 0.085 cm/s) and multi-feeding conditions. The characteristics of FGS and the performance of the FGS system for organic matter and nutrients removal were investigated. The results showed that chemical oxygen demand (COD) and total organic carbon (TOC) removal efficiencies were relatively stable, while COD removal efficiency increased from 82% to 94% in the presence of sulfamethoxazole (SMZ) at low concentration (1 mg/L). At the same time, the TP removal efficiency could be improved and maintained at around 75%, while TN removal efficiency was flocculated at around 50%. The analysis of microbial diversity showed that Thiothrix and Trichococcus as typical filamentous species were detected and dominant in the FGS system. The abundance of Thiothrix increased from 15% to 34%, while Trichococcus decreased from 23% to 3% in the presence of SMZ.

Startup and stable operation of advanced continuous flow reactor and the changes of microbial communities in aerobic granular sludge.

In this study, the granular sludge was operated under low aeration condition in sequencing batch reactor (SBR) and advanced continuous flow reactor (ACFR), respectively. Through increasing the sludge retention time (SRT) from 22 days to 33 days, the ACFR was successful startup in 30 days and achieved long term stable operation. Under SBR operation condition, the aerobic granular sludge (AGS) showed good nitrogen (60%), phosphorus (96%) and COD removal performance. During stable operation of continuous-flow, the nitrogen removal efficiency was increasing to 70%, however, the phosphorus removal efficiency could only be restored to 65%. Meanwhile, the sludge discharge volume from ACFR was about half of that in SBR. Results of high-throughput pyrosequencing illustrated that methanogenic archaea (MA), ammonia oxidizing archaea (AOA), denitrifying bacteria (DNB), denitrifying polyphosphate-accumulating organisms (DPAOs) played an important role in the removal of nutrients in ACFR. This study could have positive effect on the practical application of AGS continuous flow process for simultaneous biological nutrient removal (SBNR).

Boosting productivity of heterotrophic microalgae by efficient control of the oxygen transfer coefficient using a microbubble sparger

We isolated and characterized an oleaginous Thraustochytrid microalga. Based on 18 s rRNA sequencing, we classified this strain as Schizochytrium sp. SH103. Growth of these cells in a 5 L fermentor with a microbubble-type sparger (MTS) efficiently increased the oxygen transfer coefficient (OTC), while inducing only low external stress. For fermentation under conditions of low agitation (100 rpm) and aeration (3 L/min), the OTC was 8.6-fold higher using a MTS than a general type sparger (GTS). Under these same conditions, dry cell weight (DCW) productivity (0.54 vs. 1.26 g/L·h) and total lipid productivity (0.29 vs. 0.61 g/L·h) were each about 2.2-fold higher using a MTS than a GTS. Under conditions that optimized the OTC in a MTS fermentor (876 h−1), DCW productivity, total lipid productivity and DHA productivity were each 1.3-fold greater than non-optimized condition. Fed-batch fermentation experiments indicated the maximal DCW was 100.6 g/L at 48 h, maximal DCW productivity was 2.1 g/L·h, maximal lipid content was 48.6 g/L at 48 h, and lipid productivity was 1.0 g/L·h. Additionally, the results of experiments using a 50 L MTS fermentor (96.0 g/L DCW and 44.3 g/L total lipids at 48 h) were similar to those using the 5 L fermentor. Our results suggest that use of a MTS may help to achieve the efficient fermentation of heterotrophic microalgae for high cell densities.

Myxococcus xanthus truncated globin HbO: in silico analysis and functional characterization.

Truncated globins are 20-40 amino acids shorter than full length globins. Till date, globins have been characterized predominantly from bacteria involved in pathogenicity, nitrogen fixation and photosynthesis, where they are implicated in bacterial virulence within the host, protection of nitrogenase from oxygen inactivation and prevention of oxidative damage to the photosynthetic machinery respectively. Myxococcus xanthus, the model myxobacterium, is an obligate aerobe with a multicellular stage in its life cycle where cells encounter oxygen limitation. This work was undertaken to investigate the potential role of the truncated globin in M. xanthus. To examine the role of globins in this unique group of bacteria, the gene coding for a putative truncated globin (HbO) was identified in the genome of M. xanthus DK 1622. The sequence analysis by bioinformatics approaches revealed that HbO from M. xanthus (Mx-HbO) likely adopts a 2-on-2 alpha helical fold of the truncated globins. The gene coding for Mx-HbO was cloned and its expression in E. coli imparted reddish tinge to the cells. The spectral analysis confirmed it to be a functional globin. The expression of Mx-HbO in the heterologous host improved its growth, resulting in the attainment of higher cell density in culture. The transcript of Mx-hbO was induced threefold in the host cells when grown under low aeration condition as compared to the cells grown under high aeration condition. In M. xanthus, an obligate aerobe, where cell growth accompanies swarming, there is a higher density of cells in the middle of the swarm. Our results suggest that Mx-HbO is a functional globin and could facilitate the growth of cells facing oxygen deprivation, the condition prevailing in the middle of the swarm.

