Nutrients - Marine Science (0697) - Cambridge IGCSE

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🔬 Marine Science Study Notes: Chapter 4.1 - Nutrients

Hello Future Marine Biologists!

Welcome to the "Nutrients" chapter! If you think of the ocean as a giant city, nutrients are the essential resources—the groceries, the steel, and the cement—that keep everything growing and running smoothly. Without them, life stops!
In this section, we will explore what nutrients are, what they do, and how they constantly move around the ocean in critical cycles. Don't worry if the terminology seems tricky; we will break it down into simple, digestible pieces. Let's dive in!

1. Defining Nutrients: The Building Blocks of Marine Life

1.1 What Are Nutrients?

A nutrient is simply a substance required by an organism for growth, repair, and to provide chemical energy.
Think of nutrients like the ingredients needed to bake a cake. If you run out of flour (protein), you can't build the structure!

1.2 Major Groups of Nutrients in the Ocean (4.1.1)

Nutrients can be categorized into three main groups that marine organisms need:

Gases: Primarily carbon dioxide (required for photosynthesis by producers).
Dissolved Salts (Minerals): These are chemical compounds that dissolve in water, like nitrogen or phosphorus salts, which are crucial micronutrients.
Organic Compounds: These are complex molecules made by living things (or their remains), including carbohydrates, lipids, and proteins.

2. Functions of Major Nutrient Groups (4.1.3)

2.1 The Essential Roles

Each type of nutrient plays a unique and vital role in the life of a marine organism:

Protein:
Function: Used primarily for tissue repair and growth. It is the structural material (like muscle and skin). It can also be used for energy supply if needed.
(PA Link: Tested using Biuret solution.)
Carbohydrates (e.g., Starch and Sugars like Glucose):
Function: The main and fastest source of energy supply for all cell activities (like movement or thinking).
(PA Link: Tested using Iodine solution for starch, or Benedict's solution for reducing sugars.)
Lipids (Fats and Oils):
Function: Excellent long-term energy supply, insulation (think whale blubber), and are a source of lipid-soluble vitamins.
(PA Link: Tested using the ethanol emulsion test, resulting in a cloudy white layer.)
Vitamins and Mineral Salts:
Function: These are needed in tiny amounts (called micronutrients) but are essential for many cell processes, such as building enzymes or skeletons.

💡 Quick Tip: When studying food tests (4.1.2), focus on remembering the colour change for a positive result: Biuret goes blue to purple/lilac (Protein). Iodine goes orange-brown to blue-black (Starch).

3. Essential Elements and Their Biological Roles (4.1.4)

3.1 Why Specific Elements Matter

Certain elements found in dissolved salts are absolutely essential for marine life to build specific structures:

Element 1: Nitrogen (N)

Role: Used to make proteins and nucleic acids (like DNA).
Analogy: Nitrogen is the "glue" that holds amino acids together to build complex body structures.

Element 2: Carbon (C)

Role: Used to make all organic compounds (carbohydrates, lipids, proteins). Carbon is the backbone of all life.

Element 3: Magnesium (Mg)

Role: Used to make chlorophyll. This is the green pigment in producers (like phytoplankton) that captures light energy for photosynthesis.

Element 4: Calcium (Ca)

Role: Used to make bones, shells, and coral skeletons. (E.g., shells of molluscs or the hard structure of a coral reef).

Element 5: Iron (Fe)

Role: Used to make haemoglobin (in the blood of vertebrates and some invertebrates) to carry oxygen around the body.

Key Takeaway: Producers (like algae) need Mg for chlorophyll. Consumers (like fish) need Fe for oxygen transport and Ca for bones/shells.

4. The Nutrient Cycle in Marine Ecosystems (4.1.5 & 4.1.6)

4.1 How Nutrients Move

Nutrients are not unlimited, especially in the surface waters of the ocean where light is available. They must be constantly recycled. This process is a continuous cycle:

Step 1: Uptake
Producers (phytoplankton) take up dissolved mineral salts from the water.

Step 2: Passing up the Food Chain
The nutrients are converted into organic compounds (biomass) and are passed along when primary consumers (zooplankton) eat the producers, and secondary consumers eat them, and so on.

Step 3: Death and Decay
When organisms die, or when they produce waste (like faeces), these organic materials begin to sink through the water column.

Step 4: Sinking as Marine Snow
The sinking particles of dead organisms, waste material, and other debris form what scientists call marine snow. This snow sinks deeper, carrying nutrients away from the surface (the photic zone) to the deep ocean floor.

Step 5: Decomposition and Release
The crucial role here is played by bacteria and other decomposers (4.1.6). As the organic matter sinks, decomposers break it down (decay), releasing the essential mineral salts (like nitrate and phosphate) back into the water. Because this happens in the deep water, the nutrients are now concentrated near the seabed.

4.2 How Nutrients Return to the Surface: Upwelling

Since the surface layer needs light for photosynthesis but loses nutrients as they sink, there must be a way to get them back up. This process is called upwelling.

Upwelling is the movement of cold, deep, nutrient-rich water that rises to replace warmer surface water (4.1.5).
This process brings the mineral salts (released by decomposition in the deep ocean) back into the sunlight zone, where producers can use them again.

Did you know? Areas of major upwelling are often the most productive fishing grounds in the world because the constant supply of nutrients supports huge populations of phytoplankton, forming the base of a massive food web. (See 4.3.8 - Importance of local upwellings).

Quick Review Box

Key Terms to Master

Nutrient: Required for growth, repair, energy.
Micronutrients: Vitamins and mineral salts (needed in small amounts).
Marine Snow: Sinking organic debris (dead organisms, waste).
Decomposers (Bacteria): Break down organic matter, releasing mineral salts.
Upwelling: Brings deep, nutrient-rich water back to the surface.