Plankton - Ecological Processes and Biogeochemical Roles

Ecological Processes Involving Plankton Primary Production and the Foundation of Ocean Life Plankton—the microscopic organisms drifting through ocean waters—are responsible for the majority of the ocean's primary production. These organisms, particularly phytoplankton (photosynthetic plankton), use sunlight to fix carbon dioxide into organic compounds, a process called photosynthesis. In doing so, they release oxygen as a byproduct. This is fundamentally important: approximately half of Earth's atmospheric oxygen comes from ocean phytoplankton. When you breathe, you're partially breathing oxygen produced by these tiny marine organisms. The image above shows global chlorophyll distribution—the green pigment that phytoplankton use for photosynthesis. The concentration of chlorophyll indicates where phytoplankton are most abundant and productive, revealing that the ocean's primary production is not evenly distributed. Coastal areas and certain open ocean regions show the highest productivity. Carbon Sequestration: The Biological Pump One of the most important ecological processes in the ocean is the biological pump—a mechanism that transports carbon from the atmosphere and surface ocean down to the deep sea, effectively locking it away for centuries or longer. Here's how it works: Phytoplankton absorb carbon dioxide from surface waters When phytoplankton die, or when zooplankton (small animals that eat plankton) produce fecal pellets, this organic matter sinks toward the seafloor This sinking material—dead cells, fecal pellets, and clumpy aggregates called marine snow—carries carbon downward In the deep ocean, this carbon is largely isolated from the atmosphere This process makes the oceans the largest carbon sink on Earth. Without the biological pump, atmospheric carbon dioxide levels would be significantly higher. The image shows the remarkable diversity of plankton involved in this process—from tiny bacteria to larger zooplankton—all playing roles in transferring carbon through the water column. <extrainfo> Temperature and Acidification Effects Rising seawater temperatures can alter how efficiently the biological pump works. Additionally, ocean acidification—caused by increased carbon dioxide absorption—may cause Antarctic phytoplankton to become smaller and less effective at storing carbon as this century progresses. These changes could reduce the ocean's capacity to sequester carbon. </extrainfo> The Microbial Loop: Recycling Nutrients at the Small Scale While the biological pump exports carbon downward, the microbial loop recycles organic matter within surface waters. This process is critical for maintaining nutrient availability and supporting continued plankton growth. The microbial loop works like this: Phytoplankton photosynthesize and produce dissolved organic matter (DOM)—organic compounds released directly into the water Heterotrophic bacteria consume this dissolved organic matter Small zooplankton (particularly protozoa) graze on the bacteria When zooplankton excrete, they release dissolved organic matter back into the water, and the cycle continues This recycling is efficient—it allows nutrients to be used many times before eventually sinking to the deep ocean. The microbial loop is particularly important in nutrient-limited regions where recycling of scarce nutrients is essential for phytoplankton growth. The Viral Shunt: An Alternative Recycling Pathway Not all phytoplankton reach zooplankton mouths. Viruses—specifically bacteriophages and viruses that infect phytoplankton—dramatically alter nutrient cycling through a process called the viral shunt. When viruses infect and lyse (burst open) phytoplankton cells, they release the cell contents directly into the water as dissolved organic matter. This viral lysis diverts carbon and nutrients away from the traditional food web that would feed zooplankton, instead directing them into the microbial loop where bacteria consume them. The viral shunt is ecologically significant because: It prevents carbon from reaching higher trophic levels It redirects energy toward bacterial growth instead It affects how quickly carbon is recycled versus exported to depth The Mycoloop: Fungal Intermediaries in Nutrient Transfer Many large phytoplankton are defended by cell walls or toxic compounds that make them inedible to zooplankton. The mycoloop describes how fungal parasites called chytrids overcome this problem. Chytrids infect large, otherwise indigestible phytoplankton. This infection causes the algal cells to fragment or produces zoospores (motile spores) that are small enough for zooplankton to consume. In essence, the fungus acts as an intermediary, breaking down defended phytoplankton into edible pieces. This process transfers nutrients and energy from large phytoplankton (which would otherwise be unavailable to zooplankton) to higher trophic levels through fungal intermediaries. The mycoloop is particularly important where large, defended phytoplankton dominate, such as certain diatoms. Marine Snow: Sinking Particles and the Deep-Sea Food Web Marine snow consists of organic particles—including dead phytoplankton, zooplankton fecal pellets, and aggregates of mucus and cellular debris—that sink from productive surface waters. Despite its simple-sounding name, marine snow is a major food source for deep-sea organisms. The characteristics of marine snow vary depending on feeding conditions: When zooplankton feed at low rates, they absorb food very efficiently (high absorption efficiency), producing small, compact, dense fecal pellets that sink quickly When zooplankton feed at high rates, they absorb food less efficiently (lower absorption efficiency), producing larger, less dense pellets that sink more slowly and may be more nutritious to deep-sea organisms This image shows the diversity of zooplankton that produce these particles—from copepods to larval fish to gelatinous organisms. Absorption Efficiency and Fecal Pellet Characteristics Absorption efficiency is the proportion of ingested food that a planktonic organism actually assimilates (uses for energy and growth), rather than passing through unchanged. This is not constant—it depends on several factors: Feeding rate: High feeding rates reduce absorption efficiency Prey size: Smaller prey may be processed more efficiently Diet composition: Omnivorous diets (eating both phytoplankton and zooplankton) may yield different efficiency than strict herbivory The fecal pellets these organisms produce reflect their diet and feeding rates. When absorption efficiency is high, pellets are small and dense; when it's low, pellets are larger and looser. This matters for the deep sea because larger, less dense pellets sink more slowly, potentially providing more opportunities for bacterial decomposition before reaching the seafloor. <extrainfo> Iron Fertilization: Stimulating Plankton Growth In some ocean regions, iron is the limiting nutrient preventing phytoplankton growth. Adding iron to these iron-limited regions can stimulate phytoplankton growth, potentially increasing carbon uptake. However, large-scale iron fertilization faces significant practical challenges: Logistical difficulties: It's expensive and complex to fertilize vast ocean areas Possible side effects: Iron fertilization might deplete deep-water oxygen or trigger methane release from sediments Uncertain outcomes: The full ecosystem consequences remain poorly understood The Great Calcite Belt and Coccolithophores Coccolithophores are phytoplankton that form calcium carbonate plates (coccoliths) on their cell surfaces. When these organisms calcify, they lower seawater alkalinity and release carbon dioxide—essentially a chemical process separate from respiration. When coccolithophores die, their calcite shells sink to the seafloor, contributing to long-term carbon sequestration in oceanic sediments. The "Great Calcite Belt" refers to regions where coccolithophores are particularly abundant and productive. This electron microscope image shows the intricate calcium carbonate structure of a coccolithophore. </extrainfo>