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SUMMARY: A new class of micro-organisms, the Krassilnikoviae, found in the Black Sea, the Pacific and the Arctic Oceans, is described. These organisms developed on slides immersed in the waters ; they have not been cultivated in the laboratory. They consist of non-branching, non-septate filaments with a head at one end con- sisting of a cluster of round bodies. The filaments often become interlaced and form strings which can just be seen by the naked eye. They may or may not be contained in a sheath. During an expedition to the Pacific Ocean on the S.S. Vitijux a number of slides were immersed for 8-25 hr. at various depths from 50 to 1000 m. near the Kurile Islands. Upon removal a peculiar filamentous micro-organism with heads composed of a cluster of rounded bodies was found on the surface of these slides. Subsequent deep-water investigations in the Black Sea, the Pacific Ocean and the Arctic Ocean (near the North Pole) showed that this form of micro-organism was widely spread. They developed on the slides immersed in the oxygen zone and at maximum depths in the Black Sea, where the hydrogen sulphide concentration was very high. These organisms could not be cultivated from many hundreds of water samples taken at the same stations and the same horizons where they had been found on slides, and only in single cases could they be found on membrane filters through which 15- 50ml. of the same water had been passed. In sea water therefore these micro-organisms, as a rule, do not occur in a free-floating state, but only develop their characteristic forms when attached to objects suspended in the water. METHODS Glass slides, cleaned with ethanol or chromic-sulphuric acid mixture, washed with distilled water, dried and flamed, were inserted at one end into slits in large rubber bungs 5-6 cm. diameter. The bungs were then fixed to a cable and immersed at various horizons for periods ranging from a few hours to several days. The slides were then removed, dried, fixed by heating or in absolute ethanol, and stained with 1 yo (wlv) erythrosine in 5 yo (w/v) phenol aqueous solution, or with 5 yo (wlv) erythrosine in 3 yo (wlv) phenol aqueous solution. The morphological examination of the organisms was carried out at a magnification of x 1350. Counts of the entire slide, except the portion in contact with the bung, were also made at a magnification of x 300, a filament with a cluster head being taken as a single organism. In large and dense accumulations only separate filaments with heads were counted. Forms with I G. Microb. xx

Krassilnikoviae: A New Class of Micro-Organisms found in Sea and Ocean Depths