SUMMARYThe photosynthetic marine flagellate Micromonas pusilla (Butch.) Manton et Parke (Prasinophyceae) and its cytoplasmic virus, M. pusilla virus (MPV), were cloned. Host cells were maintained in liquid culture. Infectivity titration was by endpoint dilution, using loss of host chlorophyll as an indicator of the presence of infective virus. The virus growth cycle was characterized by an eclipse period of 3 h, a latent period of 7 h and a total lytic cycle of 14 h. The average burst size was 72 infective particles per cell. Inhibition of CO2 photoassimilation began 2 h after inoculation with virus. An almost immediate decrease in in vivo chlorophyll a fluorescence in infected cells was lightdependent. M. pusilla cells can mutate to virus resistance at the cell surface. Host range mutants of MPV exhibited variable infectivity in different strains of M. pusilla.
Cell division rates and chlorophyll a and protein contents for ten diatom and dinoflagellate species were measured. Species were chosen to include a wide range of cell size in terms of both cell volume and cell protein: from 0.004 ng protein/cell for a small Chaetoceros sp. to 2.2 ng protein/cell for Prorocentrum micans Ehrenberg. Experiments were conducted in batch or semi‐continuous cultures at 21 C under continuous illumination from 8–256 μEin .m‐2'.s‐1. Light saturation of cell division occurred at 32–80 μEin m‐1 s‐1 for all species, with no observable difference between the two phylogenetic groups. When the light‐saturated cell division rates were plotted against cell size as protein/cell, the diatoms and dinoflagellates fell on two separate lines with the diatoms having higher rates. Chl a /protein ratios (μg/μg) decreased with increasing irradiance. The diatoms had higher chl a per unit protein. The relationship between cell division rate and the chl a/protein ratio is discussed.
Photosynthetic rates, growth rates, cell carbon, cell protein, and chlorophyll a content of two diatom and two dinoflagellate species were measured. The microalgae were chosen to have one small and one large species from each phylogenetic group; the two size categories differed from each other by 1.5 orders of magnitude in terms of cell carbon or cell protein. The cultures for the experiments were grown under continuous light at an irradiance high enough for the light‐saturation of growth for all four species. The four species were found to have similar maximum photosynthetic rates per unit chlorophyll a. The diatom species showed lower carbon/chlorophyll a ratios and higher photosynthetic rates per unit carbon than the dinoflagellates. The higher growth rates of the diatoms were shown to be related to their higher photosynthetic rates per unit carbon. The ecological significance of the physiological difference between these two groups of microalgae is discussed.
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