Growth, nutrition, and the life cycle of the marine prasinophyte, Platymonas impellucida

The marine prasinophyte, Platymonas impellucida, although isolated in 1959, was established in axenic culture. Investigation of this unicellular alga was divided into five sections: Section One, growth in culture; Section Two, nitrogen, phosphorus, and carbon assimilation; Section Three, salinity and ionic requirements; Section Four, effects of heavy metals and boron; Section Five, life cycle and formation of cysts. Section One. Platymonas impellucida was grown in batch cultures in 125 ml erlenmeyer flasks. Optimum growth occurred at 25°C. Light was important to the alga with 60 and 90 uE m⁻²sec⁻¹ saturating growth and photosynthesis respectively. Daylength Of 18:6 resulted in optimal rates of growth and synchronized cell division. A diurnal pattern of photosynthesis resulted in optimal rates of net photosynthesis eleven hours after the initiation of the light period. Section Two. Platymonas impellucida was shown to assimilate nitrate, nitrite and ammonia at equal efficiencies. When presented with ammonium nitrate, the ammonia was utilized preferentially. Maxium yields were obtained when the cell was presented with uric acid, while urea supported reduced growth. Amino acids were unavailable as nitrogen sources with the exception of L-glycine and L-glutamine. Addition of organic sources of carbon neither stimulated nor inhibited P. impellucida to any great extent. The two sources of phosphorus, NaH₂PO₄ and Na-B-glycerophosphate, were assimilated at equivalent efficiencies. Section Three. Platymonas impellucida was shown to be a euryhaline organism. Optimal growth occurred from 0.3X to 2X salinity, with reduced growth at 3X and no growth at 0.1X. The drastic change in growth between 0.3X and 0.1X was the result of limiting concentrations of calcium and low ionic concentrations. Reduced growth at higher salinities was the result of ion inhibition and not increased osmotic pressures. Requirements for Cacl₂, MgCl₂, KCl, NaCl, and MgSO₄ were investigated with optimum final yields of cells with the addition of 10 mM CaCl₂, 10 mM MgCl₂, O.2 mM KCl, 100 mM NaCl, and 0.06 mM MgSO₄. Section Four. The tolerance to boron, zinc, copper, cadmium, cobalt, lead, and mercury of Platymonas impellucida grown in batch culture was established. Mercury and lead were inhibitory at the lowest concentrations (2 uM and 8 uM respectively). High concentrations of cobalt, copper and cadmium were required before growth was affected. At high concentrations, cobalt and cadmium were absorbed and concentrated within seven days, and 24 h respectively. A requirement for iron and boron could not be established, however, growth responses indicated that these two compounds were supplied as contamination in the synthetic medium. Section Five. An asexual life cycle of Platymonas impellucida was established. Cells in an 18:6 light cycle, began to settle two hours before initiation of the dark cycle. Cells divided during darkness, with release of the two motile daughter cells occuring shortly thereafter. There was no indication of a sexual cycle. Platymonas impellucida formed thick-walled cysts after two or eight weeks depending on whether the cells were grown in an enriched Seawater medium (SWM-1) or a synthetic medium (ASP-M). Attempts to accelerate formation of cysts in ASP-M indicated that general nutrient deficiency, variation of pH from the norm, and gradual desiccation all induced cysts. Differences between the two media were not accounted for. Tolerance of cysts to temperature and desiccation was no greater than tolerance of motile cells.