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Marine Viruses and Climate Change
Transcript of Marine Viruses and Climate Change
95% of all living biomass within the oceans.
The estimated 10^30 viruses in the ocean, if stretched end to end, would span farther than the nearest 60 galaxies. Why are marine viruses important? How do marine viruses affect their hosts? (Panels a and b courtesy of D. Raoult, Centre National de la Recherche Scientifique, Marseille, France. Panels c and d courtesy of W. Wilson, Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine. Panel e reproduced, with permission, from ref. 58.) The role of viruses in marine food webs and biogeochemical cycles. Viral infections are frequently followed by death of the host cells, thus representing an important source of mortality of marine microorganisms.
Virus-mediated mortality of prokaryotes, in both water column and sediments, is often in the range of 10–30% and can reach 100% (Heldal & Bratbak, 1991; Suttle, 1994; Fuhrman & Noble, 1995; Wommack & Colwell, 2000; Corinaldesi et al., 2007a, b).
The integration of viruses into microbial food web models has shown, moreover, that viral lysis of microbial cells enhances the transfer of microbial biomass into the pool of dissolved organic matter (DOM)
These viral-induced alterations of organic matter flows, within microbial food webs, have been termed ‘viral shunt’ (Wilhelm & Suttle, 1999). Prime candidates for marine viral infections: Epidemiological models predict that viral infection rates increase with increasing host cell density (Wiggins & Alexander, 1985) because infection is a direct function of the encounter rate between a pathogen and its host.
Recent studies reported that viral production in deep-sea benthic ecosystems worldwide is extremely high, and that viral infections are responsible for the abatement of 80% of prokaryotic heterotrophic production (Danovaro et al., 2008b). Impact of viruses on photosynthetic organisms: Before the identification of marine viruses as agents of mortality of photoautotrophic organisms, the death of primary producers was ascribed to zooplankton grazing and sedimentation below the photic zone.
To quantify the impact of viruses on photosynthetic organisms, it is essential to determine the extent to which they impose mortality on their host.
Compelling evidence that viruses are involved in algal bloom demise, comes from observations that high proportions (10–50%) of cells were visibly infected (using transmission electron microscope) at the end of blooms of A. anophagefferens (Sieburth et al., 1988; Gastrich et al., 2004), Heterosigma akashiwo (Nagasaki et al., 1994; Lawrence et al., 2002) and Emiliania huxleyi (Bratbak et al., 1993, 1996; Brussaard et al., 1996b). Group Discussion Activity Groups of three, answering questions from the reading to relate to this presentation. Evidence that rising sea-surface temperatures will affect the associated virus communities can be inferred from examining the relationships between viral abundance and temperature for different oceanic regions (Fig. 3a). The strongest temperature effect was observed in temperate-open oceans (ancova, P<0.01), where a temperature increase of only a few degrees was associated with a doubling of viral abundance. Case study 4: Interactive roles of marine viruses on CO2 sequestration and biological carbon pump The gas equilibrium at the ocean–atmosphere interface facilitates the exchange of gases in both directions. Gases such as CO2 escape to the atmosphere when partial pressures of the gas in water are higher than those in the air.
Two main CO2 sequestration processes in the ocean interior are known: (1) the physical pump (or solubility pump) and (2) the biological pump, accounting for about 1/3 and 2/3 of the sequestration, respectively. Physical Pump The physical pump is driven by chemical and physical processes (i.e. cooling and deep water formation) and it maintains a sharp gradient of CO2 between the atmosphere and the deep oceans Biological Pump Photosynthesis, through the conversion of CO2 into biomass and the subsequent sinking of organic particles in the ocean interior lowers the partial pressure of CO2, thus promoting the drawdown of atmospheric CO2.
Much of the carbon ‘fixed’ within the phytoplankton during photosynthesis is converted back to CO2 and released to the atmosphere by the respiration of phytoplankton, bacterioplankton and zooplankton grazing in the mixed surface layers. Case study 7: Potential impact of viruses on cloud formation The ocean contributes over 30% of the atmospheric sulfur budget (Nguyen et al., 1978).
Concerned mostly with atmospheric and ocean concentrations of DMS (dimethyl sulfide) and DMSP (dimethylsulfoniopropionate).
Viruses, by inducing the lysis of algal cells, have been reported to contribute to DMSP release to the dissolved pool where it is rapidly converted to DMS by bacteria possessing DMSP lyase (Malin et al., 1998; Niki et al., 2000). Questions from the reading: What is a relative estimate of the concentration of viruses in the ocean?
Do marine viruses grow at a faster or slower rate than larger organisms?
What type of viruses are the most common in the upper photic zones of subtropical gyres? Why is this important to climate change?