The flagellate condition
ABSTRACT Flagellates comprise a taxonomic diversity of unicellular and colonial organisms (or flagellate stages in life-cycles) in the size range 2-2000 pm which are predominantly free-swimming in a very wide range of free-living and symbiotic situations. They variously exhibit photolithotrophy and the phagotrophic and saprotrophic manifestations of chemoorganotrophy, as well as combinations of these modes, in addition to associations with other organisms in symbiosis sensu lato. Flagellates compete with organisms having the same range of trophic modes, but with alternative life forms, such as non-motile or gliding unicellular or filamentous (including mycelial) organisms in sediments, and non-flagellate organisms in the plankton. In comparison with these other life forms occupying similar habitats we can identify costs of the flagellate condition and benefits which presumably outweigh the costs in extant flagellates. Identifiable costs of flagella are those of construction of flagella (energy, C, N, P), of flagellar operation (energy, with an upper limit on energy cost imposed by the maximum catalytic capacity for energy transformation by the measured content of dynein ATPases, and a lower limit imposed by the minimum energy dissipated against drag forces) and of flagellar maintenance. There does not seem to be a measurable cost of the flagellate condition in reducing specific growth rate relative to comparable (in size and phylogeny) organisms under either resource-saturated or resource-limited conditions.