Marine plankton are tiny, often microscopic organisms that form the foundation of aquatic food webs. They play a vital role in the Earth's environment by producing about half of the planet's oxygen and acting as carbon sinks, absorbing carbon dioxide from the atmosphere. When plankton die, their decomposing cells release dissolved organic carbon, which can remain in the ocean for thousands of years. Understanding how and when plankton die is key to grasping the dynamics of marine ecosystems. However, studying the death of individual plankton species within a complex community of hundreds of species presents a significant scientific challenge. To tackle this, a team of researchers at Kyoto University focused on a specific process: viral infection and cell lysis, in which a cell's membrane breaks down, releasing its genetic material into the surrounding environment. To measure this process, the researchers grew cultures of two phytoplankton species—diatoms and raphidophytes—in seawater-based medium. They then extracted ribosomal RNA (rRNA) from the samples and used digital PCR to quantify its levels. Because rRNA degrades over time in seawater, the team added a known quantity of "spike-in" ribosomes to the medium and measured how quickly they broke down. This helped them determine a degradation rate constant, which was then used to create a model that accurately assessed the rate of cell lysis. The study, published in the journal MicrobiologyOpen, found that viral infection and subsequent cell lysis significantly increased the production of cell-free rRNA. Specifically, viral infection increased rRNA production by about 46 times, while cell lysis increased it by 302 times compared to noninfected samples, where only minimal rRNA was released. In the case of diatoms, the production of dissolved rRNA peaked before the population density began to decline due to viral infection. This suggests that active cell lysis was already occurring while the population was still growing steadily. From a biogeochemical standpoint, this finding indicates that the release of dissolved organic matter through cell death can happen during the growth phase, not just when populations visibly decline. "Surprisingly, we found that the decline in plankton populations and the process of cell lysis were not happening at the same time," said Hisashi Endo, the study's corresponding author. "Just observing living plankton isn't enough to understand how much dissolved organic carbon they contribute to the ocean." The new method for quantifying this "invisible death" of plankton offers important insights into the flow of materials within marine ecosystems. However, the study did not distinguish between different causes of cell lysis, such as viruses or other environmental factors. In the future, the team plans to explore ways to track the causes and impacts of cell lysis at the species level. Endo hopes this research will help reveal the true complexity of the plankton ecosystem, especially given the diversity of viruses found in seawater.