Heterotrophs

Heterotroph Definition

We talked about carbon fixation above but what does it mean?

We define carbon fixation as the biosynthetic pathway through which plants fix atmospheric carbon to produce organic compounds. Heterotrophs are incapable of producing food by carbon fixation as it requires pigments like chlorophyll. This is why only certain organisms like plants, algae, bacteria, and other organisms can do carbon fixation as they are able to photosynthesize food. The conversion of carbon dioxide to carbohydrates is an example of this.

All animals, fungi, and numerous protists and bacteria are heterotrophs. Plants, at large, belong to another group, although some exceptions are heterotrophic, which we will discuss shortly.

So, again, do humans also create their food by sitting under the sun via photosynthesis? Sadly, no, because humans and other animals do not have the mechanism to synthesize their food and, as a result, must consume other organisms to sustain themselves! We call these organisms heterotrophs.

Heterotrophs consume food in the form of solids or liquids and break it down through digestive processes into its chemical components. Afterwards, cellular respiration is a metabolic process that takes place within the cell and releases energy in the form of ATP (Adenosine Triphosphate) which we then use to perform tasks.

Where are heterotrophs in the food chain?

You must be aware of the hierarchy of the food chain: at the top, we have the producers, mainly plants, who derive energy from the sun to produce food. These producers are consumed by primary consumers or even secondary consumers.

Primary consumers are also called herbivores, as they have a plant-based diet. Secondary consumers, on the other hand, ‘consume’ herbivores and are called carnivores. Both herbivores and carnivores are heterotrophs as, even though they differ in their diet, they still consume one another to derive nutrition. Therefore, heterotrophs can be primary, secondary, or even tertiary consumers in nature in the food chain.

Heterotroph vs autotroph

Now, let's talk about the difference between autotrophs and heterotrophs. Heterotrophs consume other organisms for nourishment since they are unable to synthesize their food. On the other hand, autotrophs are “self-feeders” (auto means “self” and trophos means “feeder”). These are organisms that do not derive nourishment from other organisms and produce their food from organic molecules like CO₂ and other inorganic materials that they obtain from the surrounding environment.

Autotrophs are referred to as “producers of the biosphere” by biologists, as they are the ultimate sources of organic nutrition for all heterotrophs.

All plants (except a few) are autotrophic and only need water, minerals, and CO₂ as nutrients. Autotrophs, usually plants, synthesize food with the help of a pigment called chlorophyll, which is present in the organelles called chloroplasts. This is the main difference between heterotrophs and autotrophs (Table 1).

Heterotroph examples

You have learned that primary or secondary consumers may either have a plant-based diet or a meat-based diet. In some cases, some consume both plants and animals, called omnivores.

What does this tell us? Even among this category of consumers, there are organisms that feed differently. Therefore, there are different types of heterotrophs that you should be familiar with:

• Photoheterotrophs

• Chemoheterotrophs

Photoheterotrophs

Photoheterotrophs use light to produce energy, but still need to consume organic compounds to fulfil their carbon nutrition requirements. They are found in both aquatic and terrestrial environments. Photoheterotrophs mainly comprise microorganisms that feed on carbohydrates, fatty acids, and alcohols produced by plants.

Non-sulfur bacteria

Rhodospirillaceae, or purple non-sulfur bacteria, are microorganisms that reside in aquatic environments where light can penetrate and use that light to produce ATP as a source of energy, but feed on organic compounds made by plants.

Similarly, Chloroflexaceae, or green non-sulfur bacteria, are a type of bacteria that grow in a really hot environment like hot springs and use photosynthetic pigments to produce energy but rely on organic compounds made by plants.

Heliobacteria

Heliobacteria are anaerobic bacteria that grow in extreme environments and use special photosynthetic pigments called bacteriochlorophyll g to produce energy and consume organic compounds for nourishment.

Chemoheterotrophs

Unlike Photoheterotrophs, chemoheterotrophs can’t produce their energy using photosynthetic reactions. They get energy and organic as well as inorganic nourishment from consuming other organisms. Chemoheterotrophs constitute the largest number of heterotrophs and include all animals, fungi, protozoa, archaea, and a few plants.

These organisms ingest carbon molecules such as lipids and carbohydrates and obtain energy by the oxidation of molecules. Chemoheterotrophs can survive only in environments that have other forms of life due to their dependence on these organisms for nourishment.

Animals

All animals are chemoheterotrophs, largely due to the fact they lack chloroplasts and, therefore, are incapable of producing their energy through photosynthetic reactions. Instead, animals consume other organisms, such as plants or other animals, or in some cases, both!

Herbivores

Heterotrophs that consume plants for nourishment are called herbivores. They are also called primary consumers because they occupy the second level in the food chain, with producers being the first.

Herbivores typically have mutualistic intestinal microbes that help them break down cellulose present in plants and make it easier to digest. They also have specialized mouth parts that are used to grind on or chew leaves to make digestion easier. Examples of herbivores include deer, giraffes, rabbits, caterpillars, etc.

Carnivores

Carnivores are heterotrophs that consume other animals and have a meat-based diet. They are also called secondary or tertiary consumers because they occupy the second and third levels of the food chain.

Most carnivores prey on other animals for consumption, while others feed on dead and decaying animals and are called scavengers. Carnivores have a smaller digestive system than herbivores, as it is easier to digest meat than plants and cellulose. They also have different types of teeth like incisors, canines, and molars, and each tooth type has a different function like slicing, grinding, or tearing meat. Examples of carnivores include snakes, birds, lions, vultures, etc.

Fungi

Fungi are heterotrophic organisms that can't ingest other organisms. Instead, they feed on the absorption of nutrients from the surrounding environment. Fungi have root structures called hyphae that network around the substrate and break it down using digestive enzymes. The fungi then absorb the nutrients from the substrate and gain nourishment.

• The word substrate here is a broad term that can range from cheese and wood to even dead and decaying animals. Some fungi are highly specialized and feed on only a single species.

Fungi can be parasitic, meaning they latch onto a host and feed on it without killing it, or they can be saprobic, meaning they will feed on a dead and decaying animal called a carcass. Such fungi are also called decomposers.

Heterotrophic plants

Although plants are largely autotrophic, there are a few exceptions that are unable to produce their own food. Why is this? For starters, plants need a green pigment called chlorophyll to make food by photosynthesis. Some plants don’t have this pigment, and therefore, can’t produce their own food.

Plants can be parasitic, meaning they derive nutrition from another plant and, in some cases, can cause harm to the host. Some plants are saprophytes, and gain nutrition from dead matter, since they lack chlorophyll. Perhaps the most famous or well-known heterotrophic plants are insectivorous plants, which, as the name suggests, means they feed on insects.

Archaebacteria: heterotrophs or autotrophs?

Archaea are prokaryotic microorganisms that are quite similar to bacteria and are separated by the fact that they lack peptidoglycan in their cell walls.

These organisms are metabolically diverse, as they can be either heterotrophic or autotrophic. Archaebacteria are known to live in extreme environments, like high pressure, high temperature, or sometimes even high concentrations of salt, and are called extremophiles.

Archaea are generally heterotrophic and use their surrounding environment to meet their carbon needs. For example, methanogens are a type of archaea that uses methane as its carbon source.