Virally induced mortality of Phaeocystis globosa during two spring blooms in temperate coastal waters

This study reports virally mediated mortality rates of Phaeocystis globosa single cells in the southern North Sea during 2 consecutive spring blooms (2003 and 2004). An adapted dilution method was used to estimate viral lysis and microzooplankton grazing simultaneously. Parallel dilution experiments were performed with 30 kDa ultrafiltrate (virus and grazer-free diluent) and 0.2 m filtered seawater (grazer-free, but virus-containing diluent). Specific viral lysis rates were calculated from the difference in P. globosa growth rates between the 2 dilution series after 24 h incubation under natural conditions. The validity of this method was tested using a culture of P. globosa infected with a known P. globosa virus (PgV). The field data show that virally induced mortality can be a substantial loss factor for P. globosa single cells (up to 0.35 d -1 ), comparable to that due to microzooplankton grazing (up to 0.4 d -1 ). Viral lysis was the major cause of total P. globosa cell lysis. Assuming no loss due to sinking, viral lysis accounted for 5 to 66% of the total mortality of P. globosa single cells. Viral lysis and total putative PgV abundance increased concomitantly with abundance of P. globosa single cells whilst the increase in infective PgV was delayed. This delay may be caused by the formation of transparent exopolymeric particles that are generated when P. globosa colonies disrupt and are known to passively adsorb viruses. Viruses and microzooplankton were shown to be major controlling agents of P. globosa single cells, although their relative significance varied over the course of the bloom and between years.