A direct contact bioassay using immobilized microalgal balls to evaluate the toxicity of contaminated field soils.

The present study used a bioassay of immobilized microalgae (Chlorella vulgaris) via direct contact to assess the toxicity of eleven uncontaminated (reference) and five field contaminated soils with various physicochemical properties and contamination. Photosynthetic oxygen concentration in the headspace of the test kit by Chlorella vulgaris in the reference soils ranged between 12.93% and 14.80% and only 2.54%-7.14% in the contaminated soils, respectively. Inherent test variability (CVi) values ranged between 2.90% and 9.04%; variation due to soil natural properties (CVrs) ranged between 0.33% and 13.0%; and minimal detectable difference (MDD) values ranged from 4.69% to 11.6%. A computed toxicity threshold of 15% was established for microalgae soil toxicity tests based on calculations of the maximal tolerable inhibition (MTI). All contaminated soils were considered toxic to microalgae because their levels of inhibition ranged between 39.5% and 82.9%, exceeding the 15% toxicity threshold. It can be concluded that the elevated concentrations of heavy metals and organic contaminants in the contaminated soils induced the higher inhibitory levels. Overall, direct contact soil toxicity tests using immobilized microalgae provided coherent and repeatable data and can be utilized as a simple and suitable tool for the toxicity testing of contaminated field soils.

Enhanced biomass production through optimization of carbon source and utilization of wastewater as a nutrient source.

The study aimed to utilize the domestic wastewater as nutrient feedstock for mixotrophic cultivation of microalgae by evaluating appropriate carbon source. The microalgae Chlorella vulgaris was cultivated in municipal wastewater under various carbon sources (glucose, glycerol, and acetate), followed by optimization of appropriate carbon source concentration to augment the biomass, lipid, and carbohydrate contents. Under optimized conditions, namely of 5 g/L glucose, C. vulgaris showed higher increments of biomass with 1.39 g/L dry cell weight achieving biomass productivity of 0.13 g/L/d. The biomass accumulated 19.29 ± 1.83% total lipid, 41.4 ± 1.46% carbohydrate, and 33.06 ± 1.87% proteins. Moreover, the cultivation of Chlorella sp. in glucose-supplemented wastewater removed 96.9% chemical oxygen demand, 65.3% total nitrogen, and 71.2% total phosphate. The fatty acid methyl ester obtained showed higher amount (61.94%) of saturated fatty acid methyl esters associated with the improved fuel properties. These results suggest that mixotrophic cultivation using glucose offers great potential in the production of renewable biomass, wastewater treatment, and consequent production of high-value microalgal oil.

Enhancing anaerobic digestion for rural wastewater treatment with granular activated carbon (GAC) supplementation.

Notwithstanding many efforts to increase the efficiency of anaerobic digestion at low-temperature (winter) conditions, a cost-effective and efficient method is lacking. This study proposes a low-cost method of low-temperature (<35 °C) anaerobic digestion of wastewater, involving supplementation with granular activated carbon (GAC). Supplementation with GAC was found to reduce the lag time by 29.8% (from 15.1 to 10.6 days) and increase the maximum methane production rate by 23.4% (from 6.4 to 7.9 mL/day) at 25 °C. Network analysis demonstrated a strong co-occurrence of Syntrophobacteriales and hydrogenotrophic methanogens (Methanobacteriaceae; WSA2; Methanoregulaceae). GAC supplementation can drastically reduce the time required for organic matter decomposition and methane production, thereby increase the efficiency of wastewater treatment.

Effects of wire-type and mesh-type anode current collectors on performance and electrochemistry of microbial fuel cells.

Carbon-based material is commonly used for anodes in MFCs, but its low conductivity often limits anodic performance. Application of corrosion-resistive current collector to carbon-based anode can be a promising strategy for increasing the anodic performance. In this study, it was hypothesized increasing metal current collector improved anodic performance. Two different carbon-felt anodes with titanium wires (CF-W) or stainless steel mesh (CF-M) as a current collector were tested in a single chamber MFC. In the short-term tests such as polarization and impedance tests, CF-M with the larger current collector area (21.7 cm2) had 33% higher maximum power (2311 mW/m2), 81% lower anodic resistance (3 Ω), and 92% lower anodic impedance (1.1 Ω). However, in the long-term tests, CF-W with the smaller current collector area (0.6 cm2) showed higher performance in power and current generation, COD removal, and CE (51%, 10%, 11%, and 5% higher, respectively) and produced 41% higher net current in cyclic voltagramm (20.0 mA vs. 14.2 mA). This result shows that larger current collector is advantageous in short-term performance and disadvantageous in long-term performance, because the larger current collector is good for current collection, but interferes with mass transfer and microbial growth.

Polymer Film-Based Screening and Isolation of Polylactic Acid (PLA)-Degrading Microorganisms.

Polylactic acid (PLA) has been highlighted as an alternative renewable polymer for the replacement of petroleum-based plastic materials, and is considered to be biodegradable. On the other hand, the biodegradation of PLA by terminal degraders, such as microorganisms, requires a lengthy period in the natural environment, and its mechanism is not completely understood. PLA biodegradation studies have been conducted using mainly undefined mixed cultures, but only a few bacterial strains have been isolated and examined. For further characterization of PLA biodegradation, in this study, the PLA-degrading bacteria from digester sludge were isolated and identified using a polymer film-based screening method. The enrichment of sludge on PLA granules was conducted with the serial transference of a subculture into fresh media for 40 days, and the attached biofilm was inoculated on a PLA film on an agar plate. 3D optical microscopy showed that the isolates physically degraded the PLA film due to bacterial degradation. 16S rRNA gene sequencing identified the microbial colonies to be Pseudomonas sp. MYK1 and Bacillus sp. MYK2. The two isolates exhibited significantly higher specific gas production rates from PLA biodegradation compared with that of the initial sludge inoculum.

Assessment of microbial diversity bias associated with soil heterogeneity and sequencing resolution in pyrosequencing analyses.

It is important to estimate the true microbial diversities accurately for a comparative microbial diversity analysis among various ecological settings in ecological models. Despite drastically increasing amounts of 16S rRNA gene targeting pyrosequencing data, sampling and data interpretation for comparative analysis have not yet been standardized. For more accurate bacterial diversity analyses, the influences of soil heterogeneity and sequence resolution on bacterial diversity estimates were investigated using pyrosequencing data of oak and pine forest soils with focus on the bacterial 16SrRNA gene. Soil bacterial community sets were phylogenetically clustered into two separate groups by forest type. Rarefaction curves showed that bacterial communities sequenced from the DNA mixtures and the DNAs of the soil mixtures had midsize richness compared with other samples. Richness and diversity estimates were highly variable depending on the sequence read numbers. Bacterial richness estimates (ACE, Chao 1 and Jack) of the forest soils had positive linear relationships with the sequence read number. Bacterial diversity estimates (NPShannon, Shannon and the inverse Simpson) of the forest soils were also positively correlated with the sequence read number. One-way ANOVA shows that sequence resolution significantly affected the α-diversity indices (P<0.05), but the soil heterogeneity did not (P>0.05). For an unbiased evaluation, richness and diversity estimates should be calculated and compared from subsets of the same size.