Removal of large viruses and their dispersal through fecal pellets of the appendicularianOikopleura dioicaduringEmiliania huxleyibloom conditions

Abstract

Despite their importance in shaping the structure and function of marine microbial food webs, little is known about factors regulating marine virus abundance. Previous work demonstrated clearance of laboratory-cultured Emiliania huxleyi virus by the appendicularian Oikopleura dioica; however, the applicability of this interaction to natural virus assemblages was not investigated. Here, we conducted controlled laboratory experiments using O. dioica and mesocosm water containing natural virus assemblages with high densities of virus, and measured removal of virus by O. dioica using both flow cytometry and molecular methods. Bayesian models based on flow cytometry quantification of virus particles demonstrated efficient removal of viruses (mean 90.3 mL ind À1 d À1 ), with a clearance efficiency of 42.6% relative to food algae. Molecular detection of virus removal by quantification of viral mcp gene copies revealed a mean clearance rate of 68.1 mL ind À1 d À1 . Fecal pellets from these experiments demonstrated that viruses in fecal pellets retain infectivity despite passage through the O. dioica gut. Shotgun metavirome analysis demonstrated O. dioica removal of large virus groups, notably the Phycodnaviridae. The results demonstrate the removal of E. huxleyi virus from natural virus assemblages by O. dioica and the maintenance of viral infectivity when incorporated into fecal pellets, prompting further investigation on the fate of fecal-packaged viruses and their impact on host dynamics. Furthermore, our results indicate the generality of this interaction for other large algal viruses, raising questions about the implications of this mechanism of marine virus redistribution on the broader marine virus community.

Grazing on Marine Viruses and Its Biogeochemical Implications

“…Similarly, using another large algal virus ( Emiliania huxleyi virus 99B1 [EhV 99B1]; 180 nm in diameter), it was shown that the globally distributed pelagic (free-swimming) tunicate Oikopleura dioica was able to trap viruses at high rates, leading to almost 100% clearance of viral particles within ~1 day in a closed system ( 43 ). This was also shown for O. dioica feeding on natural viral communities ( 44 ). Many of the above-mentioned organisms are filter feeders, suggesting that viral grazing may be common to this feeding class of predators.…”

“…As reviewed above, different organisms have been observed to feed on giant viruses; however, the extent to which these particles may contribute to host nutrition is unexplored. Using O. dioica and EhV as a pelagic model system ( 43 , 44 ), we estimated the C, N, and P contents that predators would gain from viral particle grazing. In this system, the nutritional gain can be 24.2 ng C, 3.8 ng N, and 0.2 ng P individual −1 day −1 (see Text S1 for calculations).…”

“…In a recent screening of freshwater ciliates to graze upon viruses, Halteria was found to not only ingest the viral particles, but to also grow and divide when consuming chloroviruses as sole food source ( 100 ). In two marine zooplankton genera, namely, Calanus and Oikopleura , fecal pellets were found to contain infectious viruses, suggesting that some viruses can pass through the grazer gut undigested and remain viable ( 44 , 101 ). Transmission electron microscopy images of fecal pellets and radiolarian food vacuoles from the Ross Sea reported the occurrence of hollow capsids, suggesting at least a partial digestion of viral particles ( 102 ).…”

“…In addition, understanding the breakdown of viral capsids and lipid membranes can shed light on the fate of ingested viruses within predators. This could be through tracking the fate of fluorescent viral particles ( 33 ), monitoring the grazer’s gene expression ( 135 ) and activity of digestive enzymes (e.g., lipases and proteases) ( 136 ), or determining if viruses passing through organisms remain infectious ( 44 , 101 ). Expanding culture-based experiments to different model systems utilizing the techniques discussed above will provide us with necessary information on the strength of the viral sweep, as well as how widespread this mechanism is likely to be.…”