New environmental model for thermodynamic ecology of biological phosphine production

Insight into the evolution of phosphorous conversion, microbial community and functional gene expression during anaerobic co-digestion of food waste and excess sludge with spicy substances exposure

Garlic and chili are widely used as food flavoring agents in food cooking, therefore might be accumulated in large amounts in food waste (FW). The effects of garlic and chili on the dissolution, hydrolysis, acidification and methanation in an anaerobic co-digestion system were investigated during the combined co-digestion of FW and excess sludge (ES). Additionally, the transformation of phosphorus form and microbial metabolism changes during the process were analyzed. The results showed the addition of garlic and chili promoted the release of protein in the soluble chemical oxygen demand. Secondly, the addition of garlic and chili up-regulated the relative abundances of key coding genes pstS, pstA, pstB and pstC. The relative abundances of the pstS and pstC genes increased by 0.0113% and 0.0021%, respectively, when 10 g garlic was added compared with no garlic. Furthermore, with respect to phosphorus conversion, the addition of garlic inhibited the conversion of solid phosphorus to gaseous phosphorus, whereas the addition of chili had the opposite effect. Meanwhile, garlic and chili inhibited the expression of key coding genes in phosphofructokinase. This work provides new insights into the phosphorus conversion and microbial metabolism in the process of anaerobic co-digestion of FW and ES under the influence of spicy substances.

Characteristics and mechanisms of phosphine production in sulfur-based constructed wetlands

Phosphine (PH3) is an important contributor to the phosphorus cycle and is widespread in various environments. However, there are few studies on PH3 in constructed wetlands (CWs). In this study, lab-scale CWs and batch experiments were conducted to explore the characteristics and mechanisms of PH3 production in sulfur-based CWs. The results showed that the PH3 release flux of sulfur-based CWs varied from 0.86±0.04 ng·m-2·h-1 to 1.88±0.09 ng·m-2·h-1. The dissolved PH3 was the main PH3 form in CWs and varied from 2.73 μg·L-1 to 4.08 μg·L-1. The matrix-bound PH3 was a staging reservoir for PH3 and increased with substrate depth. In addition, the sulfur-based substrates had a significant improvement on PH3 production. Elemental sulfur is more conducive to PH3 production than pyrite. Moreover, there was a significant positive correlation between PH3 production, the dsrB gene, and nicotinamide adenine dinucleotide (NADH). NADH might catalyze the phosphate reduction process. And the final stage of the dissimilatory sulfate reduction pathway driven by the dsrB gene might also provide energy for phosphate reduction. The migration and transformation of PH3 increased the available P (Resin-P and NaHCO3-P) from 35 % to 56 % in sulfur-based CW, and the P adsorption capacity was improved by 12 %. The higher proportion of available P increased the plant uptake rate of P by 17 %. This study improves the understanding of the phosphorus cycle in sulfur-based CW and provides new insight into the long-term stable operation of CWs.

Integration of molecular and computational approaches paints a holistic portrait of obscure metabolisms

Microorganisms are essential drivers of earth's geochemical cycles. However, the significance of elemental redox cycling mediated by microorganisms is often underestimated beyond the most well-studied nutrient cycles. Phosphite, (per)chlorate, and iodate are each considered esoteric substrates metabolized by microorganisms. However, recent investigations have indicated that these metabolisms are widespread and ubiquitous, affirming a need to continue studying the underlying microbiology to understand their biogeochemical effects and their interface with each other and our biosphere. This review focuses on combining canonical techniques of culturing microorganisms with modern omic approaches to further our understanding of obscure metabolic pathways and elucidate their importance in global biogeochemical cycles. Using these approaches, marker genes of interest have already been identified for phosphite, (per)chlorate, and iodate using traditional microbial physiology and genetics. Subsequently, their presence was queried to reveal the distribution of metabolic pathways in the environment using publicly available databases. In conjunction with each other, computational and experimental techniques provide a more comprehensive understanding of the location of these microorganisms, their underlying biochemistry and genetics, and how they tie into our planet's geochemical cycles.

