Robert L. Leonard

Lamellar and hexagonal lipid phases visualized by freeze-etching

Abstract 1. 1. Rapid freezing of lipid-water preparations preserves the structure of the high temperature phases. Both lamellar and hexagonal phases can be readily demonstrated by electron microscopy and X-ray observations. 2. 2. Without careful controls, contaminants are readily deposited on fresh fracture faces. The contamination can take the form of particles resembling those found on natural membranes. 3. 3. Neither degree of saturation, degree of hydration, nor cholesterol admixture significantly affects the appearance of lamellar fracture faces which appear uniformly smooth. On uncontaminated specimens, no structures were found which resembled the particulate material of natural membranes.

Nutrient Transport in Surface Runoff from a Subalpine Watershed, Lake Tahoe Basin, California

The watershed of Ward Creek, a tributary to oligotrophic Lake Tahoe in the Sierra Nevada, has been investigated since 1971 with the objective of improving our knowledge of processes of nutrient and sediment release and transport to the lake. Quantitative data on selected stream water parameters were collected for 3 yr (1972-1975) at three stations on Ward Creek, two on the main upper tributaries and one near the stream mouth. Comparative data were collected at a stream mouth station on adjacent Blackwood Creek in the 3rd yr. The parameters were selected on the basis of their significance to eutrophication of Lake Tahoe. Precipitation in a normal year is over 90% snow but annual patterns vary widely and rainfall at any time of year can be highly important in sediment and nutrient transport. Water discharge and the flux of suspended sediments, N03-N, phosphorus, iron and trace metals were dominated by the spring snowmelt runoff from mid-April to mid-June. However, in 1974 heavy fall and summer rains accounted for a large percentage of the annual flux of sediments and nutrients in a total of only 14 d. The spring runoff was characterized by distinct diel water discharge patterns. Similar but not coincident patterns were found to exist for sediments and nutrients, including N03-N, but not for soluble phosphorus. The Ward watershed has 87% the area of Blackwood but discharged proportionately much lower quantities of sediment and nutrients in comparable water yields per hectare in water year 1975. This contrast in fluxes was probably accounted for in the history of greater disturbance by man in Black- wood Canyon. The principal source of suspended sediments in Ward Creek was streambank erosion in the lower reaches of the channel. The dominant form of inorganic nitrogen in Ward was N03-N derived from precipitation, symbiotic nitrogen fixation and nitrification of organic nitrogen in forest soil. Phosphorus and iron were almost entirely in particulate form and thus their periods of flux occurred during high flows and sediment transport. Sediment and nutrient loading of Lake Tahoe from the Ward and Blackwood watersheds reflects a history of soil disturbance and vegetation removal. Logging, fire and stream channel diversion have been the dominant perturbations. Conservative extrapolation of annual loading data from this study to the entire basin indicates that algal nutrient levels in the lake probably have increased sufficiently in the century of mans intensive disturbance of the basin watersheds to account for increased phy- toplankton and periphyton production that have been measured and observed since 1958.

Some measurements of the pH and chemistry of precipitation at Davis and Lake Tahoe, California

Precipitation samples were collected at Davis and Lake Tahoe, California, in 1972–73 and 1977–78 and analyzed for pH and major cations and anions. Rain and snow in this region of northern California are derived primarily from winter cyclonic storms which move easterly from the Pacific Ocean over Davis and then Lake Tahoe. Precipitation at both sites was found to be more acid than water in equilibrium with atmospheric CO2. Acidity at Lake Tahoe apparently increased over the 5-yr period of the study. Sulfate was the dominant acid anion in 1972–73 (not measured in 1977–78). A major source of sulfate in precipitation was probably industry in the San Francisco Bay Area, upwind of the study sites. Automobile exhaust emissions throughout the region, which contains three major interstate highways and several large urban centers, contribute both sulfate and nitrate precursors to the atmosphere. As in the Eastern United States, these strong acid anions are influencing precipitation chemistry in northern California, including the Sierra Nevada mountains.

Freeze-etched surfaces in potato starch

Native and Lintnerized potato starch grains were freeze-fractured and freezeetched. All samples showed a granular surface. The Lintnerized samples, in addition, presented unique structures which have been described as microfibrils. The question, whether the granules and the microfibrils seen here are real structures in the starch grain or artifacts produced by the freeze-etch technique is discussed.

Amphibian yolk platelet ultrastructure visualized by freeze-etching

Amphibian yolk platelets are composed of lipoprotein subunits arranged in an ordered crystalline structure. Freeze-etch electron microscopy of isolated Xenopus platelets provides a direct view of the structure of the crystal and aids the interpretation of fracture phenomena in lipoprotein systems. A study has been made both of fracture faces and of faces produced by fracturing and etching following partial dissolution of platelets in electrolyte solutions. In freeze-etch replicas, main body crystals appear to be composed of dimers. Rectangular and semihexagonal patterns are seen in fracture faces. Rectangular patterns are seen also in faces produced by partial dissolution and revealed by fracturing and etching. Dissolution faces with possible semihexagonal patterns are distinct but infrequent. Based on this evidence, a new closest-packing model of platelet structure is proposed using lipovitellin dimers as building blocks, with one molecule of the second major protein component, phosvitin, associated with each monomer of the lipovitellin dimer.

Residual Nutrient Discharge in Streamwaters Influenced by Sewage Effluent Spraying

The disposal of secondary sewage effluent on a subalpine forest floor was in- vestigated in terms of its relationship to nutrient discharge in a Tahoe basin watershed. Heavenly Valley Creek showed significant increases in N03-1-N and ultraviolet absorbance downstream from a site that had been used for land disposal 5 yr prior to this study. These increases were traceable to subsurface discharge from the sprayed area. Evidence indicated that substances present in the spray area discharge and the streamwater are inhibitory to algal photosynthesis at in situ concentrations. This effect may be due in part to N03-1-N which undoubtedly becomes stimulatory through dilution in the lake. Discharge from the spray area can be traced into Lake Tahoe by following patterns of UV absorbance which correlate closely with patterns of primary productivity. This correlation, combined with estimates of continuing heavy NO3-A-N loading of Lake Tahoe by Heavenly Valley Creek, reveals the long-term impact of land disposal of effluent on the ultra-oligotrophic lake.