Viral lysis and grazing loss of bacteria in nutrient- and carbon-manipulated brackish water enclosures

A 3 week enclosure experiment was carried out at the Gulf of Finland, the Baltic Sea. After additions of inorganic nutrients nitrogen (N) + phosphorus (P)] and a carbon source (sucrose), we followed bacterial, viral and heterotrophic nanoflagellate (HNF) abundances, as well as bacterial production and the frequency of bacteria visibly infected with viruses. Furthermore, the decay rate of virus particles was measured three times during the enclosure experiment from the KCN-treated water samples. Bacterial mortality caused by viral lysis was estimated using the decay rates and the fraction of bacteria infected. Nutrient (N + P) additions stimulated phytoplankton growth the chlorophyll (Chl) a concentration increased from <5 g l^-1 up to 19 g l^-1], while sucrose additions increased bacterial production (from 4-6 x 10⁷ l^-1 h^-1). The phytoplankton blooms affected bacterial production only slightly. Bacterial mortality that was explained by viruses ranged from <2% to 13% when estimated from the visibly infected cells, and from 8% to 808% when the decay rates (range 0.052-0.765 h^-1) were used. Assuming a clearance rate of 5 nl flagellate^-1 h^-1, the HNF community could graze 16-135% of total bacterial loss.