SummaryIn experiments with one freshwater (Chlorella pyrenoidosa) and three marine organisms (Phaeodactylum tricornutum, Cyclotella mna, and Chaetoceros galvestmensis), mercury was more toxic than the other metals tested (silver, cadmium, lead, and copper) ; and its toxicity is comparatively irreversible. Growth was monitored by changes in fluorescence of the cultures over a 3-day test period. The toxicity of the mercury varied inversely with the concentrations of nutrients present. Preliminary experiments indicate that mercury in the form of mercuric chloride is more toxic than as dimethylmercury.

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A laboratory evaluation has been made of water-soluble biocides which might be effective in controlling microbial contamination in water-compensated fuel storage tanks on naval ships. Higher concentrations of biocides were generally required for control of sulfate-reducing bacteria in steel drums simulating ships' tanks than in test-tube scale assays. At least tenfold higher concentrations were necessary when biocide additions were made after, rather than before, bacterial growth had developed extensively. A mercaptopyridine biocide was effective at the lowest concentration followed by methylene bisthiocyanate and a triazine derivative; an organoboron compound was comparatively ineffective. Attention has also been given to the possible environmental impact of discharging biocide-treated water. An algal assay has been used to study the loss in toxicity of candidate biocides in seawater solution on exposure to sunlight. The mercaptopyridine biocide was particularly susceptible to photodegradation. Thus it may be possible to control microbial contamination indefinitely in a dark fuel tank with a biocide which photodegrades to nontoxic products when discharged into sunlit surface waters.

Cultures of Phaeodactylum tricornutum were exposed to combinations of visible and ultraviolet-B (UV-B) light intensities in the laboratory, the ultraviolet light (UV) sources being fluorescent lamps whose peak emission was at approximately 310 nm. Filters of cellulose acetate and Mylar were used to attenuate the UV-B intensity, the latter absorbing all the radiation in the 310 nm region. The growth rates of the algae were calculated from chlorophyll analyses made during the exponential growth of the cultures, and trace gas measurements were made after a fixed time (44 h). Algal cells exposed to a UV-B flux approximately one-fifth that of average sunlight intensity, with visible light intensities optimal for photosynthesis, underwent a 20 percent reduction in growth rate and were completely inhibited by UV fluxes half that of sunlight. A major finding of the trace gas analyses was that carbon monoxide (CO) formation was relatively constant over the range of UV fluxes imposed. Previous experiments with outdoor exposures had produced CO in concentrations of an order of magnitude higher. It appears that UV-B was not the causative agent for the unsually high CO production resulting from exposure of the algae to sunlight.

Gas exchange with mass cultures of algae. II. Reliability of a photosynthetic gas exchanger. Appl. Microbiol. 11:450-452. 1963.-Comparisons of oxygen production and carbon dioxide absorption by an algal gas exchanger were made over a 3-month period. The data do not represent a continuous test, but they do represent results obtained when identical light intensities, CO2 supply rates, and dilution rates with fresh culture medium had been used for more than 1 day. Steady-state conditions were thus assured, and the agreement in the data was excellent. Under the same experimental conditions, the unit was operated continuously for a 5-day period, and the daily variability in this test was less than in the results obtained from month to month. The variation between the average 02 production during the 5-day test and the average of the tests over a several-month period was less than 3N. It is concluded, therefore, that the reliability of the algae in producing oxygen is sufficient to warrant their use in either submarine or space ship use.