BACTERIAL DECOMPOSITION PROCESSES IN LAKE WINGRA SEDIMENTS DURING WINTER1

Throughout winter, viable counts of bacteria from sediments of Lake Wingra (Madison, Wisconsin) at several different incubation temperatures were always higher at 25°C than 4”C, although the temperature of the sediments remained below 4°C for over 3 months. All of the organisms isolated and initially cultivated at 4°C grew better at 25°C; no obligately psychrophilic bacteria were found. Isotope studies to measure the temperature optima of the resident bacterial flora showed that the optimunl tcmpcraturc for incorporation of 14C-glucosc into cell material and conversion of l”C-glucose into l*C-COT remained at 25°C or greater all winter. A true psychrophilic flora does not develop in these sediments in winter, and bacterial decomposition processes occur at a much slower rate in winter than in summer. Lakes in temperate climates show marked variations in temperature through the year. In Wisconsin, inland lakes are covered with ice for 3 or 4 months, and during this period temperatures of the water and sediments remain below 4°C. It was of interest to see whether bacteria in sediments became adapted to these low temperatures, since if they did, rates of bacterial metabolism might be expected to be higher than if adaptation did not occur. The fact that microorganisms are capable of adapting to low temperatures has been well documented. Obligately psychrophilic bacteria (with temperature optima below 2O’C) have been isolated from constantly cold marine environments ( Morita 1966)) at high altitudes (Moiroud and Gounot 1969)) in the Arctic ( Morita and Burton 1970)) and in the Antarctic (Straka and Stokes 1960; Stanley and Rose 1967). The lowest temperature for growth of bacteria has been reported at -10°C (Larkin and Stokes 1968). However, little direct study has been made of the temperature adaptation of microbial communities found in habitats of l This work was supported by U.S. Atomic Energy Commission Contract COO-2161-6. B. Mejchar assisted in some of this work. 2 Present address: Department of Biology, Mason Laboratory, Rensselaer Polytechnic Institute, Troy, New York 12181. wide seasonal temperature variation. Temperate lake sediments constitute natural habitats where temperatures vary by the season but where the winter low of about 1°C and the summer high of 30°C remain relatively constant for several months each year. Does the microbial flora show a single optimum temperature toward either end of this temperature range or is it in a continual state of adaptation to the seasonal temperature changes in the environment? Possible answers to these questions were sought by determining temperature optima directly on the resident bacterial populations using radioisotope techniques and indirectly by culture techniques. MATERIALS AND METHODS Lake Wingra is a small calcareous lake in the city of Madison, Wisconsin, at an elevation of 254 m. It is shallow, with a maximum depth of about 5 m; much of the lake was created by dredging from a marsh to provide fill for the construction of Vilas Park. We chose a single station at the southwest corner of the lake, in about 1 m of water, because it was amidst beds of water weeds which had contributed heavy organic matter to the sediments. Most of these weeds had become fragmented or decomposed before our study was made, although occasionally a fragment of plant material was present in the sample. IceLIMNOLOGY AND OCEANOGRAF’IIY 628 JULY 1973, v. H(4) DECOMPOSITION IN LAKE WINGRA SEDIMENTS 629 over occurred by mid-December and iceout by mid-April. At each sampling during this period, a single hole about 20 cm in diameter was cut in the ice with an auger. Maximum ice thickness averaged 50 dm during the winter. After ice-out the station was marked by a buoy and subsequent spring and summer samples were taken from a boat.