Microorganisms oxidize glucose through distinct pathways in permeable and cohesive sediments.

In marine sediments, microbial degradation of organic matter under anoxic conditions is generally thought to proceed through fermentation to volatile fatty acids, which are then oxidized to CO2 coupled to the reduction of terminal electron acceptors (e.g. nitrate, iron, manganese, and sulfate). It has been suggested that, in environments with a highly variable oxygen regime, fermentation mediated by facultative anaerobic bacteria (uncoupled to external terminal electron acceptors) becomes the dominant process. Here, we present the first direct evidence for this fermentation using a novel differentially labeled glucose isotopologue assay that distinguishes between CO2 produced from respiration and fermentation. Using this approach, we measured the relative contribution of respiration and fermentation of glucose in a range of permeable (sandy) and cohesive (muddy) sediments, as well as four bacterial isolates. Under anoxia, microbial communities adapted to high-energy sandy or bioturbated sites mediate fermentation via the Embden-Meyerhof-Parnas pathway, in a manner uncoupled from anaerobic respiration. Prolonged anoxic incubation suggests that this uncoupling lasts up to 160 h. In contrast, microbial communities in anoxic muddy sediments (smaller median grain size) generally completely oxidized 13C glucose to 13CO2, consistent with the classical redox cascade model. We also unexpectedly observed that fermentation occurred under oxic conditions in permeable sediments. These observations were further confirmed using pure cultures of four bacteria isolated from permeable sediments. Our results suggest that microbial communities adapted to variable oxygen regimes metabolize glucose (and likely other organic molecules) through fermentation uncoupled to respiration during transient anoxic conditions.

Scalable Bright and Pure Single Photon Sources by Droplet Epitaxy on InP Nanowire Arrays.

High-quality quantum light sources are crucial components for the implementation of practical and reliable quantum technologies. The persistent challenge, however, is the lack of scalable and deterministic single photon sources that can be synthesized reproducibly. Here, we present a combination of droplet epitaxy with selective area epitaxy to realize the deterministic growth of single quantum dots in nanowire arrays. By optimization of the single quantum dot growth and the nanowire cavity design, single emissions are effectively coupled with the dominant mode of the nanowires to realize Purcell enhancement. The resonance-enhanced quantum emitter system boasts a brightness of millions of counts per second with nanowatt excitation power, a short radiation lifetime of 350 ± 5 ps, and a high single-photon purity with g(2)(0) value of 0.05 with continuous wave above-band excitation. Finite-difference time-domain (FDTD) simulation results show that the emissions of single quantum dots are coupled into the TM01 mode of the nanowires, giving a Purcell factor ≈ 3. Our technology can be used for creating on-chip scalable single photon sources for future quantum technology applications including quantum networks, quantum computation, and quantum imaging.

Stable isotopes of nitrate (δ15N and δ18O) as functional indicators of nitrogen processing in constructed wetlands.

The effectiveness of nitrogen removal in wetlands relies heavily on the biological processes that control its removal. Here, we used δ15N and δ18O of nitrate (NO3-) to assess the presence and the dominance of transformation processes of nitrogen in two urban water treatment wetlands in Victoria, Australia over two rainfall events. Laboratory incubation experiments were undertaken in both light and dark to measure the isotopic fractionation factor of nitrogen assimilation (by periphyton and algae) and benthic denitrification (using bare sediment). Highest isotopic fractionations were observed for nitrogen assimilation by algae and periphyton in the light, 15ε = -14.6 to -25 ‰ while the 15ε = -1.5 ‰ in bare sediment, consistent with that of benthic denitrification. Transect water samplings of the wetlands showed different rainfall patterns (discrete versus continuous) affect the removal capability of the wetlands. During the discrete event sampling, the observed 15ε of NO3- (an average of 3.0 to 4.3 ‰) within the wetland falls between the experimental 15ε of benthic denitrification and assimilation; coinciding with the decrease in NO3- concentrations, suggesting that both denitrification and assimilation were important removal pathways. Depletion of δ15N-NO3- throughout the whole wetland system also suggested the influence of water column nitrification during this time. In contrast, during continuous rain events, no fractionation effect was observed within the wetland and was consistent with limited NO3- removal. The difference in fractionation factors within the wetland during different sampling conditions suggested that nitrate removal was highly likely limited by changes in overall nutrient inputs, residence time and water temperature which impeded biological uptake or removal. These highlight that consideration of sampling condition is crucial when assessing the efficacy of a wetland in removing nitrogen.

