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.

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 cop doctor, 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.

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    Generally, the "life" of bacteria is considered the time until their next division... which is anything between a couple of minutes and about 24 hours. So, the answer to "how many bacteria in the ocean today will live to see tomorrow?" is, basically, "none". ;-) – DevSolar Sep 30 '20 at 12:35

According to Bacteriophage Distributions and Temporal Variability in the Ocean’s Interior mBio Nov/Dec 2017 Volume 8, e01903-17:

Phages have been shown to kill hosts at rates of up to 20 to 40% of the total population per day, potentially strongly impacting bacterioplankton populations (7,8).

Where reference 7 is Ecology of prokaryotic viruses. FEMS Microbiol Rev 28:127–181 and reference 8 is Are viruses driving microbial diversification and diversity? Environ Microbiol 6:1–11

  • Based on the quote in the answer, I'd say the quote in the question, as written, is questionable at best. This says "up to 20 to 40%", which means it's almost certainly not going to be 40% every day. It's also not clear what "the total population" refers to here, but it seems like it's referring to controlled tests rather than observing the ocean as a whole (which probably won't be possible with particularly high certainty anyway). So rather than "40% every day", it's probably more like "possibly up to 40% on some days, but also possibly just a fraction of that". – NotThatGuy Sep 30 '20 at 20:08
  • -1 Because this only speaks to the fatality rate of phages, not the fatality rate of the entire population of ocean bacteria, as in the title. – tuskiomi Oct 13 '20 at 17:22
  • @tuskiomi it speaks of the fatality rate of the hosts, not the phages. – DavePhD Oct 13 '20 at 19:00
  • @DavePhD my mistake. Still, to say that all ocean bacteria is a host to phages would be falsidical. – tuskiomi Oct 14 '20 at 2:11
  • @tuskiomi reference 7 at page 162 says "...average phage-induced mortality of prokaryotes is 25% in oceanic and 58% in coastal waters suggests that the strongest impact of viral lysis on bacterioplankton should occur in coastal systems. " – DavePhD Oct 14 '20 at 12:15

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