Untangling the complex issue of dissolved organic carbon uptake: a stable isotope approach

1. We estimated uptake of stream water dissolved organic carbon (DOC) through a whole-stream addition of a ¹³C-DOC tracer coupled with laboratory measurements of bioavailability of the tracer and stream water DOC.
2. The tracer, a leachate of ¹³C-labelled tree tissues, was added to the head waters of White Clay Creek, Pennsylvania, U.S.A., over a 2-h period and followed 1.27 km downstream to generate mass transfer coefficients for DOC lability classes within the tracer.
3. From the longitudinal ¹³C uptake curve, we resolved labile and semi-labile DOC classes within the ¹³C-DOC tracer comprising 82% and 18% of the tracer respectively.
4. Plug-flow laboratory bioreactors colonized and maintained with stream water were used to determine the concentration of stream water DOC fractions that had a similar lability to the labile and semi-labile classes within the tracer and we assumed that stream water DOC and tracer DOC with comparable lability fractions in the bioreactors behaved similarly in the stream, i.e. they had the same mass transfer coefficients.
5. A small fraction (8.6%) of the stream water DOC was labile, travelling 238 m downstream before being taken up. The remaining bioavailable stream water DOC was semi-labile and transported 4.5 km downstream before being taken up. These uptake lengths suggest that the labile DOC is an energy source within a stream reach, while the semi-labile DOC is exported out of the reach to larger rivers and the downstream estuary, where it may provide energy for marine microbial communities or simply be exported to the oceans.     

Neurophysiological Genetics in Drosophila melanogaster

Neurophysiological genetics is the study of the mechanisms bywhich genes control nervous function and behavior. The transductionof genetic information into neural information is studied atthe level of the neuron through genetic and physiological techniques. The neurons responsible for the leg-shaking action specificto a single-gene mutant of Drosophila melanogaster, Hk¹, havebeen located in three pairs of small regions in the thoracicganglion. The activity pattern of these neurons is coded bythe mutant Hk¹ gene. The center for the specifically patternedleg-shaking action is composed of several motor neurons whoseactivity is governed by the pacemaking activity of at leastone interneuron. As it is most likely that the mutant gene isexpressed autonomously in this interneuron, there is a possibilityof investigating ways in which genes may influence the propertiesof neurons. The activity of the mutant neuron was monitoredintracellularly, and the pattern formation mechanism was studied.The amplitude, duration, and periodicity of the pacemaker potentialand the spike initiation site determine the activity patternresulting in the specific leg-shaking action.