Relative Contribution of Viral Lysis and Grazing to Bacterial Mortality in Tropical Coastal Waters of Peninsular Malaysia

We investigated the relative importance of viral lysis and heterotrophic nanoflagellate (HNF) grazing in bacterial mortality as part of a study to understand the microbial loop functioning in tropical waters of Straits of Malacca and South China Sea above the Sunda Shelf. Bacterial abundance ranged from 0.8 × 10⁶ to 2.9 × 10⁶ cells ml^–1 and bacterial production ranged from 0.7 × 10⁵ to 4.2 × 10⁵ cells ml^–1 hr^–1. HNF and viral abundance ranged from 0.6 × 10³ to 10.1 × 10³ cells ml^–1 and 0.3 × 10⁷ to 1.4 × 10⁷ virus particles ml^–1, respectively. HNF grazing rates were from 1.1 × 10⁴ to 16.8 × 10⁴ cells ml^–1 hr^–1, whereas viral lysis rates ranged from 0.7 × 10⁴ to 3.9 × 10⁴ cells ml^–1 hr^–1. There was predator-prey coupling across the stations with both viral lysis and HNF grazing rates significantly correlated with bacterial production. HNF grazing accounted for 26.1% ± 14.6 of bacterial production, whereas viral lysis was 9.0% ± 3.5 of bacterial production. Bacterial mortality by both HNF and viruses averaged 35.5% ± 13.9, and was outpaced by bacterial production. Other loss factors, such as grazing by ciliates, sedimentation, benthic filter feeders, autolysis, or predatory bacteria, could be important.