Direct and indirect observations of bacteria on marine pebbles

Abstract

Numbers and kinds of bacteria growing on marine pebbles were studied. Rotating pebbles slowly for 3 h in 80% seawater in porcelain jars brought most of the bacteria into suspension so they could be counted by membrane filter culture technique. Pebbles stored at −15 °C showed little change in density of bacterial population.Pebbles from three Nova Scotian coasts had similar numbers of bacteria, an average of 3.45 × 105 bacteria/cm2, or about 1 bacterium per 300 μ2. Those from a fourth coast had a smaller population.Direct counts using incident-light fluorescent microscopy revealed 7 times the number detected by membrane filter counts, and that about 2% of the pebble surface was occupied by bacteria. There was greater variance in counts from pebble to pebble than on a given pebble.Eleven isolates indicated the predominant bacteria on pebble surfaces to be motile, Gram-negative, asporogenous rods with polar flagella, mostly Pseudomonas. Pigmented forms predominate and pleomorphism is common.