Every living thing on Earth falls into one of two groups: those that build their own carbon compounds, and those that get them by eating something else. The split sounds simple, and it is — until you meet the communities living in total darkness on the deep sea floor.
📘 What you need to know
Autotrophs synthesise carbon compounds from inorganic sources such as carbon dioxide and water.
Heterotrophs obtain carbon compounds by consuming other organisms or dead organic matter.
Photoautotrophs use light energy through photosynthesis.
Chemoautotrophs use energy from inorganic chemical reactions through chemosynthesis.
Types of heterotroph: herbivores, carnivores, omnivores and decomposers.
Chemoautotrophs are the primary producers in extreme environments such as hydrothermal vents.
The basic split
The word roots do the work here. Autotroph is self-feeding; heterotroph is different-feeding, meaning it feeds on something other than itself.
Types of autotroph
Photoautotrophs
These use light as their external energy source to produce organic compounds through photosynthesis, converting carbon dioxide and water into glucose and oxygen. Green plants, algae and some bacteria such as cyanobacteria all belong here. They are the base of almost every food chain on the planet.
Chemoautotrophs
These use energy released by exothermic inorganic chemical reactions to produce organic compounds — a process called chemosynthesis. No light required. They oxidise substances such as hydrogen sulfide or ammonia to release the energy they need.
You find them where sunlight cannot reach: deep-sea hydrothermal vents, and other extreme environments where ordinary food chains cannot exist. In those places, chemoautotrophs are the primary producers, creating the energy-rich compounds that support everything else in the food web.
The giant tube worm.Riftia pachyptila lives on or near hydrothermal vents and has no digestive system at all. It survives through a symbiotic relationship with chemosynthetic bacteria living inside its body. The bacteria use hydrogen sulfide from the vent, plus oxygen and carbon dioxide from the surrounding water, to make sugars — and the worm lives on those.
Vent communities are the clearest possible demonstration that ecosystems need an energy source, not specifically sunlight. If a question asks whether all food chains begin with the Sun, this is your counter-example.
Types of heterotroph
Type
How they obtain carbon compounds
Examples
Herbivores
By consuming plants
Deer, rabbits, caterpillars
Carnivores
By consuming other animals
Wolves, owls, sharks
Omnivores
By consuming both plants and animals
Bears, humans, crows
Decomposers
By breaking down dead organic material, returning nutrients to the ecosystem
Fungi, bacteria, earthworms
Decomposers deserve a second look. They are heterotrophs, but their role is different from any consumer — they are the reason matter cycles instead of accumulating in dead bodies.
WORKED EXAMPLE
Explain how a hydrothermal vent community can exist with no sunlight, and identify the primary producers.
Step 1: Identify the energy sourceEnergy comes from exothermic inorganic reactions, not lightStep 2: Name the process and organismschemosynthesis, carried out by chemoautotrophic bacteriaStep 3: Follow it into the food webThey oxidise hydrogen sulfide from the vent to make sugars, which support tube worms and other organismsChemoautotrophic bacteria are the primary producers
WORKED EXAMPLE
Classify each of the following: a cyanobacterium, an earthworm, a bear, a vent bacterium oxidising hydrogen sulfide.
Step 1: Ask whether it makes its own carbon compoundscyanobacterium: yes, using light → photoautotrophearthworm: no, eats dead matter → heterotroph (detritivore)bear: no, eats plants and animals → heterotroph (omnivore)vent bacterium: yes, using chemical energy → chemoautotrophTwo autotrophs, two heterotrophsAlways give the subtype if you know it
💡 Exam tip
Define autotrophs using inorganic sources, not just “make their own food”.
Name the sub-category: photoautotroph or chemoautotroph, herbivore or decomposer.
Have the hydrothermal vent example ready — it is the standard HL question.
Say chemosynthesis by name when describing chemoautotrophs.
Use the word roots if you are stuck: auto is self, hetero is different, trophic is feeding.
⚠ Common mix-up
Assuming all autotrophs are plants. Algae and several groups of bacteria are autotrophs too.
Saying all food chains start with the Sun. Vent communities start with chemical energy.
Calling chemoautotrophs consumers. They are producers — they build organic compounds from inorganic ones.
Leaving decomposers out of the heterotroph list. They obtain carbon from dead organic matter, so they belong.
Confusing chemosynthesis with respiration. Chemosynthesis builds organic compounds; respiration breaks them down.
Up next: Calculating Primary and Secondary Productivity — the four equations, and how to tell which one a question wants.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
