The permanently ice-covered lakes of the McMurdo Dry Valleys region of Antarctica present unique environments for study of nutrient cycling (Priscu, Ward, and Downes, Antarctic Journal, in this issue).
Annual hydrologic recharge from streams contains negligible amounts of nutrients (Greene, Angle, and Chave 1988), and annual phytoplankton blooms constitute the major source of organic matter for subsequent heterotrophic breakdown. A lack of vertical mixing has resulted in highly stratified zones of phytoplankotn and bacterial productivity (Ward, Cockcroft, and Priscu, Antarctic Journal, in this issue). Little is known, however, about the stratification and degree of heterotrophic breakdown in these systems (Smith and Howes 1990). In this study, 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) was used to estimate the fraction of phytoplankton and bacterioplankton showing respiration potential at selected depths in both lobes of Lake Bonney. CTC forms intracellular, fluorescent-formazan deposits upon reduction by active succinate dehydrogenase, indicating respiratory activity. Cells were counterstained with 4.6-diamidino-2-phenylindole (DAPI) and enumerated for CTC-deposit containing, and total microorganisms by epifluorescence microscopy. One thousand cells were counted from at least 10 fields for each sample.
Samples were collected using a 1-liter Niskin sampler on 8 December 1992 from the east and west lobes of Lake Bonney at depths of 13, 17, 25 and 30 meters (m) (hereafter designated as "E" or "W" for east and west lobes, respectively). Nine-milliliter subsamples were incubated with 5 millimolar CTC (as described by Rodriguez et al. 1992), for 8 hours in the dark. Samples E13, E17, and E25 were incubated at 5°C; E30 and W13 were incubated at 2°C; and W17, W25, and W30 were incubated at -2°C. These temperatures were near those from which the samples were collected.
Total microbial numbers ranged between 105 and 106 per milliliter in both lobes at all depths (figure 1). Although total cell numbers remained relatively constant for west-lobe samples, the fraction indicating respiratory activity decreased with depth from 35 percent at 13 m to 10 percent at 25-30 m (figures 1 and 2). This is in contrast to east-lobe samples where respiring cell numbers decreased from 60 to 45 percent between 13 and 17 m, then increased to a maximum of 73 percent at 30 m (figure 2). In addition, respiring fractions were higher in 13-m samples compared to 17-m samples in both lobes.
These results indicate that the fraction of microorganisms maintaining a low cellular reduction potential via respiration (using tetrazolium salt reduction as an indicator) is much lower in the west lobe of Lake Bonney than in the east lobe (particularly below 17 m). It should be noted that the CTC method yields the fraction of organisms showing respiration potential: absolute respiratory activity remains unknown. Detailed determinations of microbial activity are required to elucidate the types of organisms (for example, photoautotrophs, chemoautotrophs, and heterotrophs) responsible for the respiration potential we observed with the CTC method.
We would like to thank Barb Kelley, Rich Bartlett, and VXE-6 for their assistance in this project. This research was supported by National Science Foundation grant OPP 91-17907 to John C. Priscu.
References
Greene, W.J., M.P. Angle, and K.E. Chave. 1988. The geochemistry of antarctic streams and their role in the evolution of four lakes of the McMurdo Dry Valleys. Geochimica et Cosmochimica Acta, 52(5), 1247-1265.
Priscu, J.C., B.B. Ward, and M.T. Downes. 1993. Water column transformations of nitrogen in Lake Bonney, a perennially ice-covered antarctic lake. Antarctic Journal of the U.S., 28(5).
Rodriguez, G.G., D. Phipps, K. Ishiguro, and H.F. Ridgeway. 1992. Use of a fluorescent redox probe for direct visualization of actively respiring bacteria. Applied and Environmental Microbiology, 58(6), 1801-1808.
Sharp, T.R., and J.C. Priscu. 1990. Ambient nutrient levels and the effects of nutrient enrichment on primary productivity in Lake Bonney. Antarctic Journal of the U.S., 25(5), 226-227.
Smith, R.L., and B.L. Howes. 1990. Bacterial biomass and heterotrophic activity in the water column of an amictic antarctic lake. Antarctic Journal of the U.S., 25(5), 233-235.
Ward, B.B., A.R. Cockcroft, and J.C. Priscu. 1993. Nitrifying and denitrifying bacteria in Lake Bonney. Antarctic Journal of the U.S., 28(5).