Phototransformation of phosphite induced by zinc oxide nanoparticles (ZnO NPs) in aquatic environments

Phosphite, an essential component in the biogeochemical phosphorus cycle, may make significant contributions to the bioavailable phosphorus pool as well as water eutrophication. However, to date, the potential impacts of coexisting photochemically active substances on the environmental fate and transformation of phosphite in aquatic environments have been sparsely elucidated. In the present study, the effect of zinc oxide nanoparticles (ZnO NPs), a widely distributed photocatalyst in aquatic environments, on phosphite phototransformation under simulated solar irradiation was systematically investigated. The physicochemical characteristics of the pristine and reacted ZnO NPs were thoroughly characterized. The results showed that the presence of ZnO NPs induced the indirect phototransformation of phosphite to phosphate, and the reaction rate increased with increasing ZnO NPs concentration. Through experiments with quenching and trapping free radicals, it was proved that photogenerated reactive oxygen species (ROS), such as hydroxyl radical (•OH), superoxide anion (O2•-), and singlet oxygen (1O2), made substantial contributions to phosphite phototransformation. In addition, the influencing factors such as initial phosphite concentration, pH, water matrixes (Cl-, F-, Br-, SO42-, NO3-, NO2-, HCO3-, humic acid (HA) and citric acid (CA)) were investigated. The component of generated precipitates after the phosphite phototransformation induced by ZnO NPs was still dominated by ZnO NPs, while the presence of amorphous Zn3(PO4)2 was identified. This work explored ZnO NPs-mediated phosphite phototransformation processes, indicating that nanophotocatalysts released into aquatic environments such as ZnO NPs may function as photosensitizers to play a beneficial role in the transformation of phosphite to phosphate, thereby potentially mitigating the toxicity of phosphite to aquatic organisms while exacerbating eutrophication. The findings of this study provide a novel insight into the comprehensive assessment of the environmental fate, potential ecological risk, and biogeochemical behaviors of phosphite in natural aquatic environments under the condition of combined pollution.

Role of phosphite in the environmental phosphorus cycle

In modern geochemistry, phosphorus (P) is considered synonymous with phosphate (Pi) because Pi controls the growth of organisms as a limiting nutrient in many ecosystems. The researchers therefore realised that a complete P cycle is essential. Limited by thermodynamic barriers, P was long believed to be incapable of redox reactions, and the role of the redox cycle of reduced P in the global P cycling system was thus not ascertained. Nevertheless, the phosphite (Phi) form of P is widely present in various environments and participates in the global P redox cycle. Herein, global quantitative evidences of Phi are enumerated and the early origin and modern biotic/abiotic sources of Phi are elaborated. Further, the Phi-based redox pathway for P reduction is analysed and global multienvironmental Phi redox cycle processes are proposed on the basis of this pathway. The possible role of Phi in controlling algae in eutrophic lakes and its ecological benefits to plants are proposed. In this manner, the important role of Phi in the P redox cycle and global P cycle is systematically and comprehensively identified and confirmed. This work will provide scientific guidance for the future production and use of Phi products and arouse attention and interest on clarifying the role of Phi in the environmental phosphorus cycle.

Analysis of the process and factors influencing microbial phosphine production

The process of phosphine production by phosphate-reducing bacteria Pseudescherichia sp. SFM4 has been well studied. Phosphine originates from the biochemical stage of functional bacteria that synthesize pyruvate. Stirring the aggregated bacterial mass and supplying pure hydrogen could lead to an increase of 40 and 44% phosphine production, respectively. Phosphine was produced when bacterial cells agglomerated in the reactor. Extracellular polymeric substances secreted on microbial aggregates promoted the formation of phosphine due to the presence of groups containing phosphorus element. Phosphorus metabolism gene and phosphorus source analysis implied that functional bacteria used anabolic organic phosphorus, especially containing carbon-phosphorus bonds, as a source with [H] as electron donor to produce phosphine.

Phosphine gas in the dark induces severe phytotoxicity in Arabidopsis thaliana by increasing a hypoxia stress response and disrupting the energy metabolism: Transcriptomic approaches