Water turbulence effect on egg survival and characteristics of hatched larvae of the Black Sea turbot Scophthalmus maeoticus (Pallas, 1814)

The impact of different level of turbulence on developing eggs and prelarvae of the Black Sea turbot Scophthalmus maeoticus obtained from the pair of wild spawners at the end of natural spawning season has been studied. The incubation was carried out at three different levels of turbulence generated by diffused aeration: without aeration (calm water), low aeration (34 ml per minute) and high aeration (75 ml per minute). In calm water hatching rate (HR) of turbot eggs was 20 % and the hatched prelarvae had the longest standard length (SL = 3.13 mm) and the biggest volume of the yolk sac (VYS = 0.37 mm³) as compared to those of low aeration (HR = 18 %; SL = 3.10 mm; VYS = 0.32 mm³) and high aeration conditions (HR = 14 %; SL = 3.10 mm; VYS = 0.29 mm³). One day after hatching the percentage of survived prelarvae from the number of hatched was the highest – 86 % at high aeration, compared with that of low aeration (72 %) and calm water (61 %). Application of turbulence leads to elimination of “poor” quality eggs during incubation, presumably enables hatching of more viable larvae and can reduce mortality of more advanced larvae.

Oxygen-dependent regulation of SPI1 type three secretion system by small RNAs in Salmonella enterica serovar Typhimurium.

Salmonella Typhimurium induces inflammatory diarrhea and uptake into intestinal epithelial cells using the Salmonella pathogenicity island 1 (SPI1) type III secretion system (T3SS). Three AraC-like regulators, HilD, HilC and RtsA, form a feed-forward regulatory loop that activates transcription of hilA, encoding the activator of the T3SS structural genes. Many environmental signals and regulatory systems are integrated into this circuit to precisely regulate SPI1 expression. A subset of these regulatory factors affects translation of hilD, but the mechanisms are poorly understood. Here, we identified two sRNAs, FnrS and ArcZ, which repress hilD translation, leading to decreased production of HilA. FnrS and ArcZ are oppositely regulated in response to oxygen, one of the key environmental signals affecting expression of SPI1. Mutational analysis demonstrates that FnrS and ArcZ bind to the hilD mRNA 5' UTR, resulting in translational repression. Deletion of fnrS led to increased HilD production under low-aeration conditions, whereas deletion of arcZ abolished the regulatory effect on hilD translation aerobically. The fnrS arcZ double mutant has phenotypes in a mouse oral infection model consistent with increased expression of SPI1. Together, these results suggest that coordinated regulation by these two sRNAs maximizes HilD production at an intermediate level of oxygen.

Dialogue between Staphylococcus aureus SA15 and Lactococcus garvieae strains experiencing oxidative stress

BackgroundStaphylococcus aureus is an important foodborne pathogen. Lactococcus garvieae is a lactic acid bacterium found in dairy products; some of its strains are able to inhibit S. aureus growth by producing H2O2. Three strains of L. garvieae from different origins were tested for their ability to inhibit S. aureus SA15 growth. Two conditions were tested, one in which H2O2 was produced (high aeration) and another one in which it was not detected (low aeration). Several S. aureus genes related to stress, H2O2-response and virulence were examined in order to compare their level of expression depending on the inoculated L. garvieae strain. Simultaneous L. garvieae H2O2 metabolism gene expression was followed.ResultsThe results showed that under high aeration condition, L. garvieae strains producing H2O2 (N201 and CL-1183) inhibited S. aureus SA15 growth and impaired its ability to deal with hydrogen peroxide by repressing H2O2-degrading genes. L. garvieae strains induced overexpression of S. aureus stress-response genes while cell division genes and virulence genes were repressed. A catalase treatment partially or completely restored the SA15 growth. In addition, the H2O2 non-producing L. garvieae strain (Lg2) did not cause any growth inhibition. The SA15 stress-response genes were down-regulated and cell division genes expression was not affected. Under low aeration condition, while none of the strains tested exhibited H2O2-production, the 3 L. garvieae strains inhibited S. aureus SA15 growth, but to a lesser extent than under high aeration condition.ConclusionTaken together, these results suggest a L. garvieae strain-specific anti-staphylococcal mechanism and an H2O2 involvement in at least two of the tested L. garvieae strains.

Formation of Proton Motive Force Under Low-Aeration Alkaline Conditions in Alkaliphilic Bacteria.

