Light scattering by viral suspensions

Viruses represent one of the most abundant, ocean‐borne particle types and have significant potential for affecting optical backscattering. Experiments addressing the light‐scattering properties of viruses have heretofore not been conducted. Here we report the results of laboratory experiments in which the volume‐scattering functions of several bacterial viruses (bacteriophages) were measured at varying concentrations with a laser light‐scattering photometer using a He‐Ne and/or Argon ion laser (632.8 and 514.0 nm, respectively). Four bacterial viruses of varying size were examined, including the coliphages MS‐2 (capsid size 25–30 nm) and T‐4 (capsid size ∼100 nm), and marine phages isolated from Saco Bay, Maine (designation Y‐1, capsid size 50–80 nm) and Boothbay Harbor, Maine (designation C‐2, capsid size ∼110 nm). Volume‐scattering functions (VSFs) were fitted with the Beardsley‐Zaneveld function and then integrated in the backward direction to calculate backscattering cross section. This was compared to the virus geometric cross section as determined by transmission electron microscopy and flow‐field fractionation. Typical backscattering efficiencies varied from 20 × 10 −6 to 1,000 × 10 −6 . Data on particle size and backscattering efficiencies were incorporated into Mie scattering calcula‐tions to estimate refractive index of viruses. The median relative refractive index of the four viruses was ∼1.06. Results presented here suggest that viruses, while highly abundant in the sea, are not a major source of backscattering.