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Notes on “Viruses and nutrient cycles in the sea” by Wilhelm and Suttle (1999)

“The pool of viruses in the ocean is dynamic because viruses in surface waters are rapidly destroyed or dam- aged by sunlight as well as other factors.”

This means that the effect of viruses may be more important starting below the euphotic zone.

Question: How do viruses replicate?

“The flux of some DOC through the microbial loop in marine waters is rapid, and heterotrophic bacterial production is probably often limited by the flux of labile DOC. Consequently, the supply and removal of DOC are tightly coupled. The relative rates of formation of different carbon pools is thus important for analyzing carbon budgets in aquatic systems. Virus-mediated cell lysis alters these budgets by diverting carbon from the POC pool to the DOC pool.”

“These data demonstrate that viral lysis of phytoplankton shifts organic carbon from phytoplankton to heterotrophic bacteria. Similar evidence from Middelboe et al. (1996) showed that viral lysis of heterotrophic bacteria increased DOC uptake by nonhost bacteria by 72%. The addition of viruses, however, led to a 66% decrease in growth efficiency (i.e., the ratio of biomass produced to substrate utilized) of the nonhost bacteria, reflecting the increased energy requirements needed to assimilate nutrients from the complex matrix of lysis products. To generate this energy, the bacteria had to respire more carbon, thus converting less into bacterial biomass.”

“The direct effects of viral lysis on the transfer of carbon through the food web are difficult to measure but can be modeled. Fuhrman (1992) approached the problem of the impact of viruses on DOC cycling in aquatic systems by contrasting two models of carbon flux. The first model assumed that all bacterial mortality was due to grazing by zooplankton, whereas the second model assumed an equal distribution of mortality between grazers and viruses. From these models, Fuhrman deduced that the presence of viruses led to a 27% increase in bacterial production and carbon mineralization rates. [...] Overall, Fuhrman suggested, viral lysis leads to an increase in bacterial production but a decrease in the transfer of carbon to higher trophic levels. The experimental measurements of Middelboe et al. (1996) and Gobler et al. (1997) are consis- tent with Fuhrman’s conclusion that viral lysis leads to enhanced bacterial production.”

“We have modified the static food web model ofJumars et al. (1989) to account for the influence of viral lysis.”

“Heterotrophic bacteria contain lower C:N and C:P ratios than phytoplankton. Gobler et al. (1997) suggested that if bacteria obtained all of their nutrients from phytoplankton lysis products, then the bacteria would need to obtain additional nitrogen and phosphorus from other sources to satisfy their requirements for these nutrients. However, bacteria do not convert all of the carbon they assimilate into biomass. A significant amount of this carbon is converted into energy (by respiration) to drive cellular processes.”