In Mitchell’s chemiosmotic theory, a proton (H+) motive force across the membrane (Δp), generated by the respiratory chain, drives F1Fo-ATPase for ATP production in various organisms. The bulk-base chemiosmotic theory cannot account for ATP production in alkaliphilic bacteria. However, alkaliphiles thrive in environments with a H+ concentrations that are one-thousandth (ca. pH 10) the concentration required by neutralophiles. This situation is similar to the production of electricity by hydroelectric turbines under conditions of very limited water. Alkaliphiles manage their metabolism via various strategies involving the cell wall structure, solute transport systems and molecular mechanisms on the outer surface membrane. Our experimental results indicate that efficient ATP production in alkaliphilic Bacillus spp. is attributable to a high membrane electrical potential (ΔΨ) generated for an attractive force for H+ on the outer surface membrane. In addition, the enhanced F1Fo-ATPase driving force per H+ is derived from the high ΔΨ. However, it is difficult to explain the reasons for high ΔΨ formation based on the respiratory rate. The Donnan effect (which is observed when charged particles that are unable to pass through a semipermeable membrane create an uneven electrical charge) likely contributes to the formation of the high ΔΨ because the intracellular negative ion capacities of alkaliphiles are much higher than those of neutralophiles. There are several variations in the adaptation to alkaline environments by bacteria. However, it could be difficult to utilize high ΔΨ in the low aeration condition due to the low activity of respiration. To explain the efficient ATP production occurring in H+-less and air-limited environments in alkaliphilic bacteria, we propose a cytochrome c-associated “H+ capacitor mechanism” as an alkaline adaptation strategy. As an outer surface protein, cytochrome c-550 from Bacillus clarkii possesses an extra Asn-rich segment between the region anchored to the membrane and the main body of the cytochrome c. This structure may contribute to the formation of the proton-binding network to transfer H+ at the outer surface membrane in obligate alkaliphiles. The H+ capacitor mechanism is further enhanced under low-aeration conditions in both alkaliphilic Bacillus spp. and the Gram-negative alkaliphile Pseudomonas alcaliphila.

Key role of alternative oxidase in lovastatin solid-state fermentation.

Lovastatin is a commercially important secondary metabolite produced by Aspergillus terreus, either by solid-state fermentation or by submerged fermentation. In a previous work, we showed that reactive oxygen species (ROS) accumulation in idiophase positively regulates lovastatin biosynthetic genes. In addition, it has been found that lovastatin-specific production decreases with aeration in solid-state fermentation (SSF). To study this phenomenon, we determined ROS accumulation during lovastatin SSF, under high and low aeration conditions. Paradoxically, high aeration caused lower ROS accumulation, and this was the underlying reason of the aeration effect on lovastatin production. Looking for a mechanism that is lowering ROS production under those conditions, we studied alternative respiration. The alternative oxidase provides an alternative route for electrons passing through the electron transport chain to reduce oxygen. Here, we showed that an alternative oxidase (AOX) is expressed in SSF, and only during idiophase. It was shown that higher aeration induces higher alternative respiration (AOX activity), and this is a mechanism that limits ROS generation and keeps them within healthy limits and adequate signaling limits for lovastatin production. Indeed, the aox gene was induced in idiophase, i.e., at the time of ROS accumulation. Moreover, exogenous ROS (H2O2), added to lovastatin solid-state fermentation, induced higher AOX activity. This suggests that high O2 availability in SSF generates dangerously high ROS, so alternative respiration is induced in SSF, indirectly favoring lovastatin production. Conversely, alternative respiration was not detected in lovastatin-submerged fermentation (SmF), although exogenous ROS also induced relatively low AOX activity in SmF.

Can graminoids used for mine tailings revegetation improve substrate structure?

The seeding of agronomic graminoid species that are tolerant to the compacted and low aeration conditions associated with mine tailings allows for rapid cover of mine waste, which in turn controls erosion. These graminoids can be used as primer-species on mine tailings to improve the rooting of other plant species, which may not tolerate soil compaction and low aeration. Tailings colonization by graminoid roots could improve ecological filters such as low air-filled porosity and elevated bulk density. The effect of above- and below-ground development of graminoid species used for hay-field seeding on the macroporosity and density of gold mine tailings was studied under controlled conditions as well as in situ. All of the graminoid species tested improved the macroporosity of the tailings after only 2 months of growth under greenhouse conditions, but had no effect on the density of the tailings. The perennial Bromus inermis Leyss. was most efficient in improving the macroporosity of tailings, having greater root diameter, biomass, and volume. The annual Avena sativa L. also produced high root biomass and length, which improved the macroporosity of the tailings. However, under field conditions, graminoids had low cover and no effect on macroporosity, which highlights that their growth should be improved to make them usefull as primer-plants.