Distribution of heterotrophic bacteria in relation to the concentration of particulate organic matter in seawater

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Abstract
The relationship between the number of bacteria and the concentration of particulate organic carbon (POC) in seawater was investigated. In coastal seawater in summer, the POC concentration showed better correlation to the density of bacteria obtained by the viable plate count method (viable count, V.C.) than by the total direct count method (total count, T.C.). The number of attached bacteria (A) showed significant fluctuation, both laterally on a geographical scale and vertically in the water column; on the other hand, the number of free-living bacteria (F) was relatively constant. The POC concentration had a much higher correlation with A (r = 0.8795) than with T.C. (r = 0.7339), and had a low correlation with F (r = 0.6935). Moreover, a very good correlation was observed between the density of bacteria obtained by V.C. and A (r = 0.9153). These results indicate that when the concentration of particulate organic matter (POM) increases, some free-living bacteria become attached to POM, and grow on the surface of POM. These communities of attached bacteria have the ability to make colonies on plate media and can be counted as the "viable plate count."