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751.
Abstract
Invasions of aquatic ecosystems by exotic bivalves are known to cause dramatic changes in phytoplankton and some other groups,
but their effect on the microbial component is unknown. The invasion of the tidal freshwater Hudson River by the exotic zebra
mussel (Dreissena polymorpha) has caused large changes in several components of the Hudson's food web. Planktonic bacteria in the tidal freshwater Hudson
are a major part of the food web, and mediate important processes in the carbon budget. We used a long-term data set, spanning
four years prior to the zebra mussel (ZM) invasion and four years post-invasion, to describe ZM effects on planktonic bacteria.
Small and meso-scale experiments were conducted to specifically examine direct consumption of bacteria by ZM, as well as effects
on protozoans. Bacterial abundances in the Hudson have increased roughly 2× since the ZM arrived, making it clear that direct
consumption by Dreissena is a minor process. Experiments show that ZM do not remove bacteria from Hudson River water, but are very effective at clearing
flagellated protozoans, the major predator of bacteria. The observed changes in bacterial abundance have not been accompanied
by equally large changes in bacterial productivity, suggesting growth is primarily limited by carbon supply. Bacterial production
has not declined despite a dramatic decline of phytoplankton, confirming previous suggestions that bacteria and phytoplankton
are not strongly linked in the Hudson. As a result of the increase in bacterial abundance and removal of phytoplankton, the
absolute and relative contributions of bacterial carbon to living particulate organic carbon (POC) standing stocks have increased
dramatically. The maintenance of the bacterial component of the Hudson River's food web may be one mechanism whereby consumers
are ``insulated' from effects of zebra mussel consumption of phytoplankton carbon.
Received: 24 October 1997; Accepted: 9 February 1998 相似文献
752.
Deletion of DNA lying close to the glkA locus induces ectopic sporulation in Streptomyces coelicolor A3(2) 总被引:1,自引:1,他引:0
Gabriella H. Kelemen Kitty A. Plaskitt Cinzia G. Lewis Kim C. Findlay Mark J. Buttner 《Molecular microbiology》1995,17(2):221-230
Streptomyces coelicolor A3(2) J1668 sporulated ectopically in the substrate hyphae (the Esp phenotype) with the same time course as sporulation in the aerial hyphae. Examination of related strains implied that the Esp phenotype was caused by the deletion of DNA that lies close to, but is distinct from, the glucose kinase gene ( glkA ). Co-transduction of the Esp phenotype with the deletion present in J1668 confirmed this hypothesis. The size of the deletion was found to be 7.4 kb. Construction of a strain carrying both the J1668 deletion and a whiG mutation demonstrated that the Esp phenotype depends on at least one of the genes required for the differentiation of aerial hyphae into spores. 相似文献