Effect of heavy metals on natural populations of bacteria from surface microlayers and subsurface water

The effect of the heavy metals copper and cadmium on natural bacterial populations of surface microlayer and subsurface water was investigated. Two parameters, number of colony-forming bacteria and 14C-glucose uptake rate, were evaluated. The 2 natural bacterial populations showed different responses to the heavy metals. Results support the existence of autochthonous bacterioneuston populations in the marine environment. Although a large number of metals are essential for growth, some can be harmful for living cells. This is mainly due to the fact that heavy metals form complexes with protein molecules which render them inactive, for example, inactivation of enzymes. Many heavy metals are detrimental to microorganisms even at the low concentrations present in natural waters (Mills & Colwell 1977). There are also reports on the adaptation of microorganisms to heavy metals (Azam et al. 1977). This phenomenon has important implications for microbial ecology in polluted ecosystems. Accumulations of bacteria in the surface microlayer have been reviewed by many authors (e.g. Sieburth 1979, Norkrans 1980). Enrichment of toxic substances such as heavy metals in surface microlayers has also been described from various aquatic ecosystems (Baker & Zeitlin 1972, Lion & Leckie 1982). Thus, the physiological state of neuston may be considerably affected by toxic substances. The present study attempted to improve understanding of the biological function of neuston in the marine environment, by investigation of the influence of high heavy metal concentrations on natural populations of bacterioneuston. Seawater was obtained on calm days only from the innermost Kiel Fjord (Baltic Sea, Federal Republic of Germany). Surface microlayer samples were collected with a sterile circular screen (0.1256 m2, 16 mesh, wire diameter 0.35 mm, stainless steel). The thickness of the Present address: Dept. of Microbiology, College of Natural Sciences, Seoul National University, Seoul 151, Korea O Inter-ResearchIPrinted in F. R. Germany sampled surface microlayer was about 430 pm. Sampling from a depth of l m was conducted with a sterile 5 1 glass bottle. The 2 water samples were filtered through a stainless steel sieve (mesh size 75 pm) prior to use to remove larger zooplankton and detritus. Experiments to investigate the effects of copper and cadmium on natural bacterial populations were independently undertaken immediately after collection of seawater samples. The influence of Cd and Cu salts was determined on the basis of bacterial survival (number of colony-forming units) and glucose uptake rate. Sterile standard solutions of heavy metals, Cd(N03), . 4H20 and CuCl, 2H20, were individually mixed with 500 m1 of either neuston sample or subsurface sample. Two different Cd concentrations and a control were tested for each sample. The final concentrations in the sample were 1 and 10 ppb(w/v) (10 ppb Cd = 10 pg Cd 1-' = 27.444 pg Cd(N03),.4H20 1-l). Two concentrations were similarly tested for Cu and reflected 10 and 100 ppb(w/v) (100 ppb Cu = 100 pg Cu 1-' = 268.3 pg CuC1,.2H20 I') . These concentrations are 10 and 100 times greater than natural concentrations at 2 m depth in the investigative area (Kremling et al. 1979). The samples were incubated for 24 h at room temperature and continually shaken. Just prior to the addition of heavy metals, the number of colony-forming units and 14C-glucose uptake rate of the bacteria were determined as initial values. Colony-forming units on 3 parallels of ZoBell agar plates with salinity 17 %O were counted after incubation for 14 d. For the determination of glucose uptake rates, 0.2 m1 of D-(U-14C)-glucose solution (= 2.5 pg C) were added to 3 parallel 10 m1 samples from each experimental set and were incubated for l h at room temperature. One of the 3 parallels was set up as a control (fixed with 0.05 m1 formalin at the start of the incubation) in order to determine the amount of radioactive substances adsorbed. After incubation the samples were fixed with formalin, then filtered 204 Mar. Ecol. Prog. Ser. 26: 203-206, 1985