Seawater contains an extraordinary abundance of nanoparticles. Among biotic nanoparticles are the viruses which are known to affect carbon cycling and composition of microbial communities. A growing body of evidence has recently shown that membrane vesicles could be as abundant as viruses in the oceans. Discrimination between these different particles is therefore crucial.
We developed a new interferometric detection method coupled with the analysis of Brownian motion to detect, quantify, and differentiate a number of marine biotic nanoparticles. We validated this method through the analysis of calibrated nanoparticles and known homogeneous virus suspensions. This method allows to detect viruses, irrespective of their genetic material; double or single stranded DNA or RNA. The scattering signal amplitude that we measured is proportional to the third power of the virus diameter, which makes virus size determination accurate. The virus size that we could characterize with a suitable signal-to-noise ratio was as small as around 30 nm in diameter. Analysis of Brownian motion of viral particles, which are not only simple spheres, revealed for the first time peculiar trajectories for myoviruses that can provide additional signatures for their identification. We further applied the interferometric method for the detection of vesicles secreted by the marine diatom Phaeodactylum tricornutum. Based on their scattering signals and Brownian motion, we present the analysis of biotic nanoparticles from mixed marine environments, i.e. from coastal and oligotrophic samples from the Tara Oceans circumnavigation.