Biocides in hydraulic fracturing fluids: a critical review of their usage, mobility, degradation, and toxicity.

Biocides are critical components of hydraulic fracturing ("fracking") fluids used for unconventional shale gas development. Bacteria may cause bioclogging and inhibit gas extraction, produce toxic hydrogen sulfide, and induce corrosion leading to downhole equipment failure. The use of biocides such as glutaraldehyde and quaternary ammonium compounds has spurred a public concern and debate among regulators regarding the impact of inadvertent releases into the environment on ecosystem and human health. This work provides a critical review of the potential fate and toxicity of biocides used in hydraulic fracturing operations. We identified the following physicochemical and toxicological aspects as well as knowledge gaps that should be considered when selecting biocides: (1) uncharged species will dominate in the aqueous phase and be subject to degradation and transport whereas charged species will sorb to soils and be less bioavailable; (2) many biocides are short-lived or degradable through abiotic and biotic processes, but some may transform into more toxic or persistent compounds; (3) understanding of biocides' fate under downhole conditions (high pressure, temperature, and salt and organic matter concentrations) is limited; (4) several biocidal alternatives exist, but high cost, high energy demands, and/or formation of disinfection byproducts limits their use. This review may serve as a guide for environmental risk assessment and identification of microbial control strategies to help develop a sustainable path for managing hydraulic fracturing fluids.

On the influence of the culture conditions in bacterial antifouling bioassays and biofilm properties: Shewanella algae, a case study.

BACKGROUND: A variety of conditions (culture media, inocula, incubation temperatures) are employed in antifouling tests with marine bacteria. Shewanella algae was selected as model organism to evaluate the effect of these parameters on: bacterial growth, biofilm formation, the activity of model antifoulants, and the development and nanomechanical properties of the biofilms.The main objectives were: 1) To highlight and quantify the effect of these conditions on relevant parameters for antifouling studies: biofilm morphology, thickness, roughness, surface coverage, elasticity and adhesion forces. 2) To establish and characterise in detail a biofilm model with a relevant marine strain. RESULTS: Both the medium and the temperature significantly influenced the total cell densities and biofilm biomasses in 24-hour cultures. Likewise, the IC50 of three antifouling standards (TBTO, tralopyril and zinc pyrithione) was significantly affected by the medium and the initial cell density. Four media (Marine Broth, MB; 2% NaCl Mueller-Hinton Broth, MH2; Luria Marine Broth, LMB; and Supplemented Artificial Seawater, SASW) were selected to explore their effect on the morphological and nanomechanical properties of 24-h biofilms. Two biofilm growth patterns were observed: a clear trend to vertical development, with varying thickness and surface coverage in MB, LMB and SASW, and a horizontal, relatively thin film in MH2. The Atomic Force Microscopy analysis showed the lowest Young modulii for MB (0.16 ± 0.10 MPa), followed by SASW (0.19 ± 0.09 MPa), LMB (0.22 ± 0.13 MPa) and MH2 (0.34 ± 0.16 MPa). Adhesion forces followed an inverted trend, being higher in MB (1.33 ± 0.38 nN) and lower in MH2 (0.73 ± 0.29 nN). CONCLUSIONS: All the parameters significantly affected the ability of S. algae to grow and form biofilms, as well as the activity of antifouling molecules. A detailed study has been carried out in order to establish a biofilm model for further assays. The morphology and nanomechanics of S. algae biofilms were markedly influenced by the nutritional environments in which they were developed. As strategies for biofilm formation inhibition and biofilm detachment are of particular interest in antifouling research, the present findings also highlight the need for a careful selection of the assay conditions.

Influence of triphenyltin exposure on the hypothalamus-pituitary-gonad axis in male Sebastiscus marmoratus.

Both triphenyltin (TPT) and tributyltin (TBT) have been used as ingredients of antifouling biocides. However, far fewer studies addressing the reproductive toxicity of TPT on fishes are available than for TBT. The present study was conducted to investigate the effects of TPT at environmentally relevant concentrations on testicular development in male rockfish Sebastiscus marmoratus and to gain insight into its mechanism of action. After exposure for 48 days, the gonadosomatic index had decreased, and there was a reduced number of mature sperm and an abundance of the late stages of spermatocysts in the testes. Although the testosterone levels in the testes were elevated and the 17ß-estradiol levels were decreased, spermatogenesis was suppressed. The activity of γ-glutamyl transpeptidase (which is used as a Sertoli cell marker) was decreased after TPT exposure, and serious interstitial fibrosis was observed in the interlobular septa of the testes exposed to TPT. The increased expression of cGnRH-II (chicken-II type gonadotropin-releasing hormone) and sGnRH (salmon-type GnRH), and the decreased expression of LHß (luteinizing hormone) in the fish brains were detected. The expression of FSHß (follicle-stimulating hormone) was decreased at day 21, while was increased slightly at day 48. The changes of cGnRH-II, sGnRH, FSHß and LHß mRNA levels might have mainly resulted from the alteration of the sex steroids via feedback mechanisms. The decrease of the FSHß mRNA might have been one of the reasons causing the dysfunction of Sertoli cells, which play a critical role during spermatogenesis. The results suggested that TPT could perturb the function of hypothalamus-pituitary-gonad axis, and inhibiting the spermatogenesis.