Phosphine (PH₃) is an ideal fumigant alternative on methyl bromide (MB) as MB has been classified as an ozone-depleting substance. However, several challenges limit its efficient use in crop production, including the emergence of PH₃-resistant insect pests and the incidence of phytotoxic effects on nursery plants. Therefore, this study aims to elucidate the mechanism underlying PH₃ phytotoxicity in plants using transcriptomic techniques. Fumigation with 2 g/m³ PH₃ induced phytotoxic effects in A. thaliana, as evidenced by a decrease in growth and vegetation indices compared to the control group. Transcriptomic analysis revealed that PH₃ fumigation phytotoxicity responses in A. thaliana involve genes related to hypoxia stress and energy metabolism. Additionally, pretreatment with ethylene induced pre-adaptation to hypoxia under light conditions during fumigation effectively suppressed the phytotoxic effects of PH₃ in A. thaliana by increasing the expression of hypoxia-adaptive genes. Moreover, the phytotoxicity of PH₃ was also confirmed in pumpkin (Cucurbita moschata Duch.), and was dependent on light. Overall, our findings showed that fumigation under light conditions and ethylene pretreatment could be used to minimize PH₃-induced phytotoxic effects in plants.

New insights into the growth response of the macrophyte Vallisneria natans exposed to phosphite

Renewed interest in phosphite, an analog of phosphate, has increased due to its widespread distribution and increasing abundance in many waterbodies. However, up until recently very little is known about their ecological effects on aquatic organisms. Herein we studied the effects of phosphite via root and foliar exposure on the growth responses of the dominant pioneer macrophyte V. natans. Overall, both exposures of phosphite to V. natans resulted in significant reductions in the leaf length, root length, relative growth rate (RGR) and photosynthetic pigments, suggesting phosphite had an inhibitory effect on the plant growth. Our results further confirmed phosphite could induce the oxidative stresses in the V. natans cells, as indicated by the significantly increased intracellular enzyme activities i.e. superoxide dismutase activity (SOD) and malondialdehyde (MDA). Microscopic evidence also showed phosphite penetrated the cell membrane and destroyed membrane integrity under high phosphite stress. Besides, V. natans leaves exhibited intuitive deterioration symptoms, which seemed to be more sensitive to phosphite toxicity than roots. It is concluded that the increased abundance of phosphite in waterbodies cannot be utilized as a bioavailable P source but impose adverse physiological and metabolic limitations to plant growth, which should be receive more attention in the ecological risk assessment. Our result is necessary to build a comprehensive understanding of phosphite biogeochemical behaviors in aquatic ecosystems.

Enhanced biological phosphorus removal from wastewater by current stimulation coupled with anaerobic digestion

The integrated wastewater discharge standard for phosphorus has become increasingly strict. In this study, a synergetic current stimulation system coupled with anaerobic digestion was used to enhance phosphorus removal from wastewater. The effects of current intensity, pH, and methane (CH₄) synthesis on phosphorus removal were investigated. As direct current was supplied to an anaerobic bioreactor, the removal of sewage total phosphorus was significantly enhanced. The conditions of weak acid and low negative oxidation-reduction potential facilitated the phosphorus removal from wastewater. The optimal parameters for the dephosphorisation process were a current intensity of 100 mA and a pH of 6.0. When the anaerobic digestion process was inhibited by the reagent 2-bromoethanesulphonic acid sodium (BES), abundant metabolic intermediates accumulated and methanogenesis clearly decreased. Affected by the current stimulation and the inhibition of CH₄ synthesis, the formation of gaseous phosphine (PH₃) was greatly improved, and then PH₃ escaped from the digestion mixture after it was absorbed by microbial cells. The maximum PH₃ content of the digestion gas was 41.8 mg m^-3 in the reactor supplied with a current of 100 mA and BES addition of 10 mmol L^-1, and the phosphorus removal in this digestion system reached 55.2% at 6 d; however, the removal in the conventional anaerobic digestion system was only 17.7% after the same amount of time. Finally, a pathway of enhanced anaerobic biological phosphorus removal was proposed to better understand the inherent synergistic mechanism.

Phosphine on Venus Cannot Be Explained by Conventional Processes

The recent candidate detection of ∼1 ppb of phosphine in the middle atmosphere of Venus is so unexpected that it requires an exhaustive search for explanations of its origin. Phosphorus-containing species have not been modeled for Venus' atmosphere before, and our work represents the first attempt to model phosphorus species in the venusian atmosphere. We thoroughly explore the potential pathways of formation of phosphine in a venusian environment, including in the planet's atmosphere, cloud and haze layers, surface, and subsurface. We investigate gas reactions, geochemical reactions, photochemistry, and other nonequilibrium processes. None of these potential phosphine production pathways is sufficient to explain the presence of ppb phosphine levels on Venus. If PH₃'s presence in Venus' atmosphere is confirmed, it therefore is highly likely to be the result of a process not previously considered plausible for venusian conditions. The process could be unknown geochemistry, photochemistry, or even aerial microbial life, given that on Earth phosphine is exclusively associated with anthropogenic and biological sources. The detection of phosphine adds to the complexity of chemical processes in the venusian environment and motivates in situ follow-up sampling missions to Venus. Our analysis provides a template for investigation of phosphine as a biosignature on other worlds.

