Phytoplankton Productivity and Standing Crop in the Vicinity of the Lake Huron-Saginaw Bay Front

Continuous underway sampling was used in four research cruises near the mouth of Saginaw Bay in order to locate the bay-lake front, where eutrophic bay water mixes with oligotrophic Lake Huron water. Although mixing between these water masses normally occurs in outer Saginaw Bay, the distribution was variable, depending on wind force and direction as well as water temperature. Large algal crops in inner Saginaw Bay had reduced soluble nutrients, particularly silicate and nitrate-nitrogen, in the outer bay to levels lower than the adjacent lake water. Nonetheless, outer bay water supported high algal biomass (up to 14 μg/L chlorophyll a) and ¹⁴C uptake (up to 73 μg /L/h). The phytoplankton community at the nutrient front between bay and lake waters was characterized by high chlorophyll a biomass ($\overline{x}$ = 4.47 μg/L), low ¹⁴C uptake ($\overline{x}$ = 7.12 μg/L/h), and high bacterial activity (V_max$\overline{x}$ = 1.33) indicating the presence of a senescent assemblage. Decomposition and settling of phytoplankton in the vicinity of the front apparently served to remove much of the bay plankton from the water column before mixing into the open lake. But, during mid-summer, occasional large inputs of nutrients and/or phytoplankton from the bay into the open lake were observed.     

Measurements of Attenuation of Third Sound: Evidence of Trapped Vorticity in Thick Films of Superfluid 4He

We present results of a study of third sound in thick ⁴He films in circular resonator geometry. Frequency and line shapes of third sound resonances are measured for temperatures between 0.3 and 2.1 K in saturated films approximately 30 nm thick. From these measurements we calculate the attenuation of the sound. We find that the attenuation at a given temperature is a function of history of the film, strongly affected by such events as large, sudden (more than milli-Kelvin per second) temperature spikes. We also observe variable frequency splitting of resonances, indicating trapped circulation. Our measurements, taken together with other reported attenuation experiments, are incompatible with dissipation mechanisms dependent on thermodynamic properties alone. Measurements indicate a linear dissipation mechanism, inconsistent with vortex drag and re-connection models. We conclude that high attenuation values, evidence of trapped circulation, and variation in attenuation values support the hypothesis that thick films of ⁴He contain high densities of remnant quantized vortices. The vortex populations suggested by trapped circulation are consistent with proposed linear dissipation mechanisms due to vortex-normal fluid interactions and vortex dimple drag.     

Monitoring Moisture Uptake and Delamination in CFRP-Reinforced Concrete Structures with Electrochemical Impedance Sensors

Electrochemical impedance spectroscopy (EIS) sensors have been used to inspect carbon fiber reinforced polymer (CFRP) -reinforced concrete structures exposed to a variety of test conditions. Similar specimens were also used to investigate the detection of delamination between the CFRP and the concrete using a modified wedge test configuration. The use of external electrodes attached to the CFRP surface, coupled with the embedded rebar, provided the best results. Equivalent circuit modeling was used to analyze the impedance spectra. Several circuit parameters, especially the capacitance and constant phase element (CPE) magnitude, correlated very well with both bonded area/delamination and moisture. Both parameters exhibit a linear relationship with delamination area. The capacitance also showed a linear relationship with moisture content while the CPE was more strongly dependent on moisture. Differences in the response of the specimens subjected to the different exposure conditions were seen and explained based on the moisture uptake of the various specimens.     

Photorhabdus luminescens TccC3 Toxin Targets the Dynamic Population of F-Actin and Impairs Cell Cortex Integrity

Due to its essential role in cellular processes, actin is a common target for bacterial toxins. One such toxin, TccC3, is an effector domain of the ABC-toxin produced by entomopathogenic bacteria of Photorhabdus spp. Unlike other actin-targeting toxins, TccC3 uniquely ADP-ribosylates actin at Thr-148, resulting in the formation of actin aggregates and inhibition of phagocytosis. It has been shown that the fully modified F-actin is resistant to depolymerization by cofilin and gelsolin, but their effects on partially modified actin were not explored. We found that only F-actin unprotected by tropomyosin is the physiological TccC3 substrate. Yet, ADP-ribosylated G-actin can be produced upon cofilin-accelerated F-actin depolymerization, which was only mildly inhibited in partially modified actin. The affinity of TccC3-ADP-ribosylated G-actin for profilin and thymosin-β4 was weakened moderately but sufficiently to potentiate spontaneous polymerization in their presence. Interestingly, the Arp2/3-mediated nucleation was also potentiated by T148-ADP-ribosylation. Notably, even partially modified actin showed reduced bundling by plastins and α-actinin. In agreement with the role of these and other tandem calponin-homology domain actin organizers in the assembly of the cortical actin network, TccC3 induced intense membrane blebbing in cultured cells. Overall, our data suggest that TccC3 imposes a complex action on the cytoskeleton by affecting F-actin nucleation, recycling, and interaction with actin-binding proteins involved in the integration of actin filaments with each other and cellular elements.     

Fibrillar Structure in Lintnerized Potato Starch by Scanning Electron Microscopy

A study of Lintnerized potato starch has been made with the scanning electron microscope. All fracture faces, etched with the dilute acid solution, show a radial fibrillar system as the structural foundation of the starch grain. No evidence of isodiametric granules was found. Ringlike areas, corresponding to tangentially fractured concentric lamellae of the grain, are not uncommon and are made evident by a change in slope of the fracture face at these rings. The fibrillar system, centered at the middle of a ring, radiates outward from that point and traverses the ring without obvious interruption. The longest fibrillar ridge had a length of 14 μm.