Bottom-up, Chip-Scale Engineering of Low Threshold, Multi-Quantum-Well Microring Lasers.

Integrated, on-chip lasers are vital building blocks in future optoelectronic and nanophotonic circuitry. Specifically, III-V materials that are of technological relevance have attracted considerable attention. However, traditional microcavity laser fabrication techniques, including top-down etching and bottom-up catalytic growth, often result in undesirable cavity geometries with poor scalability and reproducibility. Here, we utilize the selective area epitaxy method to deterministically engineer thousands of microring lasers on a single chip. Specifically, we realize a catalyst-free, epitaxial growth of a technologically critical material, InAsP/InP, in a ring-like cavity with embedded multi-quantum-well heterostructures. We elucidate a detailed growth mechanism and leverage the capability to deterministically control the adatom diffusion lengths on selected crystal facets to reproducibly achieve ultrasmooth cavity sidewalls. The engineered devices exhibit a tunable emission wavelength in the telecommunication O-band and show low-threshold lasing with over 80% device efficacy across the chip. Our work marks a significant milestone toward the implementation of a fully integrated III-V materials platform for next-generation high-density integrated photonic and optoelectronic circuits.

Pulses of labile carbon cause transient decoupling of fermentation and respiration in permeable sediments.

Dihydrogen (H2) is an important intermediate in anaerobic microbial processes, and concentrations are tightly controlled by thermodynamic limits of consumption and production. However, recent studies reported unusual H2 accumulation in permeable marine sediments under anoxic conditions, suggesting decoupling of fermentation and sulfate reduction, the dominant respiratory process in anoxic permeable marine sediments. Yet, the extent, prevalence and potential triggers for such H2 accumulation and decoupling remain unknown. We surveyed H2 concentrations in situ at different settings of permeable sand and found that H2 accumulation was only observed during a coral spawning event on the Great Barrier Reef. A flume experiment with organic matter addition to the water column showed a rapid accumulation of hydrogen within the sediment. Laboratory experiments were used to explore the effect of oxygen exposure, physical disturbance and organic matter inputs on H2 accumulation. Oxygen exposure had little effect on H2 accumulation in permeable sediments suggesting both fermenters and sulfate reducers survive and rapidly resume activity after exposure to oxygen. Mild physical disturbance mimicking sediment resuspension had little effect on H2 accumulation; however, vigorous shaking led to a transient accumulation of H2 and release of dissolved organic carbon suggesting mechanical disturbance and cell destruction led to organic matter release and transient decoupling of fermenters and sulfate reducers. In summary, the highly dynamic nature of permeable sediments and its microbial community allows for rapid but transient decoupling of fermentation and respiration after a C pulse, leading to high H2 levels in the sediment.

Interactive effect of temperature and plant species on nitrogen cycling and treatment in stormwater biofiltration systems.

