Photosynthesis, excretion, and growth rates of Phaeocystis colonies and solitary cells

Abstract

Unialgal cultures of the prymnesiophyte, Phaeocystis cf. pouchetii, were isolated from Norwegian and United States coastal waters. Manipulation of the nutrient medium resulted in populations overwhelmingly dominated by either colonies or solitary cells of Phaeocystis. Both morphotypcs were grown under a range of irradiances at 0°, 2°, 5°, 10° and 20°C. Photosynthesis was measured as incorporation of H14CO3 and excretion as accumulation of DO14C during 24-hour incubations; growth rates of solitary cells were determined concurrently from changes in abundance. Both morphotypes exhibited temperature-dependent asymptotic increases in pigment-specific photosynthesis with irradiance. Saturation intensities increased with temperature. Cell division by Phaeocystis solitary cells exhibited a functional response similar to photosynthesis, although growth apparently saturated at lower irradiances. C:Chla ratios were positively correlated with irradiance and inversely related to temperature, while C:N ratios were insensitive to these environmental parameters. Colonies had higher C:Chla and C:N ratios than solitary cells. Pigment-specific excretion rates were linear functions of irradiance, and exhibited temperature-dependent positive correlations with photosynthesis. Percent extracellular release (PER) by both morphotypes was inversely related to temperature. At low temperatures (0-5 °C), solitary cells had higher photosynthesis rates than colonies at all irradiances. Their excretion rates, however, were also higher, such that the PER of solitary cells exceeded those of colonies at 0°C and low irradiances at 2°C. No differences were detectable at 5°C. At higher temperatures, photosynthesis by solitary cells still generally exceeded that by colonies, but the colonies excreted considerably more DOC. Thus, while solitary cells are more efficient at utilizing light for photosynthesis, they do not necessarily channel a larger proportion into biomass production. Colonies, however, appear to be particularly stressed by higher temperatures and irradiances.