Posted by: Shirley French
For August and September, 2010, I helped a student, Amy McMullin, conduct experiments in the ‘field’ at Round Lake. We measured the physical parameters (i.e. temperature, oxygen, light), sampled the zooplankton and ran experiments with live critters (Daphnia) in bottles at different depths. Our study species was D. pulicaria (a water flea, left figure), a key component in many lake food webs where they consume tiny plants, the phytoplankton, and in turn, Daphnia are food for insect larvae (such as the Phantom midge) and for larval or juvenile fish. The top predator at Round Lake seemed to be a pair of loons residing at the lake over the summer and their single offspring (young with their mother, below right).
Round Lake is appropriately named for its circular shape. The steep sides plunge down to a maximum depth of 30 meters. The more gradual shore on the SE side is where we launch the boat. One of the interesting features of this lake is the low oxygen in the bottom waters (i.e. 0.26 -0.10 mg/L from 15 m & below, Aug. 31/10). As we were setting out and retrieving the Daphnia from jars in the water at 15 and 20 meters, I kept finding lovely dark pinkish-red clusters of cells in the water from the 20 m depth (Amy worked on the 15 m samples). At first I wondered why there were red algae at a depth of 20 meters since we had measured the amount of light and it was extremely low! I realized that it was not an algae after seeing an article on ‘Bahamas Blue Holes’ in National Geographic (August, 2010). The articles’ description of diving through a zone of pink stained water (a layer of purple sulfur bacteria), made a strong impression. As it turned out, the purple sulfur bacteria are inhabitants of freshwater environments as well as saltwater. After examining some of the pinkish-red stuff under the microscope, the cells were easily identified as the purple sulfur bacteria, Amoebobacter (Pfennig and Trüper, 1989). Even though the individual round cells are only 2-3 microns across, the colony is held together by wispy strands of mucus and so the bacteria are very easy to see with your eye.
What do the purple sulfur bacteria need to persist? Like plants, they photosynthesize, but they use bacteriochlorophyll and carotenoids to capture light energy at wavelengths quite different from the ones used by algae (Pfennig and Trüper, 1989). In addition, the purple sulfurs can photosynthesize at light levels 500 times lower than that available in the water’s surface layers on a sunny summer’s day. They are not as efficient at photosynthesis as the phytoplankton but they also grow in darkness and can adjust their buoyancy depending on their needs (Overmann and Pfennig, 1992). In summary, the purple sulfur bacteria need sulfide compounds, have species specific ranges in temperature, need low oxygen or no oxygen for growth, and, have a wide range of salinity tolerances. As mentioned, purple sulfurs grow in high salinity as in the Bahamas, salty lakes (Mahoney Lake, B.C.), or, in fresher water such as Round Lake. Not surprisingly, the purple sulfur bacteria are thought to have arisen a long time ago (1.6 billion years before present; see Brocks and Schaeffer, 2008) when there was not a lot of oxygen in the oceans and the cyanobacteria were just getting started. Daphnia, on the other hand, have been around since “at least the Permian” (299 – 251 million years ago; Taylor et al., 1999).
In Round Lake Daphnia pulicaria are found throughout the water column. Some individuals are found in the low oxygen water, why are they there at all? We noticed that some live specimens have an obvious red coloration that has been noted by other researchers and studied in Daphnia. Like their predator found in Round Lake, the phantom midge Chaoborus flavicans, Daphnia produces hemoglobin in their bodies when they have been under low oxygen stress. It is their interesting adaptative plasticity and their suitability for lab experiments, which make Daphnia a good organism for studying energy budgets in Bill Nelson’s lab at Queen’s.
There are many unanswered questions about the plankton community and the physical features of Round Lake. I had hoped to determine whether the lake had ‘turned over’ in the fall (i.e. early December), as is characteristic of many lakes, so it was necessary to sample the lake in the winter. The weather was finally favorable Feb. 24th , the depth of the snow had shrunk and the air temperature was close to 0◦C. The ATV didn’t make it very far on the trail and so I want to mention that my son, Linden’s, effort in dragging most of the sampling equipment to the lake and back was greatly appreciated. Mark Conboy (Operations Manager at the Biological Station) had loaned us an ice auger and axe to cut a hole in the ice which was ~18 inches thick as Mark had said to expect. Much to my excitement the temperature of the water in Round Lake was warmest in the bottom 20 to 29 meters. The oxygen also abruptly changed from >3 mg/L in the upper 19 meters; to a low range of 0.56 – 0.43 mg/L in the bottom 9 meters. The temperature profile indicated that the ‘densest’ (4◦C ) cool water in the upper layers, did not mix with the bottom waters (max. 4.7◦C); maintaining the low oxygen environment for the bacterial community. It also suggests that the nutrients at the deep depths are not completely brought back into the water column with a fall or spring ‘turn over’ depriving the phytoplankton of a high nutrient supply. The zooplankton appeared healthy, plump and many were red with hemoglobin in the February samples.
- Brocks, J.J., and Schaeffer, P., 2008. Okenane, a biomarker for purple sulfur bacteria (Chromatiacea) and other new carotenoid derivatives from the 1640 Ma Barney Creek Formation. Geochima et Cosmochimica Acta 72: 1396-1414.
- Overmann, J. and Pfennig, N. 1992. Buoyancy regulation and aggregate formation in Amoebobacter purpureus from Mahoney Lake. Microbiology Ecology 101: 67-79.
- Pfennig,N. and Trüper,H.G. 1989. Anoxygenic phototrophic bacteria. In: Bergey’s manual of systematic bacteriology, Vol 3 (Staley, J.T., Bryant, M.P., Pfennig,N. and Holt, J.G. Eds.) pp.1635-1709. Williams and Wilkins, Baltimore,M.D.
- Taylor, D.J., Crease,T.J. and Brown, W.M. 1999. Phylogenetic evidence for a single long-lived clade of crustacean cyclic parthenogens and its implications for the evolution of sex. Proc. R. Soc. London B, 266: 791-797.