Phosphine Generation Pathways on Rocky Planets

The possibility of life in the venusian clouds was proposed in the 1960s, and recently this hypothesis has been revived with the potential detection of phosphine (PH₃) in Venus' atmosphere. These observations may have detected ∼5-20 ppb phosphine on Venus (Greaves et al., 2020), which raises questions about venusian atmospheric/geochemical processes and suggests that this phosphine could possibly be generated by biological processes. In such a claim, it is essential to understand the abiotic phosphorus chemistry that may occur under Venus-relevant conditions, particularly those processes that may result in phosphine generation. Here, we discuss two related abiotic routes for phosphine generation within the atmosphere of Venus. Based on our assessment, corrosion of large impactors as they ablate near Venus' cloud layer, and the presence of reduced phosphorus compounds in the subcloud layer could result in production of phosphine and may explain the phosphine detected in Venus' atmosphere or on other rocky planets. We end on a cautionary note: although there may be life in the clouds of Venus, the detection of a simple, single gas, phosphine, is likely not a decisive indicator.

The Case (or Not) for Life in the Venusian Clouds

The possible detection of the biomarker of phosphine as reported by Greaves et al. in the Venusian atmosphere stirred much excitement in the astrobiology community. While many in the community are adamant that the environmental conditions in the Venusian atmosphere are too extreme for life to exist, others point to the claimed detection of a convincing biomarker, the conjecture that early Venus was doubtlessly habitable, and any Venusian life might have adapted by natural selection to the harsh conditions in the Venusian clouds after the surface became uninhabitable. Here, I first briefly characterize the environmental conditions in the lower Venusian atmosphere and outline what challenges a biosphere would face to thrive there, and how some of these obstacles for life could possibly have been overcome. Then, I discuss the significance of the possible detection of phosphine and what it means (and does not mean) and provide an assessment on whether life may exist in the temperate cloud layer of the Venusian atmosphere or not.

Analysis of the characteristics of phosphine production by anaerobic digestion based on microbial community dynamics, metabolic pathways, and isolation of the phosphate-reducing strain

Although phosphine is ubiquitously present in anaerobic environments, little is known regarding the microbial community dynamics and metabolic pathways associated with phosphine formation in an anaerobic digestion system. This study investigated the production of phosphine in anaerobic digestion, with results indicating that phosphine production mainly occurred during logarithmic microbial growth. Dehydrogenase and hydrogen promoted the production of phosphine, with a maximum phosphine concentration of 300 mg/m³. The abundance of Ruminococcaceae and Escherichia was observed to promote phosphine generation. The analysis of metabolic pathways based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) and the MetaCyc pathway database revealed the highest relative abundance of replication and repair in genetic information processing; further, the cofactor, prosthetic group, electron carrier, and vitamin biosynthesis were observed to be closely related to phosphine formation. A phylogenetic tree was reconstructed based on the neighbor-joining method. The results indicated the clear evolutionary position of the isolated Pseudescherichia sp. SFM4 strain, adjacent to Escherichia, with a stable phosphate-reducing ability for a maximum phosphine concentration of 26 mg/m³. The response surface experiment indicated that the initial optimal conditions for phosphine production by SFM4 could be achieved with nitrogen, carbon, and phosphorus loads of 6.17, 300, and 10 mg/L, respectively, at pH 7.47. These results provide comprehensive insights into the dynamic changes in the microbial structure, isolated single bacterial strain, and metabolic pathways associated with phosphine formation. They also provide information on the molecular biology associated with phosphorus recycling.

