The Max Planck Institute for Marine Microbiology has been studying spring algae blooms in the ocean waters surrounding the German island of Helgoland since 2009. In a previous study, researchers observed a group of bacteria called SAR11 to grow rapidly during these blooms, but their abundance decreased by 90% over five days, suggesting predation or viral infections were occurring. To investigate further, researchers aimed to identify the phages infecting SAR11, as phages are viruses that specifically infect bacteria. Using a method that allowed them to track the phage’s genetic material inside the cell, researchers were able to detect phage-infected SAR11 cells and found that nearly 20% of the cells were infected during periods of rapid growth, explaining the low cell numbers. This discovery confirmed that phages were responsible for the decrease in SAR11 population.
Interestingly, researchers also discovered “zombie” cells among the phage-infected SAR11 cells. These cells were in a transitional state between life and death, as they no longer contained ribosomes. This phenomenon was observed in samples collected off Helgoland as well as in samples from the Atlantic, Southern Ocean, and Pacific Ocean, indicating that zombie cells are a global occurrence. The presence of zombie cells in the sea accounts for up to 10% of all cells, broadening our understanding of the viral infection cycle. It is believed that nucleic acids contained in the ribosomes of zombie cells are being broken down and recycled to make new phage DNA. This finding suggests that not only SAR11 bacteria but possibly other bacteria can also be turned into zombies, prompting further investigation into the distribution of zombie cells and their role in the viral infection cycle.
The discovery of zombie cells highlights the significant role of phages in regulating and controlling the SAR11 population, despite their rapid growth rates. SAR11 bacteria play a crucial role in global biogeochemical cycles, including the carbon cycle, and the findings from this study urge a reevaluation of their importance in the ocean ecosystem. This research emphasizes the importance of microbial interactions in the ocean and the impact of phages on marine ecosystems. Understanding the role of phages in controlling bacterial populations expands our knowledge of the intricate dynamics within marine ecosystems and highlights the complexity of microbial interactions in maintaining the balance of oceanic environments. Further research will focus on investigating the distribution of zombie cells in other bacterial populations and exploring their implications in the broader context of viral infection cycles in the ocean.