Analyses of the settling velocities of fecal pellets from the subtidal polychaete Amphicteis scaphobranchiata

Measured settling velocities of fecal pellets produced by Amphicteis scaphobranchiata, a deposit-feeding subtidal polychaete worm, are analyzed to determine the effects of pellet shapes and to arrive at improved formulae for predicting settling rates of fecal pellets in general. Drag coefficients calculated from the measured settling velocities and pellet densities decrease with increasing Reynolds numbers, the trend roughly paralleling the drag-coefficient curve for spheres but having higher values due to the nonspherical shapes of the pellets. No clear dependence could be found on the pellet shapes, however, when the shape is expressed as pellet elongation (length/width). Comparisons are also made between the measured settling velocities and equations for the prediction of settling of spheres, the relationships between the two serving as the basis for improved equations for the evaluation of settling velocities of fecal pellets of diverse origins

Dynamics and Characterization of Refractory Dissolved Organic Matter Produced by a Pure Bacterial Culture in an Experimental Predator-Prey System

We studied the effects of a bacterium (Pseudomonas chlororaphis) and a bactivorous protozoan (Uronema sp.) on transformations of labile dissolved organic carbon (DOC). In 36-day time series experiments, bacteria were grown on glucose both with and without protozoa. We measured bulk organic carbon pools and used electrospray ionization mass spectrometry to characterize dissolved organic matter on a molecular level. Bacteria rapidly utilized glucose, depleting it to nondetectable levels and producing new DOC compounds of higher molecular weight within 2 days. Some of these new compounds, representing 3 to 5% of the initial glucose-C, were refractory and persisted for over a month. Other new compounds were produced and subsequently used by bacteria during the lag and exponential growth phases, pointing to a dynamic cycling of organic compounds. Grazers caused a temporary spike in the DOC concentration consisting of labile compounds subsequently utilized by the bacteria. Grazing did not increase the complexity of the DOC pool already established by the bacteria but did continually decrease the particulate organic carbon pool and expedited the conversion of glucose-C to CO(2). After 36 days, 29% of initial glucose-C remained in pure bacteria cultures, while only 6% remained in cultures where a grazer was present. In this study the bacteria were the primary shapers of the complex DOC continuum, suggesting higher trophic levels possibly have less of an impact on the qualitative composition of DOC than previously assumed

Low-density particles as potential nitrogenous foods for benthos

The demonstrated bias of both macrobenthos and fluids to mobilize low-density particles leads to their potential importance as nutritional materials in benthic systems. We fractionated sediments from three coastal regions into low- and high-density separates, and examined both their organic geochemical characteristics and effects on ingestion rates of a deposit feeder. The low-density separates were highly enriched in total organic matter relative to the high-density phases. Enzymatically hydrolyzable protein concentrations in low-density separates were as much as 57-fold higher than the corresponding high-density separates, though some samples from Puget Sound and the Mediterranean Sea showed no enrichment at all. Low-density phases without nutritional enrichments were usually composed of woody debris. In spite of the organic richness of the low-density phase, it makes up no more than a minor fraction of either total sedimentary organic matter or its nutritional component. Addition of anomalously high concentrations of low-density materials to sediments caused a deposit-feeding spionid polychaete to reduce ingestion rates

Protozoan Migration in Bent Microfluidic Channels▿

Microfluidic devices permit direct observation of microbial behavior in defined microstructured settings. Here, the swimming speed and dispersal of individual marine ciliates in straight and bent microfluidic channels were quantified. The dispersal rate and swimming speed increased with channel width, decreased with protozoan size, and was significantly impacted by the channel turning angle