Phosphine as a Biosignature Gas in Exoplanet Atmospheres

A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O₂, only a handful of gases have been considered in detail. In this study, we evaluate phosphine (PH₃). On Earth, PH₃ is associated with anaerobic ecosystems, and as such, it is a potential biosignature gas in anoxic exoplanets. We simulate the atmospheres of habitable terrestrial planets with CO₂- and H₂-dominated atmospheres and find that PH₃ can accumulate to detectable concentrations on planets with surface production fluxes of 10¹⁰ to 10¹⁴ cm^-2 s^-1 (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and ultraviolet (UV) irradiation. While high, the surface flux values are comparable to the global terrestrial production rate of methane or CH₄ (10¹¹ cm^-2 s^-1) and below the maximum local terrestrial PH₃ production rate (10¹⁴ cm^-2 s^-1). As with other gases, PH₃ can more readily accumulate on low-UV planets, for example, planets orbiting quiet M dwarfs or with a photochemically generated UV shield. PH₃ has three strong spectral features such that in any atmosphere scenario one of the three will be unique compared with other dominant spectroscopic molecules. Phosphine's weakness as a biosignature gas is its high reactivity, requiring high outgassing rates for detectability. We calculate that tens of hours of JWST (James Webb Space Telescope) time are required for a potential detection of PH₃. Yet, because PH₃ is spectrally active in the same wavelength regions as other atmospherically important molecules (such as H₂O and CH₄), searches for PH₃ can be carried out at no additional observational cost to searches for other molecular species relevant to characterizing exoplanet habitability. Phosphine is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets from any source that could generate the high fluxes required for detection.

Trivalent Phosphorus and Phosphines as Components of Biochemistry in Anoxic Environments

Phosphorus is an essential element for all life on Earth, yet trivalent phosphorus (e.g., in phosphines) appears to be almost completely absent from biology. Instead phosphorus is utilized by life almost exclusively as phosphate, apart from a small contingent of other pentavalent phosphorus compounds containing structurally similar chemical groups. In this work, we address four previously stated arguments as to why life does not explore trivalent phosphorus: (1) precedent (lack of confirmed instances of trivalent phosphorus in biochemicals suggests that life does not have the means to exploit this chemistry), (2) thermodynamic limitations (synthesizing trivalent phosphorus compounds is too energetically costly), (3) stability (phosphines are too reactive and readily oxidize in an oxygen (O₂)-rich atmosphere), and (4) toxicity (the trivalent phosphorus compounds are broadly toxic). We argue that the first two of these arguments are invalid, and the third and fourth arguments only apply to the O₂-rich environment of modern Earth. Specifically, both the reactivity and toxicity of phosphines are specific to aerobic life and strictly dependent on O₂-rich environment. We postulate that anaerobic life persisting in anoxic (O₂-free) environments may exploit trivalent phosphorus chemistry much more extensively. We review the production of trivalent phosphorus compounds by anaerobic organisms, including phosphine gas and an alkyl phosphine, phospholane. We suggest that the failure to find more such compounds in modern terrestrial life may be a result of the strong bias of the search for natural products toward aerobic organisms. We postulate that a more thorough identification of metabolites of the anaerobic biosphere could reveal many more trivalent phosphorus compounds. We conclude with a discussion of the implications of our work for the origin and early evolution of life, and suggest that trivalent phosphorus compounds could be valuable markers for both extraterrestrial life and the Shadow Biosphere on Earth.

Natural Products Containing 'Rare' Organophosphorus Functional Groups

Phosphorous-containing molecules are essential constituents of all living cells. While the phosphate functional group is very common in small molecule natural products, nucleic acids, and as chemical modification in protein and peptides, phosphorous can form P⁻N (phosphoramidate), P⁻S (phosphorothioate), and P⁻C (e.g., phosphonate and phosphinate) linkages. While rare, these moieties play critical roles in many processes and in all forms of life. In this review we thoroughly categorize P⁻N, P⁻S, and P⁻C natural organophosphorus compounds. Information on biological source, biological activity, and biosynthesis is included, if known. This review also summarizes the role of phosphorylation on unusual amino acids in proteins (N- and S-phosphorylation) and reviews the natural phosphorothioate (P⁻S) and phosphoramidate (P⁻N) modifications of DNA and nucleotides with an emphasis on their role in the metabolism of the cell. We challenge the commonly held notion that nonphosphate organophosphorus functional groups are an oddity of biochemistry, with no central role in the metabolism of the cell. We postulate that the extent of utilization of some phosphorus groups by life, especially those containing P⁻N bonds, is likely severely underestimated and has been largely overlooked, mainly due to the technological limitations in their detection and analysis.