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infectious particles (especially in marine Synechococcus sp.) to determine the host range, in situ titers, and ecological variability of naturally occurring cyanophages (21).
In the marine environment there is certainly a continuum of size in both bacterioplankton and virioplankton. Bacterioplankton can range from large filaments > 10 µm, to small coccoid cells with diameters approaching 0.3 µm (2). Marine virus isolates range in length from about 40 nm to as large as 120 nm (5). Electron micrographs of naturally occurring infected cells suggest that some bacterial hosts are considerably less than 10-fold larger than their vital parasites, having a burst size of about 7 (6)! The very smallest bacterial cells and the very largest viral particles fall into about the same size category, raising some questions about the accuracy of currently used methods for quantifying naturally occurring virus and prokaryotes. Commonly used epifluorescence techniques are convenient and reproducible, but the identity of the fluorescently stained particles is certainly subject to some uncertainty. What fraction of VLPs are actually viruses? What fraction of VLPs are viable viruses? What fraction of DNA-containing particles < 0.1 µm are actually cells, and not viruses? If some of the < 0.1 µm DNA-containing particles are cells, are they viable? These remain open-ended questions.
With regard to the complexity of these populations, the issue of cultivability is a serious one. It still appears from available data that a large fraction of naturally occurring microbes have resisted cultivation attempts. The specific physiological traits and life histories of these microorganisms remain unknown, as does that of their vital parasites. A major challenge to contemporary microbiology is to devise and implement approaches to better characterize this large and uncharacterized biota.