Reduced daytime net growth rate of Synechococcus spp. in the East China sea in summer estimated using a dilution approach

Abstract

Viruses are thought to play a critical role in aquatic nutrient cycling, yet few field experiments have quantified their significance. We conducted fifteen modified dilution experiments in the East China Sea in summer to determine the growth rate and effect of mortality on growth rates of Synechococcus spp. They were measured in a 0.2 μm dilution series in which the abundance of Synechococcus spp. and nanoflagellates were diluted but the viruses were retained. They were also measured in a 30 kDa filtered natural samples in which the abundance of Synechococcus spp., nanoflagellates and viruses were diluted. We found positive relationships between dilution factor and Synechococcus spp. net growth rates. Instantaneous growth rate of Synechococcus spp in 100% whole water generally exceeded rates in other dilution fraction treatments, a result of net growth rates of Synechococcus spp. in undiluted incubations. Throughout the entire study area, growth rate of Synechococcus app. was highly variable and ranged from 0.01 h−1 to 0.17 h−1, with a maximum value appearing in waters with a salinity of around 31 psμ. Most importantly, net growth rates of Synechococcus spp. were suppressed in the 30 kDa dilution series compared to the 0.2 μm dilution series dilution experiments, suggesting that dissolved organic matter available to heterotrophic bacteria and viral lysis contributed to nutrient cycling and made a difference in the growth rates found in the two dilution series.