Marine Science (0697) | Photosynthesis

Hello Future Marine Scientist! Understanding Photosynthesis

Welcome to the foundation of marine life! This chapter is all about how energy first enters the ocean ecosystem. You already know that the Sun powers almost everything on Earth, but how does that solar energy get turned into food that sea turtles, whales, and fish can eat? The answer is Photosynthesis.

Don't worry if this sounds complicated. We will break down this vital process step-by-step, focusing only on what you need to master for your IGCSE exams.

What is Photosynthesis? (The Core Process)

4.3.1 The Definition of Photosynthesis

Photosynthesis is the process used by certain organisms (called producers) to convert light energy, usually from the Sun, into chemical energy in the form of food (carbohydrates).

Think of it like this: Photosynthesis is the ultimate recipe for life in the marine environment. The organism takes simple, non-food ingredients and uses solar energy to cook up sugar!

• Ingredients (Reactants): Carbon dioxide and Water.
• Energy Source: Light (usually sunlight).
• Product (Food): Glucose (a type of carbohydrate).
• By-product: Oxygen.

4.3.5 The Photosynthesis Equation

You must know the word equation for this process. It summarises the ingredients and the products perfectly:

$$ \text{carbon dioxide} + \text{water} \xrightarrow{\text{light energy}} \text{glucose} + \text{oxygen} $$

Memory Aid: Remember the key outputs: Food (glucose) and the air we breathe (oxygen)!

The Key Players: Chlorophyll and Producers

4.3.2 The Role of Chlorophyll

How do plants and algae capture light energy? They use a special substance called chlorophyll.

• Chlorophyll is a green pigment found inside the cells of photosynthetic organisms.
• Its job is to trap the energy from light.
• This trapped light energy is then used to power the chemical reactions that build organic chemicals (like carbohydrates).
• Analogy: Chlorophyll acts like a solar panel, absorbing sunlight and converting it into electrical energy (chemical energy) that the cell can use.

4.3.3 What is a Producer?

An organism that can make its own food is called a producer (or sometimes an autotroph, which means "self-feeder").

In most ecosystems, including the ocean, producers use energy from sunlight through photosynthesis to create their own organic nutrients (carbohydrates). They are the starting point of every food chain.

4.3.4 Where Does Photosynthesis Happen in the Marine Environment?

Photosynthesis can only happen where there is light – this means primarily in the Sunlight Zone (or Photic Zone), which extends down to about 200m depth (Syllabus 5.3).

The organisms carrying out photosynthesis in the ocean belong to different groups:

• Marine Plants: Examples include seagrasses (which have true roots and leaves, unlike algae).
• Marine Protoctists (Algae): These are often the most important producers.
  • Macroalgae: These are large seaweeds, like kelp.
  • Microalgae: Tiny, single-celled organisms that float in the water, such as diatoms (which have silica skeletons) and dinoflagellates. These microalgae form the base of the open ocean food web (phytoplankton).
• Marine Bacteria: Certain bacteria, known as cyanobacteria, are also capable of photosynthesis.

4.3.7 Productivity: Measuring the Rate of Food Production

Defining Productivity

Productivity is simply the measure of how successful the producers are at making food.

• Productivity is defined as the rate at which producers transfer energy into carbohydrates (organic matter).
• A high productivity ecosystem makes a lot of new food very quickly.

4.3.6 Factors Affecting the Rate of Photosynthesis (and Productivity)

The rate of photosynthesis can be limited by several factors. The syllabus asks us to consider the effect of light intensity.

Light is crucial! If there is more light, the plant can photosynthesise faster, up to a certain point. If the light intensity drops (e.g., deeper in the ocean or on a very cloudy day), the rate of photosynthesis drops too.

We can investigate this in the lab using an aquatic plant (like Elodea) or a macroalga. We measure the rate by counting the bubbles of oxygen released as light intensity changes.

4.3.8 The Importance of Upwelling to Productivity

While light is the energy source, producers also need nutrients (like nitrates and phosphates) to grow and build new cell structures (Syllabus 4.1).

In the open ocean, the surface waters (where the light is) often have very few nutrients because they are used up quickly by plankton.

This is where upwelling becomes incredibly important:

1. Strong winds push warm surface water away from the coast.
2. This gap is filled by cold, nutrient-rich water rising up from the deep ocean floor.
3. This deep water contains nutrients that have sunk as marine snow or been recycled by decomposers (Syllabus 4.1.5).
4. When these nutrients reach the surface (the Sunlight Zone), the producers (phytoplankton and macroalgae) suddenly have everything they need: Light AND Nutrients!
5. This massive influx of nutrients causes an explosion in producer growth, leading to a much higher productivity in these coastal marine ecosystems.

Did you know? Areas with consistent upwelling, such as the coast of Peru, are some of the most productive fishing grounds in the world because the base of the food web (the producers) is so rich.