Viruses rule the waves: a 2017 review of our knowledge thus far

Viruses are the most abundant biological entity in the marine environment (averaging 10⁸ viruses ml^-1). They infect everything from animals to unicellular organisms and in the last decade have been revealed as key players in marine ecosystems; driving bacterial and algal mortality and influence biogeochemical cycles. Authors Middelboe and Brussaard (2017) have produced a review of our viral knowledge thus far (2017) and their function and regulation of aquatic ecosystems.

Viruses are not homogeneously spread throughout the ocean. Availability of inorganic limiting nutrients are important predictors of host and consequently of viral abundance and thus virus-host ratios across temporal and spatial scales.  Also, trophic interactions (e.g host predation) in the microbial food web are now known to influence viral and prokaryote community structure. Understanding these interactions/ environmental influence is key to understanding/ mapping viral distribution.

A key feature of this review is that the authors provide a schematic diagram which gives a breakdown of virus-host interactions. I will summarise it here:

1)    Bacteria defend against phage infection by mutating their surface receptors or enzymatic degradation of incoming phage DNA.

2)    Aggregation/ biofilm life can act as phage defence.

3)    Phage DNA can integrate in to hosts DNA, residing as prophage and can prevent. further infection by similar phages.

4)    Prophage induction may stimulate biofilm production.

5)    Phages can manipulate host gene expression to improve infection efficiency.

6)    Phages interact with their bacterial hosts contribute to shaping the gut microbiome and thus affect symbiotic relationship between gut microbes and their host.

7)    Some coccolithophore (e.g. Emilinania huxleyi) diploid cells undergo viral lysis OR re-emerge as viral resistant haploid cells containing viral RNA (reforming in to diploid via karyogamy).

8)    Giant viruses (NCLDV) infect a large range of photosynthetic protists, thus effecting mortality and diversity of phytoplankton. Influencing the entire marine food web.

Viruses have been shown to shift in response to seasonal variation in host diversity, however persistence of viral genotypes across a 3-year study suggest virus-host co-existence. Investigation of current theories of viral-host interactions are dynamic and current understanding is constantly being challenged. Host defences have always been thought as being costly, creating a trade-off between resistance and fitness. However, recent studies using phytoplankton Prymnesium parvum (Heath et al., 2017) and Emilinania huxleyi (Ruiz et al., 2017) showed no direct cost of resistance, highlighting the complexity of interplay between virus-host co-evolution. Climate change is also predicted to impact viral-host interactions, thus far elevated pCO₂ levels have been shown to impact diversity of E. huxleyi viruses (EhV) (see Highfield et al., 2017) and Maat et al. (2017) demonstrated temperature sensitivity in virus infectivity related to Arctic picophytoplankter Micromonas polaris.

This review paper provides an excellent brief summary for recent advances in our knowledge of viruses and their influence on marine ecosystems, especially in regard to their impacts on primary productivity. This review details many topics, far too many to summarise in 500 words, therefore I have created a small reference list below of the key studies from this review.  

Reference of this paper:

Middelboe, M. and Brussaard, C. (2017). Marine Viruses: Key Players in Marine Ecosystems. Viruses, 9(10), p.302.

Reference’s from particularly interesting points in this review:

Highfield, A.; Joint, I.; Gilbert, J.A.; Crawfurd, K.J. (2017) ; Schroeder, D.C. Change in Emiliania huxleyi virus assemblage diversity but not in host genetic composition during an ocean acidification mesocosm experiment. Viruses, 9. 


Maat, D.S.; Biggs, T.; Evans, C.; van Bleijswijk, J.D.L. (2007); van Der Wel, N.N.; Dutilh, B.E.; Brussaard, C.P.D. Characterization and temperature dependence of arctic micromonas polaris viruses. Viruses, 9, 6–9. 


Haatveit, H.M.; Wessel, Ø.; Markussen, T.; Lund, M.; Thiede, B.; Nyman, I.B.; Braaen, S.; Dahle, M.K.; Rimstad, E. (2017) Viral protein kinetics of piscine orthoreovirus infection in atlantic salmon blood cells. Viruses, 9. 


Middelboe, M.; Glud, R.N. (2006). Viral activity along a trophic gradient in continental margin sediments off central Chile. Mar. Biol. Res., 2, 41–51. 


Rohwer, F.; Thurber, R.V. (2009). Viruses manipulate the marine environment. Nature, 459, 207–212.