Viral lysis

Some is released into seawater as an extracellular exudate or as a result of viral lysis and from sloppy feeding by the grazers. Consumption by grazers entrains the organic matter in the marine food web as shown in Figure 23.2. Cells that are not consumed or lysed by a virus can sink or be otherwise carried out of the euphotic zone by currents. 

Sloppy feeding by grazers also leads to lysis of microbial and eukaryotic cell membranes with a similar outcome to viral lysis, i.e., conversion of POM into DOM as the cellular matrix is released into seawater. 

Given the relatively small contribution of terrestrial organic matter to seawater, most of the DOM in the ocean is perforce of marine origin. Most of this DOM has as its ultimate source biomolecules released into seawater by excretions and exudations from microbes, plants, and animals. Equally important is the role of viral lysis in which cell membranes are ruptured, causing the DOM within the cells to be spilled into seawater. A similar effect can result from sloppy feeding by herbivorous zooplankton. 

Bratbak, G., A. Jacobsen, and M. Heldal. 1998. Viral lysis of Pbaeocystis pouchetii and bacterial secondary production. Aquatic Microbial Ecology 16 11—16. 

M. I. Gutierrez, J. G. Judde, I. T. Magrath, and K. G. Bhatia, Switching viral latency to viral lysis a novel therapeutic approach for Epstein-Barr virus-associated neoplasia, Cancer Res. 56 969 (1996). 

Table 3 Types of P. globosa free-living cells used in physiology, grazing and viral lysis experiments... 

On the contrary, lysis and particularly viral lysis could be a major process in these bioassays. In cultures of natural marine bacteria inoculated into 0.2 pm filtered sea water, Wilcox and Furhman (1994) reported that virus abundance increased after few days of bacterial growth. The high abundance and production of bacteria in the HFe bioassay could enhance viral activity and consequently increase specific mortality rates. In agreement with this, during the mesoscale Fe fertilization EISENEX, a higher viral infection of bacterioplankton was estimated in the Fe-enriched patch (Weinbauer et al. 2003). Lysis could be of significance in Fe-limited ecosystems, as Fe released via lysis can be highly bioavailable (Poorvin et al. 2004). 

Gobler CJ, Hutchins DA, Fisher NS, Cosper EM, Sanudo-Wilhelmy SA (1997) Release and bioavailability of C, N, P, Fe, and Se following viral lysis of a marine chryso-phyte. Limnol Oceanogr 42 1492-1504... 

Fig. 7 Modeled abundance of Phaeocystis globosa cells and mortality rates of P. globosa during a mesocosm study (Ruardij et al. 2005). (a) A model run with colonies, and (b) a run without colonies present. Viral lysis is represented by the outline of the black area, microzooplankton grazing by...

Impact of viral lysis of Phaeocystis on the microbial food web and element cycling... 

The theoretical models considering the influence of (algal) viruses on the carbon cycle that exist to date are steady-state models assuming a fixed percentage of the algal population dying due to viral lysis. A bloom of Phaeocystis in, for example, temperate eutrophic coastal waters is, however, clearly not a steady-state situation. Based on the ecosystem model by Ruardij et al. (2005), we established a carbon budget for the main players... 

Fig. 8 Simplified representation of a pelagic food web used to calculate the C budget during a Phaeocystis globosa bloom as presented in Table 1. The release of cellular carbon due to viral lysis as sources of DOC, as well as automortality was only modeled for P. globosa. Excretion of DOC due to photosynthetic release was taken into account for all phytoplankton groups (Phytopl. Phaeocystis and other algae). Micrograzers (Micropl.) include heterotro-phic nanoflagellates (HNF) and ciliates. Export was considered a negligible loss...

The daily C fluxes (pg C L-1 d-1 ) originate from an ecosystem model by Ruardij et al. (2005) and are averaged values over a period of 36 days. The standard run represents the situation as was observed during the mesocosm experiment, with both P. globosa single cells and colonies present. Viral lysis of phytoplankton is specific for P. globosa. Viral lysis of bacteria is a second order density-dependent mortality. Respiration and high refractory DOC were modeled but are not included in the table... 

Iron is a nutritive trace element whose role as a limiting agent for algal growth has been demonstrated in areas where certain Phaeocystis species (e.g., P. antarctica) are also commonly found. Viral lysis will affect the absolute concentration of iron that is potentially available for biological requirement, but may also directly affect the spe-ciation and bioavailability as iron is mostly com-plexed with organic ligands and colloids. 

---