Stormwater biofiltration systems (also known as biofilters, bioretention, rain gardens) are engineered nature-based solutions, which help mitigate aquatic nitrogen pollution arising from storm runoff. These systems are being increasingly used in a range of climates across the world. A decline in treatment performance is frequently observed in cold weather conditions. While plant species comprise an important design factor influencing system performance, the effect of temperature on the fate of dissolved nitrogen forms, namely ammonium (NH4+) and nitrate (NO3-), in the presence of different plant species in these systems remains unclear. A large scale laboratory experiment was undertaken that measured potential rates of nitrification, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) as well as the microbial community structure to investigate nitrogen fate and hence removal under two different temperature conditions (2 °C and 15 °C) in the presence of four distinct plant species. The results indicate that lower nitrification rates (reduced by a factor of 4) coupled with potential media NH4+ desorption could be contributing to reduced NH4+ removal during cold conditions. Planting with species exhibiting good nutrient uptake capacity can reduce the extent of this performance decline. While NO3- reduction generally remains problematic during cold weather (<0 to 55% reduction), which may not be significantly different from warmer periods, the study demonstrated that the denitrification potential and gene abundance (nap, nar, NirS, norB, nosZ) to be higher than those of nitrification (amoA). Denitrification may not proceeding at optimal rates due to lack of conducive environmental conditions. Nitrogen transformation via DNRA was found to be relatively insignificant. Future studies should investigate the potential of employing cold-resilient plant species to maintain both NH4+ and NO3- removal in cold weather conditions.

Sources and cycling of nitrogen revealed by stable isotopes in a highly populated large temperate coastal embayment.

The identification of nitrogen sources and cycling processes is critical to the management of nitrogen pollution. Here, we used both stable (δ15N-NO3-, δ18O-NO3-, δ15N-NH4+) and radiogenic (222Rn) isotopes together with nitrogen concentrations to evaluate the relative importance of point (i.e. sewage) and diffuse sources (i.e. agricultural-derived NO3- from groundwater, drains and creeks) in driving nitrogen dynamic in a shallow coastal embayment, Port Phillip Bay (PPB) in Victoria, Australia. This study is an exemplar of nitrogen-limited coastal systems around the world where nitrogen contamination is prevalent and where constraining it may be challenging. In addition to surrounding land use, we found that the distributions of NO3- and NH4+ in the bay were closely linked to the presence of drift algae. Highest NO3- and NH4+ concentrations were 315 µmol L-1 and 2140 µmol L-1, respectively. Based on the isotopic signatures of NO3- (δ15N: 0.17 to 21‰; δ18O: 3 to 26‰) and NH4+ (δ15N: 30 to 39‰) in PPB, the high nitrogen concentrations were attributed to three major sources which varied between winter and summer; (1) nitrified sewage effluent and drift algae derived NH4+ mainly during winter, (2) NO3- mixture from atmospheric deposition, drains and creeks predominantly observed during summer and (3) groundwater and sewage derived NO3- during both surveys. The isotopic composition of NO3- also suggested the removal of agriculture-derived NO3- through denitrification was prevalent during transport. This study highlights the role of terrestrial-coastal interactions on nitrogen dynamics and illustrates the importance of submarine groundwater discharge as a prominent pathway of diffuse NO3- inputs. Quantifying the relative contributions of multiple NO3- input pathways, however, require more extensive efforts and is an important avenue for future research.

Hydrodynamic disturbance controls microbial community assembly and biogeochemical processes in coastal sediments.

The microbial community composition and biogeochemical dynamics of coastal permeable (sand) sediments differs from cohesive (mud) sediments. Tide- and wave-driven hydrodynamic disturbance causes spatiotemporal variations in oxygen levels, which select for microbial generalists and disrupt redox cascades. In this work, we profiled microbial communities and biogeochemical dynamics in sediment profiles from three sites varying in their exposure to hydrodynamic disturbance. Strong variations in sediment geochemistry, biogeochemical activities, and microbial abundance, composition, and capabilities were observed between the sites. Most of these variations, except for microbial abundance and diversity, significantly correlated with the relative disturbance level of each sample. In line with previous findings, metabolically flexible habitat generalists (e.g., Flavobacteriaceae, Woeseaiceae, Rhodobacteraceae) dominated in all samples. However, we present evidence that aerobic specialists such as ammonia-oxidizing archaea (Nitrosopumilaceae) were more abundant and active in more disturbed samples, whereas bacteria capable of sulfate reduction (e.g., uncultured Desulfobacterales), dissimilatory nitrate reduction to ammonium (DNRA; e.g., Ignavibacteriaceae), and sulfide-dependent chemolithoautotrophy (e.g., Sulfurovaceae) were enriched and active in less disturbed samples. These findings are supported by insights from nine deeply sequenced metagenomes and 169 derived metagenome-assembled genomes. Altogether, these findings suggest that hydrodynamic disturbance is a critical factor controlling microbial community assembly and biogeochemical processes in coastal sediments. Moreover, they strengthen our understanding of the relationships between microbial composition and biogeochemical processes in these unique environments.

