Tuesday, 4 February 2014

Molecular and microscopic evidence for viruses in marine copepods

Viruses are ubiquitous in aquatic systems and are infectious agents of many organisms from bacteria up to fish and mammals, yet despite their huge ecological significance very few studies have been conducted on the effects of viruses on aquatic metazoans. Copepods are the most dominant members of such mesozooplankton communities and are responsible for biogeochemical cycling and fundamental to marine food webs. Whilst previous work has focussed on the ecology and population dynamics of such organisms, causes of mortality are still poorly understood.

In the lecture Colin delivered on ‘Diseases of mollusks, crustaceans and fish’ it was stated Drake and Dobbs (2005) had found no negative effects of viruses on the fecundity or survival of copepod Acartia tonsa. This, as mentioned within the lecture, seems rather strange as viruses are central to controlling algal (phytoplankton) blooms so could be predicted to act in a similar manner on zooplankton. Consistent with viral disease in phytoplankton, mortality rates of copepods coincide with population peaks, and with ~35% of total copepod mortality unexplained by predation, another underlying cause of mortality must be present.
Using molecular methods the presence of viruses within A. tonsa and Labidocera aestiva copepods was assessed.

Using metagenomic sequencing on extracted and purified viruses from the sampled copepod species, the presense of circo-like viruses was detected, named LaCopCV and AtCopCV in L. aestiva and A. tonsa respectively. Whilst these viral genomes shared characteristics with known circoviruses; namely a small genome, a stem-loop with a conserved nonanucleotide motif and two nonoverlapping ORF’s coding for replication initiatior (Rep) and capsid proteins, the genomic architecture varied slightly. Unlike the bi-directional orientation of the 2 ORF’s in known circoviruses, the ORF’s in copepod circo-like virus genomes are mono-directional, a characteristic previously observed in environmental viral communities and fecal metagenomes. Additionally, the nonanucleotide motif on the stem-loop in copepod viruses exhibited variations to the normal conserved motif present in circoviruses.

Using a phylogenetic tree based on Rep amino acid sequences, it was shown that both LaCopCV and AtCopCV are highly divergent from recognised and identified circoviruses, and are, unsurprisingly, more closely related to environmental assembled circo-like virus sequences.

Using qPCR of the capsid gene, a high prevalence of LaCopCV among wild L. aestiva populations was observed in the order of magnitude similar to that of white spot syndrome virus in postlarval crustaceans, and was found to be actively replicating, with low levels of transcription detected using a modified qRT-PCR assay. AtCopCV was also detected, though only in samples from 7 months of the year principally in spring and autumn during times of population changes. Despite being suggestive of lower prevalence in comparison to LaCopCV, the presence of AtCopCV coninciding with population peaks and previously stated nonpredatory mortality peaks could suggest that AtCopCV has a greater influence in mortality during zooplankton blooms and LaCopCV accounts for a steady rate of mortality throughout the year. This is supported by the frequent identification of LaCopCV in sediments from Tampa Bay, perhaps signifying a potential reservoir for the virus.

Contrary to the study by Drake and Dobss (2005) the results of this study provide evidence of not only the presence, but the prevalence and active replication of previously undocumented viruses within two numerically dominant species of natural zooplankton populations. Though further study is required to understand the pathology of the viruses and the larger ecological implications they may have on marine food webs, the study is an important breakthrough in understanding potential roles of viruses in the nonpredatory mortality of zooplankton. Whilst both viruses found were limited to and only detected in their respective zooplankton species, it’s not to say that other circo-like viruses don’t propagate in other zooplankton species. With the demonstrated capacity to test this, it is an area which warrants future study. 

Dunlap D., Fei Fan Ng T., Rosario K., Barbosa J., Greco A., Breitbart M. and Hewson I. (2012) Molecular and microscopic evidence of viruses in marine copepods. Proceedings of the National Academy of Sciences, 11(4), 1375-1380

Drake L. and Dobbs F. (2005) Do viruses affect fecundity and survival of the copepod Acartia tonsa Dana? Journal of Plankton Research, 27(2), 167-174


  1. Whilst evidence of the prevalence of viral replication in copepod tissue is blatant, exposing these circo-like viruses as a contributor to zooplankton mortality seems less straightforward. Mesocosms have been used examine phytoplankton mortality, but you could never exclude viruses from a control mesocosm. Another option is controlled laboratory investigations, but they will likely not reflect the true mortality rates in the ocean. Why do so many organisms insist on being too small to see?

  2. I agree, their size and ubiquitous nature does make them incredibly hard to study, and does leave a lot of uncertainty. Whilst sampling technique does make it difficult to truly assess whether circo-like viruses to contribute to mortality, i think there is a lot of reason to believe they do. If they are present, and replicating the viruses are obviously active, and most active viruses usually infer some form of structural or biochemical alteration to the host cells, often resulting in damage. Obviously, as previously mentioned, without knowing the pathology of the virus it cannot be said with 100% confidence that these circo-like viruses do cause damage, however i think it is very likely.

  3. This is a really nice gateway study. Does this paper mention any other factors other than predation? It seems to me like they're ruling out a lot of other things which could be associated with population crashes, such as competition and food availability. Also, I'm not sure if this is a moot point, but could the viruses that seem to be associated with these copepods in fact be associated with other micro-organisms found with or on copepods? such as bacteria. It'll be interesting to find out what the actual association is before assuming they interact with one another on a significant level.