In the March 9, 2018 column of newspaper columnist Cecil Adams' column The Straight Dope, he speaks about the possible benefits of treating bacterial infections with bacteriophages, viruses that infect bacteria, mentioning in passing their apparently excellent record in killing bacteria in the ocean. He doesn't provide a source for this assertion.
The report didn’t mention bacteriophages....They’re viruses that infect bacteria, sometimes modifying their activity, sometimes killing them outright. Phages are crucial in maintaining the world’s microbial equilibrium; every day, they kill off 40 percent of all bacterial cells in the ocean. What with their proven knowhow at offing microbes, you can understand why we might want to try aiming them at the infectious ones.
- Source: Cecil Adams (March 9, 2018), "Will bacteriophages save us from the antibiotics crisis?", The Straight Dope (emphasis mine)
A forty percent daily mortality rate is a pretty high claim. Is there any truth that approximately 40% of all bacteria in the ocean die every day from viral infection, or that of all the bacteria in the oceans today, only 60% will live to see tomorrow, with the rest being killed by viruses?
DevSolar mentioned a philosophical objection to the idea of bacterial death, as bacteria reproduce by division and neither of the resulting cells can truly be ontologically identified either as its ancestor or a distinct being from said ancestor. That could form the basis of a frame-challenge answer, but I'm seeing the claim as more of a practical usage of the idea of death. For example,
- At Time = T, a single bacterium, the only bacterial inhabitant of a Petri dish, divides into two bacterial cells. Whether you see these two cells as the children of a now-dead parent or as the result of some sci-fi trope where Riker gets duplicated again by another transporter accident is up to you.
- At time = T + 1, one of the resulting bacterial cells is killed (by whatever means imaginable - a virus, blunt force trauma, overdosing on roofies, suicide by
copdoctor, blasting off in a homemade spaceship with faulty o-rings, etc. It doesn't matter, the bacterium is deceased, pushing up daisies, pining for the fjords, etc. It is an ex-bacterium.).
- At time = T + 2, the remaining cell divides.
Thus, we can say that, at Time = T + 1, the bacterial mortality rate in the Petri dish is 50%. One lived, one died. No philosophical discussions on the meaning of identity and death, etc.