Rapid isothermal amplification of diatom rbcL from eDNA and eRNA reveals their abundance and photosynthetic physiology

The ocean is a vast, shifting landscape where the smallest organisms drive the largest systems. Among these, diatoms are microscopic algae that form the foundation of marine food webs, capturing sunlight and converting it into energy that sustains life from the surface to the deep. For decades, scientists have tried to understand how many of these tiny plants exist at any given moment and how actively they are growing. Traditional methods have relied on looking at the ocean from space to measure green pigments or taking water samples to count cells under a microscope. While useful, these approaches often lack the speed and detail needed to see rapid changes in the water, leaving gaps in our understanding of how these critical organisms respond to their environment.

To fill this gap, researchers have turned to the genetic material floating in the water itself. Every organism sheds bits of its DNA, the instruction manual for life, and its RNA, the active copy of those instructions that cells use to build proteins and carry out daily functions. By analyzing this environmental genetic material, scientists can detect not just who is present, but what they are doing. A new study focuses on a specific gene found in diatoms that is essential for photosynthesis, the process by which they turn light into food. The researchers developed a way to find and count this gene quickly, without needing the complex, high-heat equipment usually required for such tasks.

The team created a test that works at a steady, low temperature, allowing it to produce results in less than fifteen minutes. This method, known as recombinase polymerase amplification, acts like a molecular photocopier that can make millions of copies of a specific genetic sequence from a tiny water sample. The researchers tested this tool on a wide variety of diatom types to ensure it could recognize them all accurately. They then took it to the Bay of Naples, Italy, where they collected water samples over time and compared their new genetic measurements against traditional counts of cells, satellite images of ocean color, and direct measurements of how much carbon the diatoms were fixing.

The results showed that the new test could track the number of diatoms in the water across a massive range, from very sparse populations to dense blooms, matching the counts seen under a microscope. However, the genetic data offered a deeper layer of insight. By looking at the DNA, which represents the total number of organisms, and the RNA, which represents the active work being done by those organisms, the researchers could see how the diatoms changed with the seasons. In the winter, when light was scarce and fewer diatoms were present, the remaining cells contained more copies of the photosynthesis gene and more green pigment. This suggests that the diatoms were boosting their internal machinery to capture as much light as possible, even though their overall numbers had dropped.

Furthermore, the amount of active genetic material, or RNA, found in the water directly matched the rate at which the diatoms were fixing carbon. This connection allowed the researchers to identify specific groups of diatoms that were working harder than others, revealing differences in their metabolic activity that standard counts would miss. The study demonstrates that this rapid, low-temperature genetic test is a reliable way to monitor both the abundance and the physiological state of diatoms in the ocean. It offers a practical tool for observing marine life in real time, providing a clearer picture of how these vital organisms function and adapt to the changing conditions of the sea.