Amanda M. Tazaz

Substrate Limitation for Methanogenesis in Hypersaline Environments

Motivated by the increasingly abundant evidence for hypersaline environments on Mars and reports of methane in its atmosphere, we examined methanogenesis in hypersaline ponds in Baja California Sur, Mexico, and in northern California, USA. Methane-rich bubbles trapped within or below gypsum/halite crusts have δ¹³C values near -40‰. Methane with these relatively high isotopic values would typically be considered thermogenic; however, incubations of crust samples resulted in the biological production of methane with similar isotopic composition. A series of measurements aimed at understanding the isotopic composition of methane in hypersaline systems was therefore undertaken. Methane production rates, as well as the concentrations and isotopic composition of the particulate organic carbon (POC), were measured. Methane production was highest from microbial communities living within gypsum crusts, whereas POC content at gypsum/halite sites was low, generally less than 1% of the total mass. The isotopic composition of the POC ranged from -26‰ to -10‰. To determine the substrates used by the methanogens, ¹³C-labeled methylamines, methanol, acetate, and bicarbonate were added to individual incubation vials, and the methane produced was monitored for ¹³C content. The main substrates used by the methanogens were the noncompetitive substrates, the methylamines, and methanol. When unlabeled trimethylamine (TMA) was added to incubating gypsum/halite crusts in increasing concentrations, the isotopic composition of the methane produced became progressively lower; the lowest methane δ¹³C values occurred when the most TMA was added (1000 μM final concentration). This decrease in the isotopic composition of the methane produced with increasing TMA concentrations, along with the high in situ methane δ¹³C values, suggests that the methanogens within the crusts are operating at low substrate concentrations. It appears that substrate limitation is decreasing isotopic fractionation during methanogenesis, which results in these abnormally high biogenic methane δ¹³C values.

Developing an Assessment Instrument to Measure Early Elementary Teachers’ Mathematical Knowledge for Teaching

This study reports on the development and field study of K-TEEM, a Web-based assessment instrument designed to measure mathematical knowledge for teaching (MKT) at the early elementary level. The development process involved alignment with early elementary curriculum standards, expert review of items and scoring criteria, cognitive interviews with practicing teachers, a field test involving 405 practicing teachers, and data modeling using a Rasch model. Several examples of MKT at the early elementary level are provided, and some of the challenges and decisions made during the process of item and scale development are discussed. Rasch model results indicate good model fit and adequate reliability, and the model accounts for more than 75% of the variance in the data. The K-TEEM assessment instrument may fill an important gap in the set of tools available to researchers for program evaluation and empirical investigation of teacher knowledge.