Particulate organic matter

We often think of the ocean as a vast, empty expanse, but it is teeming with life. And when that life ends, it doesn't just disappear. Invisible particles, both living and dead, are constantly sinking into the ocean's depths. These are the remnants of surface life, carrying secrets of what once swam and floated, and they are quietly shaping our planet's carbon cycle. This is the story of particulate organic matter, or POM. Unseen fragments of life. They form a critical, ever-present conveyor belt of carbon and nutrients from the sunlit surface to the dark, cold deep ocean. This ceaseless journey influences everything from the smallest microbes to the global climate itself. Particulate organic matter is defined operationally. That means it's defined by how we measure it. Specifically, it's the organic matter that doesn't pass through a fine filter, typically one with pores between about 53 micrometers and 2 millimeters. Think of it as the tiny bits and pieces that make up the fabric of life. It includes everything from microscopic bacteria and phytoplankton to larger fragments of dead organisms, plant material, and even fecal pellets from marine animals. In soils, POM is a vital source of energy and nutrients for decomposers and plants. It helps build healthy soil structure. But it's also sensitive to how we manage the land. In aquatic environments, POM is food. It sustains the organisms that live there. It can also make the water cloudy, a phenomenon called turbidity, which can limit how deep sunlight penetrates, impacting the growth of plants and algae. But the most profound role of POM is in the ocean. The sinking of these particles is a key part of what scientists call the biological carbon pump. It's a process that moves carbon from the surface, where it's captured from the atmosphere, down to the deep ocean. There, it can be sequestered, locked away for millennia. This sinking flux is what sustains much of the life in the deep sea. The composition of these particles – what they're made of – and their size and density determine how fast they sink. This sinking speed is crucial for how much carbon is exported and how nutrients are cycled. Changes in how POM moves and behaves can have massive implications. They can affect marine ecosystems and even the amount of carbon dioxide in our atmosphere. Particulate organic carbon, or POC, is a closely related term. It refers specifically to the carbon within this particulate organic matter. While POM is the total mass of the particles, POC is just the carbon component. Typically, there's about twice as much POM as there is POC, because POM also includes other elements like nitrogen, oxygen, and hydrogen. Sometimes POM is also called suspended organic matter, or macroorganic matter. When we look at soil samples, this fraction includes partially decomposed detritus, plant material, and pollen. POM is readily decomposable. It provides energy for soil organisms and nutrients for plants. In water, it can contribute to turbidity, limiting sunlight. It also improves soil structure, helping with water infiltration and aeration, and making soil more resistant to erosion. Soil management practices, like tilling the land or adding compost, can significantly alter the amount of POM in soils and in the water that runs off. In freshwater systems, eroded soil rich in POM can enter rivers and lakes. This can lead to a rapid buildup of organic matter, potentially causing eutrophication – a process where excessive nutrients lead to algal blooms and oxygen depletion. Suspended organic matter can also act as a carrier for pollutants, bringing fecal bacteria, toxic metals, or harmful organic compounds into water bodies. Even in sewage systems, particulate organic carbon affects the viscosity and toxicity of the waste. It can build up and increase corrosion in infrastructure, turning sewage into a thick sludge. But it is in the marine environment where POM truly plays a starring role on a global scale. Life and particulate organic matter in the ocean have fundamentally shaped our planet. On the most basic level, POM includes both living and non-living matter of biological origin, larger than about 0.2 micrometers. This organic matter regulates global marine biogeochemical cycles. It's been involved in events like the Great Oxidation Event in Earth's early history and continues to be vital for sequestering atmospheric carbon dioxide in the deep ocean. Understanding POM in the ocean is fundamental to understanding marine ecosystems, from food web dynamics to global cycles. Measuring POM in the ocean is complex. Optical particle measurements are becoming increasingly important. These techniques, used from ships or autonomous platforms, can give us much better spatial and temporal coverage of particles in the ocean's mid-waters, the