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Unlock This CourseIn the vast blue oceans that cover 71% of our planet, tiny marine plants called phytoplankton are working around the clock to capture sunlight and turn it into food. This amazing process, called photosynthesis, is the foundation of almost all life in the sea. At the heart of this process is a green molecule called chlorophyll, which acts like a solar panel, capturing light energy and converting it into chemical energy that can feed entire ocean food webs.
Marine photosynthesis produces about 50% of all the oxygen we breathe and removes massive amounts of carbon dioxide from our atmosphere. Understanding how chlorophyll works in marine environments is crucial for understanding ocean ecosystems and climate change.
Key Definitions:
The basic equation for photosynthesis is:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
This means carbon dioxide plus water plus light energy creates glucose and oxygen. In marine environments, this happens billions of times every second!
Not all chlorophyll is the same! Marine organisms use different types of chlorophyll to capture light at different depths and conditions in the ocean.
Marine plants and algae contain several types of chlorophyll, each specialised for different light conditions found at various ocean depths.
The main type found in all photosynthetic organisms. It absorbs red and blue light best and appears green. This is the primary pigment that actually converts light energy into chemical energy.
An accessory pigment that helps capture additional light wavelengths. It absorbs blue and red-orange light and passes the energy to chlorophyll-a. Common in green algae.
Found in brown algae and diatoms. It helps these organisms capture light in deeper waters where red light has been filtered out by the water above.
The process of capturing and transferring light energy in marine photosynthesis is like a perfectly organised relay race, where energy is passed from molecule to molecule until it reaches its destination.
When sunlight hits chlorophyll molecules in marine phytoplankton, an amazing chain of events begins that ultimately powers most ocean life.
1. Light Absorption: Chlorophyll molecules absorb photons (particles of light energy)
2. Electron Excitation: The energy excites electrons in the chlorophyll, making them "jump" to higher energy levels
3. Energy Transfer: The excited electrons are passed along a chain of proteins called the electron transport chain
4. ATP Production: As electrons move through this chain, they release energy that's used to make ATP (the cell's energy currency)
5. Water Splitting: Water molecules are split to replace the electrons, releasing oxygen as a waste product
Marine organisms have evolved sophisticated molecular machines called photosystems that work together to capture and process light energy efficiently.
This system captures light energy and uses it to split water molecules. It's called "II" but actually works first in the process. It produces oxygen and starts the electron transport chain that powers the cell.
This system receives electrons from Photosystem II and uses additional light energy to boost them to an even higher energy level. These high-energy electrons are then used to make NADPH, another energy-carrying molecule.
Several environmental factors influence how efficiently marine organisms can carry out photosynthesis, affecting the productivity of entire ocean ecosystems.
Light behaves very differently in water compared to air and this has huge implications for marine photosynthesis.
Red light is absorbed quickly by water, while blue light penetrates deepest. This is why deep-water algae often appear blue-green or brown rather than bright green.
Most marine photosynthesis occurs in the top 200 metres of the ocean, called the photic zone. Below this, there's not enough light for photosynthesis.
Particles in the water, such as sediment or other plankton, can block light and reduce photosynthesis rates. Clear tropical waters allow deeper light penetration.
The Great Barrier Reef demonstrates how light and chlorophyll work together in marine ecosystems. The reef's zooxanthellae (symbiotic algae living in coral tissues) contain chlorophyll-a and accessory pigments that capture the intense tropical sunlight. These tiny organisms provide up to 90% of the coral's energy needs through photosynthesis. However, when water temperatures rise due to climate change, the coral expels these algae, leading to coral bleaching and potential death of the reef system.
Primary productivity measures how much organic matter is produced through photosynthesis in a given area over time. In marine environments, this is almost entirely due to phytoplankton photosynthesis.
Scientists use various methods to measure how much photosynthesis is occurring in different parts of the ocean.
Satellites can measure chlorophyll concentrations from space by detecting the green colour of phytoplankton. Areas with high chlorophyll concentrations indicate high productivity.
Scientists can measure oxygen production rates to determine photosynthesis levels. More oxygen production means more active photosynthesis.
Marine productivity isn't evenly distributed across the oceans. Some areas are incredibly productive while others are like underwater deserts.
Areas where deep, nutrient-rich water rises to the surface. These zones, like off the coast of Peru, have incredibly high productivity due to abundant nutrients for phytoplankton growth.
During summer months, polar seas can be extremely productive due to long daylight hours and nutrient-rich waters from melting ice.
The centres of tropical oceans are often low in productivity because warm surface waters prevent nutrient-rich deep water from reaching the surface.
During the Antarctic summer, massive phytoplankton blooms occur as sea ice melts and sunlight increases. These blooms can be seen from space as green swirls in the Southern Ocean. The phytoplankton use chlorophyll-a and accessory pigments adapted to the cold, nutrient-rich waters. These blooms support enormous populations of krill, which in turn feed whales, seals and penguins. The timing of these blooms is crucial for the entire Antarctic food web.
Human activities are affecting marine photosynthesis in various ways, with consequences for ocean productivity and global climate.
Rising ocean temperatures and changing ocean chemistry are altering patterns of marine photosynthesis worldwide.
Warmer waters hold less dissolved CO₂ and nutrients, potentially reducing photosynthesis rates. However, some regions may become more productive as ice melts and new areas become available for phytoplankton growth.
As oceans absorb more CO₂ from the atmosphere, they become more acidic. This can affect the ability of some marine organisms to build shells and may alter photosynthesis rates in certain species.