Garza, D.R. and C.A. Suttle. 1994. Isolation of lytic viruses which infect a marine heterotrophic nanoflagellate. Abstracts American Society Limnology and Oceanography, and Phycological Society American, Miami, FL, June 1994.

A virus which causes lysis of a marine heterotrophic nanoflagellate (E1) was isolated by adding concentrated natural virus communities from the coastal waters of Texas, to exponentially growing flagellate cultures, and waiting for culture lysis. The effect could be propagated by inoculating an uninfected culture with culture lysate. In the inoculated culture the concentration of flagellates dropped to below the detection limit (<10⁴ cells/mL) within 48 h, whereas, control cultures reached densities of ca. 10⁶ cells/mL. The pathogen was not removed by filtration through 1.0, 0.4 or 0.2 µm-pore-size filters. The pathogen is relatively host specific and did not infect Bodo sp. and Paraphysomonas sp., but did infect another flagellate that was isolated from the same water sample as E1. Observations of thin-sections by transmission electron microscopy revealed the presence of intracellular virus-like particles (VLP's) 24-48 h following infection, concomitant with a decline in E1 cell numbers; VLP's were not present in the control samples. The viruses are hexagonal in shape, 227 nm in diameter, contain electron-dense cores and have a doubled-layered capsid (inter-layer space ca. 6 nm). The viruses are morphologically similar to viruses which infect eukaryotic phytoplankton and to VLP's observed in natural nanoflagellate communities; however, to our knowledge this is the first isolation of an infective virus which causes lysis of a marine heterotrophic nanoflagellate. The presence of viruses in seawater which cause lysis of phagotrophic nanoflagellates imply another important role for viruses in the microbial loop.