Effect of viral infection on sinking rates of Heterosigma akashiwo and its implications for bloom termination

Abstract

Viruses play many important roles in the ecology of marine microbes, including that of mortality agents of phytoplankton. The widespread abundance of viruses in sediments, coupled with observations that some flagellated algae lose motility when infected, prompted this investigation into the fate of infected cells and the propagation of infection throughout a population in situ. We used 2 unrelated viruses that infect the bloom-forming alga Heterosigma akashiwo as model systems. Settling columns were used to determine the impact of viral infection on the vertical movement of H. akashiwo. Within 24 h following infection with HaRNAV 263 or H. akashiwo virus strain OIs1, 20.9 ± 5.8 and 12.2 ± 1.9% of the populations were sinking at rates of 1.25 and 0.73 m d -1 , respectively. By 48 h post-infection approximately 50% of cells infected with either virus had lysed. As sinking cells would encounter decreasing levels of light in the natural environment, we incubated infected cul- tures in complete darkness and compared the length of the lytic cycle to that of cultures incubated under a L:D cycle. Again, approximately 50% of the population lysed by 48 h, regardless of the light conditions or type of virus, and lysis of the entire population was complete by 144 h. Based on these results, we propose that, depending on the characteristics of the water column, there are 3 poten- tial fates for viruses from cells infected in situ. In water with a deep mixed layer (> 8 m), lysis of infected cells would occur within the mixed layer and the infection would be propagated within the euphotic zone. If the water is relatively deep (100s of meters), and the mixed layer shallow (<8 m), cell lysis and viral release would occur below the mixed layer, where viral propagation would not occur. Finally, in shallow waters (10s of meters) lysis of infected cells would occur at the sediment surface, resulting in the accumulation of a high abundance of viruses.