mesopelagic zone, compared to traditional methods like sediment traps. Ocean primary production, the creation of organic matter by photosynthesis, can be divided into new production and regenerated production. The total new production in the ocean roughly equals the sinking flux of particulate organic matter to the deep ocean. This amounts to about 4 billion tons of carbon annually. When these organic particles sink, they form an important part of the oceanic biological pump. This pump impacts carbon sequestration and the resupply of nutrients to the mid-ocean. As particles sink, they are remineralized, broken down by bacteria. This causes the sinking flux of organic matter to decrease with depth. Marine snow, a common term for these sinking particles, varies in shape, size, and character. It can be individual cells, fecal pellets, or larger aggregates. Most of it is rapidly colonized and consumed by bacteria, contributing to that decrease in flux with depth. The sinking velocity of these particles is crucial. It can range from floating towards the surface to several kilometers per day, as seen with fast-sinking salp fecal pellets. Stokes' law, combined with approximations, suggests sinking velocity increases with excess density and the square of particle diameter. For a long time, scientists thought size was the main predictor. But that's not always true. Particles made by different plankton communities, or under varying environmental conditions, can have very different densities and shapes. Packaging and porosity also matter. Adding dense materials like diatom shells can increase sinking speed. But sometimes, adding dense mineral particles can lead to smaller, more tightly packed aggregates that sink slower because of their reduced size. Particles rich in mucus might float despite their size, while those containing oil or plastic might sink rapidly. In natural environments, particles are formed in diverse ways, by different organisms, and under varying conditions. This makes a universal conversion of size to sinking velocity impractical. Along with dissolved organic matter, POM drives the lower aquatic food web. It provides energy in the form of carbohydrates and other polymers that can be broken down. POM in water bodies comes from various sources: terrestrial inputs like leaf litter, submerged aquatic vegetation, or algae produced right there in the water. Each source has its own chemical makeup, affecting how easily it can be used by the food web. Algal-derived POM is generally considered the most easily degradable. But there's growing evidence that POM from land can supplement the diets of organisms like zooplankton, especially when primary production is low. The dynamics of particulate organic carbon are central to the marine carbon cycle. POC is the link between surface production, the deep ocean, and the seafloor sediments. How quickly POC is degraded in the dark ocean can directly impact atmospheric carbon dioxide levels. As illustrated in many diagrams, phytoplankton in the upper ocean capture carbon dioxide using sunlight. This creates POC. This POC is then processed by marine microorganisms and zooplankton, forming aggregates – marine snow – which are exported to deeper waters by sinking and by the vertical migrations of animals. The biological carbon pump is the collection of processes involved in the production, sinking, and remineralization of organic carbon in the ocean. Photosynthesis captures inorganic carbon, turning it into biomass. When this biomass sinks, it fuels the metabolism of deep-sea organisms. Zooplankton play a critical role. They ingest and fragment particles, produce fast-sinking fecal material, and actively migrate vertically, all influencing the flux of particles. This exported organic carbon is not just food for deep-sea life. It's estimated that 5 to 20 billion tons of carbon are transported to the deep ocean each year. A portion of this, perhaps 0.2 to 0.5 billion tons, is sequestered for thousands of years. The biological carbon pump is immense, comparable in scale to current carbon emissions from fossil fuels. Any changes to its magnitude, perhaps due to a warming world, could have direct implications for deep-sea organisms and for atmospheric carbon dioxide concentrations. The efficiency of this pump, how much carbon it stores relative to how much is produced, is determined by the amount of organic matter exported and the rate at which it's broken down in the mid-ocean. Particle size and composition are key. They dictate how fast a particle sinks, how much material it carries, and which organisms can utilize it. Sinking particles can be phytoplankton, zooplankton, detritus, fecal pellets, or a mix. They range in size from a few micrometers to several centimeters. Particles larger than half a millimeter are often referred to as...