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SAR11 Decline Unveiled With Zombie Cells and Phage Attacks

Sunset over the island of Helgoland in the German Bight, where the researchers from the Max Planck Institute for Marine Microbiology obtained their samples.
Credit: Jan Brüwer/Max Planck Institute for Marine Microbiology.
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Mar­ine mi­crobes con­trol the flux of mat­ter and en­ergy es­sen­tial for life in the oceans. Among them, the bac­terial group SAR11 ac­counts for about a third of all the bac­teria found in sur­face ocean wa­ters. A study by re­search­ers from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many, now re­veals that at times nearly 20% of SAR11 cells are in­fec­ted by vir­uses, sig­ni­fic­antly re­du­cing total cell num­bers. The vir­uses can also trans­form these once thriv­ing bac­teria into zom­bies, a phe­nomenon ob­served for the first time and wide­spread in the oceans.

The ocean wa­ters sur­round­ing the Ger­man is­land of Hel­go­land provide an ideal set­ting to study spring al­gae blooms, a fo­cus of re­search at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy since 2009. In a previous study, the Max Planck scientists observed a group of bacteria called SAR11 to grow particularly fast during these blooms. However, des­pite their high growth rates, the abund­ance of SAR11 de­creased by roughly 90% over five days. This sug­ges­ted that the cells were quickly decim­ated by pred­at­ors and/​or viral in­fec­tions. Now, the Max Planck re­search­ers in­vest­ig­ated what ex­actly lies be­hind this phe­nomenon.

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Find­ing the phages in­fect­ing SAR11

“We wanted to find out if the low num­bers of SAR11 were caused by phages, that is vir­uses that spe­cific­ally in­fect bac­teria”, ex­plains Jan Brüwer, who con­duc­ted the study as part of his doc­toral thesis. “An­swer­ing this seem­ingly simple ques­tion was meth­od­o­lo­gic­ally very chal­len­ging”.

How does phage in­fec­tion work? Phages in­fect bac­teria by in­tro­du­cing their ge­netic ma­ter­ial into them. Once there, it rep­lic­ates, and util­izes the bac­terial ri­bosomes to pro­duce the pro­teins it needs. Re­search­ers from Bre­men used a tech­no­logy that en­abled them to “fol­low” the phage’s ge­netic ma­ter­ial in­side the cell. “We can stain the spe­cific phage genes and then see them un­der the mi­cro­scope. Since we can also stain the ge­netic ma­ter­ial of SAR11, we can sim­ul­tan­eously de­tect phage-in­fec­ted SAR11 cells”, ex­plains Jan Brüwer.

While this might seem straight­for­ward, the low bright­ness and small size of the phage genes made it chal­len­ging for re­searches to de­tect them. Non­ethe­less, thou­sands of mi­cro­scope im­ages were suc­cess­fully ana­lyzed, bring­ing some ex­cit­ing news.

“We saw that SAR11 bac­teria are un­der massive at­tack by phages”, says Jan Brüwer. “Dur­ing peri­ods of rapid growth, such as those as­so­ci­ated with spring al­gae blooms, nearly 20% of the cells were in­fec­ted, which ex­plains the low cell num­bers. So, phages are the miss­ing link ex­plain­ing this mys­tery.”

Brüwer and his col­leagues hy­po­thes­ize that not only SAR11 bac­teria, but also other bac­teria, can be turned into zom­bies. Thus, they want to fur­ther in­vest­ig­ate the dis­tri­bu­tion of zom­bie cells and their role in the viral in­fec­tion cycle.

“This new find­ing proves that the SAR11 pop­u­la­tion, des­pite di­vid­ing so fast, is massively con­trolled and reg­u­lated by phages”, stresses Brüwer. “SAR11 is very im­port­ant for global biogeo­chem­ical cycles, in­clud­ing the car­bon cycle, there­fore their role in the ocean must be re­defined. Our work high­lights the role of phages in the mar­ine eco­sys­tem and the im­port­ance of mi­cro­bial in­ter­ac­tions in the ocean”.

Reference: Bwer JD, Sidhu C, Zhao Y, et al. Globally occurring pelagiphage infections create ribosome-deprived cells. Nat Commun. 2024;15(1):3715. doi: 10.1038/s41467-024-48172-w

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