Sediments in the southwestern Baltic Sea showed variable correlations between heat production rate, adenosine triphosphate (ATP) concentration, and activity of the electron transport system (ETS). There was excellent correlation between heat flux and ATP for beach sand but none for subtidal sand. Heat production/ATP, heat production/ETS, and ETS/ATP ratios are 3.5, 2.8, and 1.8 times higher, respectively, for sandy beach sediments than for sandy subtidal sediments. Heat production/ATP ratio in a batch culture of the obligate anaerobic fermenter, Bacteroides sp. was relatively stable during growth phase but decreased greatly from stationary through senescent phases, indicating that at least part of the variability in the correlation between heat production rate and ATP in sediments might be due to differences in the physiological state of bacteria. Long-term storage of sediments resulted in decreases in both ATP concentration and rate of heat production, possibly due to exhaustion of food substrate, of electron acceptors, or changes in microbial composition. Inferring differences in metabolic rates of very different sediment samples from differences in ATP or ETS activity alone could be misleading. Direct metabolic rate measurements are essential in addition to ATP or ETS activity measurements for accurate understanding of the differences and changes in sediment metabolism. When dealing with unknown mixtures of metabolic types in sediments, direct calorimetry has the advantage of measuring equally well the rate of production of a common end-product of all kinds of metabolic activities.
Heat flow of closed artificial systems, consisting of flooded clean sand with added food, shows the usual growth pattern of microbes in a batch culture: rapid growth with concomitant increase in metabolic heat production to a peak level, followed by exponential decline. Heat flow from the same system with nematodes (Diplolairnella chitwoodi) and polychaetes (Capjtella capitata) indicates different rates and patterns of growth and metabolism of the microbial community in the presence of worms. The nematode prolonged the lag phase and prevented the microbial populations from reaching the peak metabolic rate without nematodes. C. capitata maintained system metabolism and microbial population at relatively steady state. The greater the food ration per mg of polychaete the greater the microbial heat production. Unfed C capitata showed an exponentially decreasing metabolic rate to a level, by the end of 24 h, about a sixth that of the worms with food. Direct calorimetry can just as easily measure the energy flow of natural soft-bottom ecosystems, and the effects of species added to it. On the other hand, results indicate the extreme difficulty of partitioning system energy flow into its metabolic compartments (by species, size, or functional groups) without uncoupling interaction effects.
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