The upper limit of viable cell count by plate counting method was studied theoretically and experimentally. The probability which k-cfus make n-colonies, A (k, n), was expressed by the following equation:
A(k, n)=n·p·A(k-1, n)+(1+(n-1)·p)·A(k-1, n-1)
where p is the ratio of colonies' average area to plate area. From this equation it was indicated that as the values of p increased, fused colonies increased and the counts of colonies decreased. Similarly this phenomenon was also observed at the experiment and calculated values coincided with the experimental values qualitatilvely. At this equation, using p/5 instead of p calcutated values coincided with the experimental values quantitatively. Therefore, it was concluded that this equation refrects the acutual plate counting and the upper limit of it had a theoretical base.
In the present report this equation gave the upper limit of plate counting method a theoretical base.

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Grain density autoradiography was examined as applied to the quantitative determination of photosynthesis by individual phytoplankton species. Silver grains over and around (30μm from the cell margin) algal cells were estimated to be formed by β-emission of ¹⁴C incorporated in the cells. The grain count in this area increased linearly with the exposure time and related significantly with the radioactivity of the cells. A good correlation was obtained between grain counts (no./cell) and radioactivity in the cell (dpm/cell). The regression was almost the same in three algal species of a similar small cell size, but considerably lower in a larger species. This indicates that the comparison of absolute radioactivity between different species should be made carefully. By using the autoradiographic technique, photosynthetic characteristics such as light-photosynthetic rate relation could be determined in individual phytoplankton species in a eutrophic pond. The technique appears to be useful especially in detecting species reaction to a change of nutrient conditions in phytoplankton ecology.

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Bacterial numbers and floras in the water and sediment of subtropical Lakes Phewa, Begnas, and Rupa in the Pokhara Valley, Nepal, were surveyed in late monsoon season. In the water column at the deepest part of Lakes Phewa, Begnas, and Rupa, total bacteria estimated by the acridine orange direct count (AODC) method changed in the range of 1.1-2.4×10⁶, 1.7-2.4×10⁶, and 3.2-3.5×10⁶ cells/ml, respectively, and heterotrophic bacteria estimated by the spread plate method changed in the range of 2.5-18×10³, 2.0-4.0×10³, and 6.9-14×10³ CFU (colony-forming units)/ml, respectively. The distribution pattern of free-living and attached heterotrophic bacteria, size-fractionated with a 3.0μm Nuclepore filter, throughout the water column differed among the three lakes. The total and heterotrophic bacteria in the water at inlet were in the orders of 10⁴-10⁵cells/ml and 10³CFU/ml, respectively, and those in the water at outlet showed a similar number to those in the surface water at each deepest part. The number of heterotrophic bacteria in the sediment at the deepest part was not different among the lakes and in the range of 5.9-8.9×10⁵ CFU/cm³ of wet sediment. All of 178 isolates purified from the three lakes, 64 from the water and 114 from the sediment, were identified to generic level. Only 20% of bacteria isolated from the water were purified, and most of those isolates belonged to Pseudomonas, Flavobacterium, and coryneforms. On the other hand, the bacterial floras in the sediment of the three lakes were dominated by Bacillus.

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Asterionella glacialis, a marine diatom, which occurred a bloom in October in Maizuru Bay, Japan, was isolated, and the effect of bacteria on the growth of this alga was examined. This alga did not grow axenically, but grew after addition of a Flavobacterium NAST (the monoxenic condition). The growth of A. glacialis was strongly stimulated by a bacterium (Pseudomonas 022) occurring during the algal bloom in Maizuru Bay. The algal growth was also stimulated by the filtrate of Pseudomonas 022-monoxenic A. glacialis culture. The results suggest that the alga is promoted by some metabolite of a concomitant bacterium.

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Two species of bacteria, a Macromonas-like bacterium and a large-celled phototrophic one, probably a new species of Chromatiaceae family, are densely populating the upper boundary of H₂S water of Lake Kaiike forming a purplish red colored bacterial plate. The large phototrophic bacterium grows slowly even under an optimal condition; the maximum growth rate is 1.0·day^-1, and its in situ growth rate is suggested to be lower than thus estimated due to a limitation by light and H₂S. Comparison of the light-limited growth between the large phototrophic bacterium and Chromatium buderi DSM 176 shows that the bacterium could maintain the dense population by keeping its physiological activity to a minimum level.

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Ecological assessment, needed for the establishment of admissible dose and the persistence of the effects of heavy metals on microbial communities is reviewed with reference to recent research. Developing concepts for ecological assessment give most emphasis to the exposure time to metals and to qualitative changes in the microbial communities. The potential ability of microbial communities for the development of metal-tolerant microflora should be considered carefully in relation to the exposure time. In this sense, studies on the qualitative changes in bacterial communities after heavy metal contamination, with consideration of the phylogenical relationships between the species, should provide valuable information for better interpretations and the identification of general principle in the potential ability of bacterial communities. In addition, analytical methods for the estimation of the bioeffective amount of heavy metals, which are surely necessary for the establishment of the admissible dose and the comparison of observed effects, were also reviewed.

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