Hidrológiai Közlöny 1967 (47. évfolyam)
7. szám - A „Szervesanyag meghatározási problémák édesvizekben” című 1966. szeptember 25–28. között Tihanyben rendezett Szimpózium előadásai - Wetzel, Róbert G.: Oldott organikus vegyületek két indiánai tóban
Wetzel, R. G.: Oldott organikus vegyületek Hidrológiai Közlöny 1967. 7. sz. 299 carrier of the labelled substrate) were added to the 12 cultures, usually in concentrations of 10, 20, 30, 40, 50, 75, 100, 250, 500, 1000, and 2,500jug l" 1. Simultaneously a small amöunt of high specific activity, sterile, C-14 labelled dissolved organic substrate of known radioactivity (0.50—1.0 uc ml1) was added rapidly witli an automatic syringe. Absolute activity was determined by aqueous persulfate combustion and radioassay of the C 1 40 2 in gas-phase (Wetzel, 1964). The samples were incubated in complete darkness at temperatures within 0.5°C of that of the laké and depth from which they were collected. Duration of incubation varied inversely with the temperature and plankton density (1 to 7 hours), but ne ver exceeded 5% of the regeneration turnover time of the substrate. Each culture series from each depth contained a control sample fixed with an iodide-acetic acid (Lugol's) solution and filtered immediately after injection of the radiocarbon. After incubation the samples were rapidly (1—2 minutes) filtered in a multiple filtration unit onto membrane filters (Millipore HA) of 0.45 ±0.02 p porosity. The experimentál samples were not fixed with Lugol's solution, contrary to the methods of Wright and Hobbié (1966), to circumvent possible loss of radioactivity from the cellular pool. Dried samples were radioassayed with a gas-flow detector (Nuclear-Chicago Model D-47; micromil window) of a counting efficiency calibrated with gasphase techniques and National Bureau of Standards samples. The velocity of uptake of the substrate at increasing concentrations permits a graphical and mathematical treatment, based on MichaelisMenten enzyme kinetics, to give an estimation of the theoretical maximai velocity (V) of substrate removal and utilization, natural substrate concentrations (Kt + Sn), (see discussion section) and the turnover time of substrate regeneration (Tt). The theroy behind these analyses is presented at great length by Wright and Hobbié (1966). The transport constant, Kt, is measure of affinity of the uptake system for a substrate and is generaly a constant independent of the biomass of the plankton. K< of uncultured bacteria is usually small (<0.005 mg l1) (Hobbié and Wright/1965); hence, Kt + S n gives an approximation of natural substrate concentrations, where S n is the natural concentration of the dissolved organic substance assayed. A factor for isotopic discrimination of 6% was used in the calculations, based on that of algae (cf. Steemann Nielsen, 1955; Sorokin, 1959). Although a good estimate of isotopic discrimination is unknown for such bacterial metabolism, it is almost certain to occur with at least this magnitude. The samples were assayed from profiles of the deepest central depressions of both Crooked and Little Crooked lakes, Noble-Whitley counties, Indiana (Stations A, Fig. 1, Wetzel, 1965). A detailed analysis of the annual rates of algal production biomass, and annual cycles of somé 30 limnological parameters of these and II other mari lakes of northern Indiana are presented elsewhere Kt+Sn 0 10 20 30 0 ? 30 0 10 20 30 Figure 1. Seasonal distribution of glucose: natural substrate concentrations (Kt + S n) X 10~ 3 mg l~ l, bacterial uptake velocity (V)xlO~* mg l~' hrand substrate renegeration (Tt) in hours. Crooked Laké, Witley-Noble counties, Indiana, 1964. Points represent sampling periods and depths. (Wetzel, 1965, 1966a, 1966b, in preparation). A1 though Crooked and Little Crooked lakes, interconnected by a few meters of channel, differ greatly in morphometry, the inorganic composition of the trophogenic zones, is similar in both lakes. However, alga biomass and annuall rates ofproduction of Little Crooked Laké are approximately twice that Crooked Lake. Results and Discussion The analyses for the utilization of glucose, acetate, galactose, fructose, and succinic acid all exhibited first order uptake kinetics, demonstrating rate limitation with increasing substrate concentrations. A linears slope occurred consistently to substrate levels of between 250—500X10~ 3 mg 1 — 1. Substrate concentrations above this quantity, from 0.5 to 10.0 mg l~ l, nearly always followed kinetics of simple diffusion in a similar manner to the results of Wright and Hobbié (1965, 1966) and H. L. Allén (personal communication) The results of the work of Wright and Hobbié indicated that the algal chemo-organotrophic assimilation of these organic substrates is inefecctive at naturallv occurring concentrations; that uptake measured is almost entirely a result of bacterial metabolism. At the existing substrate concentrations found in Crooked and Little Crooked lakes,
