Tripartite interaction between Emiliania huxleyi RCC6946, BAM7, and EhVM1
Algal blooms are hotspots for diverse microbial interactions affecting bloom fate. Viral infections of these blooms recycle >25% of oceanic fixed carbon through a process termed the viral shunt. However, while blooms collapse synchronously, only 10-30% of cells show active viral infection, suggesting additional mortality agents. Using microbial communities from natural coccolithophore blooms, we discovered that bacteria significantly enhanced virus-mediated algal mortality. We quantified this algal-viral-bacterial interaction with two novel metrics: the Synergy Index (SI) measuring killing interaction strength, and the Benefit Index (BI) assessing the increase in bacterial fitness. These measurements identified Alteromonas bacteria as key drivers of viral-bacterial coinfection: they exhibited chemoattraction towards infected algal cells leading to their enrichment in particles derived from bloom demise. This viral-bacterial synergy represents a previously unrecognized mortality agent controlling bloom termination, modulating the balance between the carbon-recycling viral shunt and the carbon-exporting viral shuttle pathways, thus significantly influencing global carbon cycling.