Arthropod Predation of Vertebrates Structures Trophic Dynamics in Island Ecosystems.

AbstractOn isolated islands, large arthropods can play an important functional role in ecosystem dynamics. On the Norfolk Islands group, South Pacific, we monitored the diet and foraging activity of an endemic chilopod, the Phillip Island centipede (Cormocephalus coynei), and used a stable isotope mixing model to estimate dietary proportions. Phillip Island centipede diet is represented by vertebrate animals (48%) and invertebrates (52%), with 30.5% consisting of squamates, including the Lord Howe Island skink (Oligosoma lichenigera) and Günther's island gecko (Christinus guentheri); 7.9% consisting of black-winged petrel (Pterodroma nigripennis) nestlings; and 9.6% consisting of marine fishes scavenged from regurgitated seabird meals. Centipede predation was the principal source of petrel nestling mortality, with annual rates of predation varying between 11.1% and 19.6% of nestlings. This means that 2,109-3,724 black-winged petrel nestlings may be predated by centipedes annually. Petrels produce a single offspring per year; therefore, predation of nestlings by centipedes represents total breeding failure for a pair in a given year. Our work demonstrates that arthropods can play a leading role in influencing vertebrate reproductive output and modifying trophic structures and nutrient flow in island ecosystems.

Effects of drift algae accumulation and nitrate loading on nitrogen cycling in a eutrophic coastal sediment.

The permeable (sandy) sediments that dominate the world's coastlines and continental shelves are highly exposed to nitrogen pollution, predominantly due to increased urbanisation and inefficient agricultural practices. This leads to eutrophication, accumulation of drift algae and changes in the reactions of nitrogen, including the potential to produce the greenhouse gas nitrous oxide (N2O). Nitrogen pollution in coastal systems has been identified as a global environmental issue, but it remains unclear how this nitrogen is stored and processed by permeable sediments. We investigated the interaction of drift algae biomass and nitrate (NO3-) exposure on nitrogen cycling in permeable sediments that were impacted by high nitrogen loading. We treated permeable sediments with increasing quantities of added macroalgal material and NO3- and measured denitrification, dissimilatory NO3- reduction to ammonium (DNRA), anammox, and nitrous oxide (N2O) production, alongside abundance of marker genes for nitrogen cycling and microbial community composition by metagenomics. We found that the presence of macroalgae dramatically increased DNRA and N2O production in sediments without NO3- treatment, concomitant with increased abundance of nitrate-ammonifying bacteria (e.g. Shewanella and Arcobacter). Following NO3- treatment, DNRA and N2O production dropped substantially while denitrification increased. This is explained by a shift in the relative abundance of nitrogen-cycling microorganisms under different NO3- exposure scenarios. Decreases in both DNRA and N2O production coincided with increases in the marker genes for each step of the denitrification pathway (narG, nirS, norB, nosZ) and a decrease in the DNRA marker gene nrfA. These shifts were accompanied by an increased abundance of facultative denitrifying lineages (e.g. Pseudomonas and Marinobacter) with NO3- treatment. These findings identify new feedbacks between eutrophication and greenhouse gas emissions, and in turn have potential to inform biogeochemical models and mitigation strategies for marine eutrophication.

Epitaxially Grown InP Micro-Ring Lasers.

