Evidence for Viral Infection in the Copepods Labidocera aestiva and Acartia tonsa in Tampa Bay, Florida

Mesozooplankton are of critical importance to marine food webs by transferring energy from the microbial food web to higher trophic levels and depositing energy to the deeper ocean layers through fecal deposition. While decades of research have shown that viruses have significant impacts in the oceans, and infect a wide range of organisms from bacteria to whales, there is still little known about the impacts of viruses on the mesozooplankton community. As copepods are the most abundant mesozooplankton group, this study sought to characterize the viruses present in natural populations of the calanoid copepods Acartia tonsa and Labidocera aestiva in Tampa Bay, Florida. Viral metagenomics revealed two virus genomes, named Acartia tonsa copepod circovirus (AcCopCV) and Labidocera aestiva copepod circovirus (LaCopCV), which were discovered in their respective copepod species. Both viruses show amino-acid similarities to known circoviruses, and phylogenetic and genomic analyses suggest they may be divergent members of the Circoviridae family. LaCopCV was found to be extremely prevalent in the L. aestiva population, with up to 100% of individuals infected. High viral loads for LaCopCV were observed by quantitative PCR, with an average viral load of 1.3x105 copies per individual. In addition, transcription of the LaCopCV replication gene was detected in L. aestiva, demonstrating active viral replication. AcCopCV could be detected sporadically in A. tonsa populations throughout the year. The circoviruses were specific to their respective hosts, and were not detected in the other copepod species or surrounding seawater. Virus-like particles were observed in A. tonsa and L. aestiva under transmission electron microscopy, demonstrating that viruses were actively proliferating in copepod connective tissue, as opposed to gut tissue, parasites, or symbionts. Preliminary results from in-situ hybridization show that the AcCopCV genome can be detected in A. tonsa tissue, linking the discovered genomes to virus propagation in copepod tissue. This is the first study describing viruses in copepods, as well as the first discovery of circoviruses infecting marine organisms. These results suggest that viruses impact marine copepod populations, necessitating further studies to determine the ecological impacts of viruses on the mesozooplankton community