Biological carbon fixation

Some living things make their own food. They take gas from the air. They turn that gas into food.

Most plants use sunlight to do this. This is a very big job. It helps plants grow tall.

Some tiny life forms do not use light. They use chemical energy instead. This helps them live in dark places.

This process helps the whole Earth. It keeps the air healthy. It helps all life stay strong.

Nature has many ways to make food.

Living things need carbon to grow. Carbon is a base part for life. Some life forms can take carbon from the air or water. They turn it into food. This is called biological carbon fixation.

Most plants and algae are autotrophs. This means they make their own food. Most use sunlight to do this. This way is called photosynthesis. They use a set of steps called the Calvin cycle. This cycle makes up 90% of all carbon fixation. It turns carbon dioxide into sugar.

Some life forms do not use light. They use chemical energy instead. This is called chemosynthesis. They can live in dark places like the deep ocean. One way they use is the reverse Krebs cycle. This helps life grow in dark areas.

Other tiny life forms use different ways. Some use the reductive acetyl-CoA pathway. This way uses very little power. This helps life in places with little energy. These cycles help keep our air and oceans healthy. They are a big part of how our world works.

Carbon is a base element for all life on Earth. Living things use it to build their bodies and store energy. This important work is called biological carbon fixation. It is the way living things turn inorganic carbon, like carbon dioxide, into organic compounds.

This process is a huge part of the global carbon cycle. It moves carbon from the air and oceans into living things. This helps regulate the climate and keeps ecosystems working well. Without this, life as we know it could not exist.

There are different ways this work happens. Most living things, like plants and algae, are called autotrophs. These organisms use sunlight to fix carbon through photosynthesis. This is often done using the Calvin cycle. The Calvin cycle is very common and accounts for 90% of all carbon fixation.

It uses energy to turn carbon dioxide into a sugar called triose phosphate. Other organisms use chemosynthesis instead of sunlight. These life forms use chemical energy to build their organic compounds.

Scientists have found seven different natural pathways for fixing carbon. The Calvin cycle is the most famous one. It is used by plants, algae, and even some bacteria. Other pathways are only found in tiny organisms like bacteria or archaea. For example, the reductive acetyl-CoA pathway is used by many bacteria.

This pathway is very efficient because it uses only one molecule of ATP for one molecule of pyruvate. This makes it a great choice for life in places with very little energy.

History shows us how much carbon has been moved by life. Since life first began, it is estimated that 200 billion billion tons of carbon have been fixed.

Every year, photosynthesis converts about 250 billion tons of carbon dioxide. Nearly half of this happens in the oceans. The rest happens on land, mostly in the tropical regions. Some of this carbon is used right away through respiration. This means the total amount of carbon fixed is actually much larger.

Understanding these cycles helps us see how everything is linked. The way plants grow affects the air we breathe. The way tiny microbes in the soil work affects the whole planet. Even the deep, dark parts of the ocean rely on these cycles. In those dark places, the reverse Krebs cycle allows life to grow. This shows that carbon fixation is happening almost everywhere on Earth. It is a beautiful, constant cycle that supports all living things.

Biological carbon fixation is the fundamental process of converting inorganic carbon into organic compounds. This process, also known as carbon assimilation, allows living things to create the building blocks for life. These organic compounds serve as energy storage and as the structural components for other biomolecules. Carbon is an essential base element for all organic life. Through fixation, carbon moves from the atmosphere, oceans, and lithosphere into the biosphere. This movement is a key part of the global carbon cycle. Understanding these cycles is vital for studying ecosystem dynamics and climate regulation.

Organisms that can fix their own carbon are called autotrophs. These are divided into two main groups based on their energy source. Photoautotrophs, such as most plants and algae, use sunlight to drive the process through photosynthesis. Lithoautotrophs use inorganic oxidation to obtain energy. In contrast, heterotrophs, including animals and fungi, cannot fix carbon themselves. They must grow by consuming the organic carbon produced by autotrophs or other heterotrophs.

The Calvin cycle is the most dominant mechanism for carbon fixation on Earth. It accounts for approximately 90% of all biological carbon fixation. In plants, this cycle occurs within the chloroplasts. It is also the primary method used by algae, cyanobacteria, and certain purple bacteria. The cycle works by consuming adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These molecules provide the energy and electrons needed to convert carbon dioxide into a sugar called triose phosphate (TP). This sugar is composed of glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP).

Other pathways allow life to thrive in environments without sunlight. The reverse Krebs cycle, or reductive citric acid cycle, is one such method. It was discovered in 1966 by scientists Evans, Buchanan, and Arnon while studying the green sulfur bacterium Chlorobium limicola. This cycle is used by certain anaerobic bacteria and archaea. It is especially important in the aphotic, or dark, zones of the ocean. In hydrothermal vents, Campylobacterota use this cycle to enable primary production. Without this dark primary production, these deep-sea habitats could not support life.

The reductive acetyl-CoA pathway, also known as the Wood-Ljungdahl pathway, is another specialized method. This pathway uses carbon dioxide as an electron acceptor and hydrogen as an electron donor to form acetic acid. It is widespread among the phylum Bacillota and is used by methanogens in the archaea domain. This pathway is remarkably efficient for organisms living in low-energy, anaerobic conditions. It requires only one molecule of ATP to produce one molecule of pyruvate. This efficiency makes it a primary choice for chemolithoautotrophs in harsh environments.

Scientists have identified several other complex pathways used by microorganisms. The 3-hydroxypropionate (3-HP) bicycle was discovered in 1989 in the Chloroflexaceae family. This pathway involves 19 different reactions and uses 13 multifunctional enzymes to fix three bicarbonate molecules. It is a costly process, consuming 7 ATP for pyruvate synthesis and 3 ATP for phosphate triose. Variants of this cycle exist, such as the 3-HP/4-hydroxybutyrate cycle found in the archaeon Metallosphaera sedula. Other versions include the dicarboxylate/4-hydroxybutyrate cycle, which was proposed for the hyperthermophile Ignicoccus hospitalis in 2008.

The scale of carbon fixation is massive and affects the entire planet. It is estimated that photosynthesis converts about 250 billion tons of carbon dioxide annually. Nearly half of this occurs in the oceans, while the rest happens in terrestrial environments, mostly in the tropics. The gross amount of fixed carbon is even higher because about 40% is consumed by respiration immediately after fixation. Since the origin of life, it is estimated that 200 billion billion tons of carbon have been fixed.

Carbon fixation is deeply connected to the chemistry of the Earth. Some enzymes, like RuBisCO, exhibit carbon isotope discrimination. This means they prefer the lighter carbon-12 isotope over the heavier carbon-13 isotope. Measuring these isotopic ratios helps scientists evaluate plant water use efficiency. It also helps them track the sources of carbon within the global cycle. From the microscopic bacteria in the soil to the massive forests of the tropics, these chemical pathways sustain the biological world.