Bacterial motility in the sea and its ecological implications

Motility could be an important adaptation of heterotrophic bacteria and arc haea, and it may have ecological and biogeochemical implications. However, the limited observations so far show that only a small fraction (less than or equal to 10%) of bacteria is motile. We report a systematic 10 mo long f ield study off the coast of La Jolla, California, as well as a mesocosm stu dy to examine bacterial motility and its relationship to environmental vari ables. Dark-field microscopy revealed periods of sustained low (fall and wi nter, < 5 to 25%) and high (spring and summer, 40 to 70%) percentages of mo tile bacteria (% motile). Bacteria in natural seawater did not swim constan tly nor at constant speeds; over 40 % swam < 20% of the time, and showed bu rsts of motility. Percent motile showed a distinct diel pattern and was not significantly correlated with tidal cycle, chlorophyll a, or the abundance of algae, bacteria, and heterotrophic nanoflagellates. However, it was pos itively related with particulate organic carbon throughout diel sampling on 24 to 26 September 1997, During a mesocosm diatom bloom % motile rose shar ply as the bloom crashed, suggesting algal detritus may elicit motility. En hanced % motile resulted in increased colonization of living and dead algal cells by bacteria. Filtering seawater through a 1 mum filter reduced % mot ile, again suggesting the importance of particulate loci. Enrichment with d issolved organic nutrients enhanced % motile only after 6 h but it rapidly (less than or equal to 1 h) increased the time individual bacteria were swi mming. Our results show that a variable fraction of marine bacteria is able to respond to loci of organic matter, e.g. organic particles and algae, an d that motility underlies dynamic patterns of ecological relationships (sym biosis, competition, parasitism) between bacteria and algae. Since motility may enhance bacteria-organic-matter coupling it is likely to be an importa nt variable in the oceanic carbon cycle.