COMPARISON OF GROWTH AND SINKING RATES OF NON-COCCOLITH-FORMING AND COCCOLITH-FORMING STRAINS OF EMILIANIA-HUXLEYI (PRYMNESIOPHYCEAE) GROWNUNDER DIFFERENT IRRADIANCES AND NITROGEN-SOURCES

We examined the effect of the presence or absence of coccoliths on the growth and sinking rates of an oceanic isolate of the coccolithophore Emiliania huxleyi (Lohmann) Hay et Mohler isolated from the northeast ern subarctic Pacific. Coccolith-forming and non-coccolith-forming (i. e. naked, nonmotile) strains were obtained from the same isolate and grown under both saturating and limiting irradiance levels with either nitrate or ammonium as the primary nitrogen source. Sinking rate, gro wth rate, and cell volume (excluding coccoliths) were measured for eac h culture. Under saturating irradiance, coccolith-forming cells grew s ignificantly slower than naked cells, had significantly higher sinking rates, and had larger cell volumes than naked cells. Under limiting i rradiance levels, growth rates of the two strains were identical, sink ing rates were higher for coccolith-forming cells in stationary-phase cultures only, and cell volumes remained greater for coccolith-forming cells. The sinking rates achieved for this ubiquitous coccolithophore ranged from <0.1 to 0.5 m . d(-1). Sinking rates were not statistical ly different between coccolith-forming and naked strains off. huxleyi under limiting irradiance conditions for log-phase cultures, but sinki ng rates were greater for coccolith-forming cells under some of the ot her conditions tested. However, the average sinking rate was never mor e than twice as great as for coccolith-forming cells, with the excepti on of nitrate-grown, senescent cells under limiting irradiance (3.4 ti mes greater). Cell volumes (excluding coccoliths) were consistently ca . 1.5 times greater for coccolith-forming cells than for naked cells. Nitrogen source had an effect on growth rate and cell volume, with amm onium-grown cultures growing faster and having larger cell volumes tha n nitrate-grown cultures of both strains. However, despite the differe nce in growth rate and cell volume, nitrogen source had little if any effects on sinking rate.