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A lipid antibiotic was extracted from whole cells of the marine diatom Chaetoceros lauderi (Ralfs) and roughly identified as an unsaturated fatty acid by thin layer chromatography. A number of Gram-positive terrestrial bacteria and several Gram-negative marine bacilli were sensitive to this antibiotic, whereas Gram-negative enteric bacteria were resistant. Solar irradiation led to its partial disappearance and to the production of more polar antibiotics with a stronger antibacterial activity. This observation is compared to similar data culled from literature, and its significance in a natural environment is discussed.

Authors tried to determine some factors of incertitude in the anlaysis of the bacterial contamination of seawater. Such analysis depends on the reliability of the methods used for bacterial numeration, and on the heterogeneity of water in the investigated area. The general numeration of organisms (GNO) and the numeration of coliforms (NC) were taken into account. The turbidity of the sample, and the operator's manner were shown to be factors of variation for the GNO. An interaction between the operator and the amount of the anlaysed sample seemed to influence NC. In a natural environment, variations with time were the most important. They were partly due to hydrological and meteorological fluctuations, but sometimes seemed to be random. During a day cycle, the bacterial contamination trended to decrease when the solar radiance was maximum. Influence of short scale variations in the location of sampling was generally random, yet with a slight increase of contamination in the more stagnant waters.