Motility could be an important adaptation of heterotrophic bacteria and archaea, and it
may have ecological and biogeochemical implications. However, the limited observations so far show
that only a small fraction (=10%) of bacteria is motile. We report a systematic 10 mo long field study
off the coast of La Jolla, California, as well as a mesocosm study to examine bacterial motility and its
relationship to environmental variables. Dark-field microscopy revealed periods of sustained low (fall
and winter, <5 to 25%) and high (spring and summer, 40 to 70%) percentages of motile bacteria (%
motile). Bacteria in natural seawater did not swim constantly nor at constant speeds; over 40% swam
<20% of the time, and showed bursts 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 positively related with particulate organic carbon
throughout diel sampling on 24 to 26 September 1997. During a mesocosm diatom bloom % motile
rose sharply as the bloom crashed, suggesting algal detritus may elicit motility. Enhanced % motile
resulted in increased colonization of living and dead algal cells by bacteria. Filtering seawater
through a 1 µm filter reduced % motile, again suggesting the importance of particulate loci. Enrichment
with dissolved organic nutrients enhanced % motile only after 6 h but it rapidly (=1 h) increased
the time individual bacteria were swimming. 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, and that motility
underlies dynamic patterns of ecological relationships (symbiosis, competition, parasitism) between
bacteria and algae. Since motility may enhance bacteria-organic-matter coupling it is likely to be an
important variable in the oceanic carbon cycle.