In the near future, technological advances driven by the Fourth Industrial Revolution will boost the demand for integrated, power-efficient miniature lasers, which are important for optical data communications and advanced sensing applications. Although top-down fabricated III-V semiconductor micro-disk and micro-ring lasers have been shown to be efficient light sources, challenges such as etching-induced sidewall roughness and poor fabrication scalability have been limiting the potential for high-density on-chip integration. Here, we demonstrate InP micro-ring lasers fabricated with a highly scalable epitaxial growth technique. With an optimized cavity design, the optically pumped micro-ring lasers show efficient room-temperature lasing with a lasing threshold of around 50 µJ cm-2 per pulse. Remarkably, through comprehensive modeling of the micro-ring laser, we demonstrate lasing mode engineering experimentally by tuning the vertical ring height. Our work is a major step toward realizing the high-density monolithic integration of III-V miniature lasers on submicrometer-scale optoelectronic devices.

Understanding Shape Evolution and Phase Transition in InP Nanostructures Grown by Selective Area Epitaxy.

There is a strong demand for III-V nanostructures of different geometries and in the form of interconnected networks for quantum science applications. This can be achieved by selective area epitaxy (SAE) but the understanding of crystal growth in these complicated geometries is still insufficient to engineer the desired shape. Here, the shape evolution and crystal structure of InP nanostructures grown by SAE on InP substrates of different orientations are investigated and a unified understanding to explain these observations is established. A strong correlation between growth direction and crystal phase is revealed. Wurtzite (WZ) and zinc-blende (ZB) phases form along <111>A and <111>B directions, respectively, while crystal phase remains the same along other low-index directions. The polarity induced crystal structure difference is explained by thermodynamic difference between the WZ and ZB phase nuclei on different planes. Growth from the openings is essentially determined by pattern confinement and minimization of the total surface energy, regardless of substrate orientations. A novel type-II WZ/ZB nanomembrane homojunction array is obtained by tailoring growth directions through alignment of the openings along certain crystallographic orientations. The understanding in this work lays the foundation for the design and fabrication of advanced III-V semiconductor devices based on complex geometrical nanostructures.

Land use and episodic rainfall as drivers of nitrogen exports in subtropical rivers: Insights from δ15N-NO3-, δ18O-NO3- and 222Rn.

Ongoing land-use intensification in subtropical catchments is expected to release more inorganic nitrogen to downstream coastal waters similar to historical changes in temperate ecosystems. Here, we examined spatial and temporal drivers of stream nitrogen loads across a subtropical land-use gradient using the isotopic compositions of nitrate (NO3--N) and radon (222Rn), a natural groundwater tracer. We investigated eleven subtropical creeks/rivers over contrasting hydrological conditions in Australia. NOx-N (nitrite (NO2--N) + nitrate (NO3--N)) accounted for 13.1%, 34.0%, and 42.6% of total dissolved nitrogen (TDN-N) in forest, peri-urban and agricultural creeks, respectively. Following an 80 mm rain event, loads of dissolved inorganic nitrogen (DIN-N) from agriculture catchments reached 368 mg N m-2 catchment area day-1. Forest and peri-urban catchments had aquatic TDN-N loads 17.8% and 31.1% of loads from agricultural catchments. Radon observations suggest that nitrogen and phosphorus loads were driven primarily by surface runoff rather than groundwater discharge. The δ15N-NO3- and δ18O-NO3- values in the agriculture, forest and peri-urban catchments indicate fertilisers and soil nitrogen as the main sources of NO3--N. However, one of the catchments (Double Crossing Creek) received a mixture of recirculated greywater and chemical nitrogen fertilisers. Isotopic signatures imply significant NO3--N losses via denitrification during dry conditions. Groundwater discharge played a minor role because regional aquifers were not contaminated by nitrogen. Overall, intensive agricultural land use and episodic rainfall events were the major spatial and temporal drivers of nitrogen loads.