Transmission electron microscopy (TEM) and epifluorescence microscopy of DAPI and Yo-Pro-1 stained samples were used to estimate viral abundance in natural communities along a transect from the oligotrophic central Gulf of Mexico to the productive
near-shore waters at Port Aransas, Texas (USA). Estimates of viral abundance based on TEM averaged only 66% (range 26 to 108%) of those made using epifluorescence microscopy and the cyanine-based dye, Yo-Pro-1. DAPI staining provided estimates that were
much closer and averaged 86% (range 72 to 109%) of those made using Yo-Pro. However, all 3 methods provided similar estimates at viral abundances <106 ml-1. The precision of the Yo-Pro and DAPI methods (coefficient of variation 8 and
11%, respectively) was much greater than for the TEM method (25%). Experiments with cultures indicated that grazing by flagellates was unlikely to be a significant source of viral-size particles that could interfere with the DAPI or Yo-Pro method.
Estimates of viral abundance made using the Yo-Pro method ranged from 0.3
Virus enumeration · Gulf of Mexico · Yo-Pro · DAPI · Virus abundance
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