SAR11 Decline Unveiled With Zombie Cells and Phage Attacks

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.

“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: Brüwer 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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