Nitrospira spp. are chemolithoautotrophic nitrite oxidizing bacteria (NOB), which are ubiquitous in natural and engineered environments. However, there exist few independent biokinetic studies on Nitrospira spp., likely because their isolation and selective enrichment from environmental consortia such as activated sludge can be challenging. Herein, planktonic Nitrospira spp. cultures closely related to Candidatus Nitrospira defluvii (Nitrospira lineage I) were successfully enriched from activated sludge in a sequencing batch reactor by maintaining sustained limiting extant nitrite and dissolved oxygen concentrations. Morphologically, the enrichment consisted largely of planktonic cells with an average characteristic diameter of 1.3 ± 0.6 µm. Based on respirometric assays, estimated maximum specific growth rate (μmax), nitrite half saturation coefficient (KS), oxygen half saturation coefficient (KO) and biomass yield coefficient (Y) of the enriched cultures were 0.69 ± 0.10 d-1, 0.52 ± 0.14 mg-N/L, 0.33 ± 0.14 mg-O2/L and 0.14 ± 0.02 mg-COD/mg-N, respectively. These parameters collectively reflect not just higher affinities of this enrichment for nitrite and oxygen, respectively, but also a higher biomass yield and energy transfer efficiency relative to Nitrobacter spp. Used in combination, these kinetic and thermodynamic parameters can help towards the development and application of energy efficient biological nutrient removal processes through effective Nitrospira out-selection.

We report the complete draft genome sequence of Cryptococcus albidus var. albidus, an oleaginous yeast, which can utilize various organic carbon sources for lipid synthesis. Availability of this genome will help elucidate factors driving lipid accumulation in C. albidus and contribute toward bioprocess development and optimization for engineered lipid production.

Nitrogen transformations in aquaponics with different edible plant species, i.e., tomato (Lycopersicon esculentum) and pak choi (Brassica campestris L. subsp. chinensis) were systematically examined and compared. Results showed that nitrogen utilization efficiencies (NUE) of tomato- and pak choi-based aquaponic systems were 41.3% and 34.4%, respectively. The abundance of nitrifying bacteria in tomato-based aquaponics was 4.2-folds higher than that in pak choi-based aquaponics, primarily due to its higher root surface area. In addition, tomato-based aquaponics had better water quality than that of pak choi-based aquaponics. About 1.5–1.9% of nitrogen input were emitted to atmosphere as nitrous oxide (N2O) in tomato- and pak choi-based aquaponic systems, respectively, suggesting that aquaponics is a potential anthropogenic source of N2O emission. Overall, this is the first intensive study that examined the role plant species played in aquaponics, which could provide new strategy in designing and operating an aquaponic system.

Lipid accumulation in the oleaginous yeast Cryptococcus albidus was evaluated using mixtures of volatile fatty acids (VFA) as substrates. In general, batch growth under nitrogen limitation led to higher lipid accumulation using synthetic VFA. During batch growth, an initial COD:N ratio of 25:1 mg COD:mg N led to maximum intracellular lipid accumulation (28.3 ± 0.7% g/g dry cell weight), which is the maximum reported for C. albidus using VFA as the carbon source, without compromising growth kinetics. At this feed COD:N ratio, chemostat cultures fed with synthetic VFA yielded statistically similar intracellular lipid content as batch cultures (29.9 ± 1.9%, g/g). However, batch cultures fed with VFA produced from the fermentation of food waste, yielded a lower lipid content (14.9 ± 0.1%, g/g). The lipid composition obtained with synthetic and food-waste-derived VFA was similar to commercial biodiesel feedstock. We therefore demonstrate the feasibility of linking biochemical waste treatment and biofuel production using VFA as key intermediates.

Despite recognition of the possible role of biological nitrogen removal (BNR) processes in nitrous oxide (N2O) emission, a measured database of N2O emissions from these processes at the national scale does not currently exist. This study focused on the quantification of N2O emissions at 12 wastewater treatment plants (WWTPs) across the United States using a newly developed U.S. Environmental Protection Agency (USEPA) reviewed protocol. A high degree of variability in field-scale measurements of N2O was observed, both across the WWTPs sampled and within each WWTP. Additionally, aerobic zones, which have hitherto not been considered in the USEPA approach of estimating N2O emissions, generally contributed more to N2O fluxes than anoxic zones from BNR reactors. These results severely qualify the conventional use of a single emission factor to “estimate” N2O emissions from BNR processes, solely by virtue of denitrification. Upon subjecting the nationwide data set to multivariate regression data mining, high nitrite, ammonium, and dissolved oxygen concentrations were positively correlated with N2O emissions from aerobic zones of activated sludge reactors. On the other hand, high nitrite and dissolved oxygen concentrations were positively correlated with N2O emissions from anoxic zones. Based on these results, it can be argued that activated sludge processes that minimize transient or permanent build up of ammonium or nitrite, especially in the presence of dissolved oxygen, are expected to have low N2O emissions.

The overall goal of this study was to determine the molecular and metabolic responses of chemostat cultures of model nitrifying bacteria to imposition of and recovery from transient anoxic conditions. Based on the study, a specific directionality in nitrous oxide (N2O) and nitric oxide (NO) production was demonstrated. N2O production was only observed during recovery to aerobic conditions after a period of anoxia and correlated positively with the degree of ammonia accumulation during anoxia. NO, on the other hand, was emitted mainly under anoxia. The production of NO was linked to a major imbalance in the expression of the nitrite reductase gene, which was overexpressed during transient anoxia. In contrast, genes coding for ammonia and hydroxylamine oxidation and nitric oxide reduction were generally under-expressed during transient anoxia. These results are different from the observed parallel expression and activity of nitrite and nitric oxide reductase in heterotrophic bacteria subjected to transient oxygen cycling. Unlike NO, the production of N2O could not be solely correlated to gene expression patterns and likely involved responses at the enzyme activity or metabolic levels. Based on experimental data, the propensity of the nitrifying cultures for N2O production is related to a shift in their metabolism from a low specific activity (q < qmax) toward the maximum specific activity (qmax).