The bloom sequence in a eutrophic lake, Linsley Pond, over a period of 3 years is correlated to the effects of cell-free filtrates of dominant blue-green algae on both their successors and their predecessors. There is unbroken correspondence between the effects of heat-labile probiotic and antibiotic filtrates and the rise and fall of bloom populations in situ. All organisms in vitro were axenic or unialgal (bacterized) isolates from Linsley Pond.
Cell-free filtrates of axenic or bacterized cultures of the dominant blue-green algae from a freshwater lake inhibited the growth of diatoms isolated from the same lake. Lake waters, collected during blue-green algal blooms, also inhibited diatom growth. In situ observations over a 5-year period indicate that diatom bloom populations vary inversely with the levels of the preceding blue-green algal populations. Blue-green algal dominance of eutrophic lakes is attributed to this allelopathy, and dilution is proposed as one cause for the limited occurrence of blue-green alga dominance in marine waters.
Daphnia pulex de Geer and Daphnia magna Straus populations cannot be maintained in defined (sensu stricto) media containing less than 0.1 part per billion (ppb) of selenium. A concentration of 1 ppb is sufficient to satisfy minimal needs in otherwise sufficient media. In the first generation with no selenium added to the medium or detected in it, media deficiency is shown by a premature cuticle deterioration visually similar to senescence, by progressive loss of distal segments of second antennae (primary swimming appendages), and by a shortened lifespan. No progeny attain reproductive maturity in the second generation. Although experimental animals in prime condition exhibit a shortened lifespan in the first generation maintained at 0.5 ppb selenium, culture lines can be maintained at 0.5 ppb for indefinite numbers of generations if established as young orthoclones. Tests in organic-rich media indicate a significant sparing effect of organic additions. This selenium requirement is reminiscent of that for the stability of feathers in domestic fowl.Selenium deficiency diseases in domesticated vertebrates are well known (1, 2) and a ubiquitous need for the element is suggested by its roles in the structure of glutathione peroxidase and in electron transport (3)(4)(5). No specific metabolic role for selenium in invertebrates has yet been reported.The recent development of the cladoceran maintenance system [the MS system (6)] has made study of the trace nutnent requirements of the Cladocera practical. The MS system (Table 1) relies on wholly defined media in which crystalline vitamin B12 is the only organic nutrient intentionally included in animal media, and biotin and thiamin are the only additional extraneous organic compounds that could be carried into animal cultures when MS-grown algae are used to feed the Cladocera. Uncertain inorganic trace contaminants are not introduced by the addition of commercially produced organics because the cladoceran's (heterotroph's) need for organic macromolecules is met by algae (autotrophs) produced within the controlled system. Unintended additions are, thereby, limited to those accompanying the best grade inorganic salts available and those inadvertently leached from equipment that comes in contact with media.Prior studies, which led to the successful elucidation of cladoceran organic nutrient requirements (7-10) depended on culture media containing such undefined organics as liver extracts and yeasts. Although D'Abramo and Baum (11) clearly demonstrated that a liver extract contributes a significant concentration of choline to the nutritional regime established in their Moina macrocopa cultures, its role as a source of trace inorganics was not explored. Since trace elements, including selenium, concentrate in the liver and kidneys of vertebrates during long-term exposure to subtoxic levels, such inclusions must at the least confound the interpretation The publication costs of this article were defrayed in part by page charge payment. This article must therefore be he...
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