An electron microscopic study of picoplanktonic organisms from a Small Lake

Picoplankton, both prokaryotic and eukaryotic, are distinguished from other aquatic organisms by their small size (0.1-2.0 μm). Such organisms were recovered from waters of a small oligotrophic lake using screens, filters, and high-speed centrifugation. The majority of the picoplankton were unable to form visible colonies on common media. Cells examined in thin sections by electron microscopy showed that 60-75% of the cells had an average diameter after dehydration of 0.48-0.51 μm. The maximum dimensions of the rest of the cells ranged from 0.56-1.81 μm. Using details of ultrastructure, cells were classified as prokaryotic or eukaryotic. Phototrophs present included two cyanobacterial morphotypes (5-6%) and two eukaryotic algae (less than I%). The arrays of intracytoplasmic membranes in 18-20% of the cells were suggestive of methanotrophic rods and chemoautotrophs. Relatively few prosthecate bacteria were observed in the water column samples. The smallest cells (1-2%) contained magnetosomes, the presence of which were confirmed by x-ray spectroscopy. Iron was also detected in the envelopes of some rod shaped cells by the same technique. The study of in situ picoplankton populations using TEM coupled with other techniques may provide better understanding of picoplankton biomass.