Microalgae, carbon dioxide and global warming

Microalgae are becoming increasingly important as a means of removing carbon dioxide from the atmosphere to help limit global warming.

The briefest of summaries of global warming and greenhouse gases


Some of the solar energy absorbed at the Earth’s surface is radiated back into the atmosphere as heat. As the heat makes its way through the atmosphere and back out to space, greenhouse gases absorb much of it.  As levels of greenhouse gases increase, so does this retention of heat.

Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities (methane – another important greenhouse gas is considered in the
Bacteria and methane page of this site). The latest measurement of carbon dioxide (at the Mauna Loa Observatory, Hawaii) shows carbon dioxide levels of 406.1 ppm (parts per million). These results show a significant increase in levels of atmospheric carbon dioxide over recent years. (See Figure to the right).

[See:  IPCC Climate Change Synthesis Report 2014] (
https://www.ipcc.ch/pdf/assessment-report/ar5/syr/SYR_AR5_FINAL_full.pdf


Carbon dioxide molecule
Carbon dioxide can enter the atmosphere by means of human activities such as deforestation, and by the burning of fossil fuels (coal, natural gas, and oil) and other solid wastes. It can also be produced by certain industrial process such as the manufacture of cement. Carbon dioxide also enters the atmosphere by natural means such as animal and plant respiration and volcanic eruptions.
Flumes of carbon dioxide released to atmosphere

Removing carbon dioxide from the atmosphere


There are many different ways being explored to remove carbon dioxide from the atmosphere to reduce its effect on the environment. They include:
  • Increasing the efficiency of processes such as fuel efficiency in vehicles
  • Fuel switching from coal-fired electric plants to natural gas-powered facilities
  • Use of wind power
  • Use of solar power
  • Use of nuclear power
  • Elimination of deforestation
  • Carbon capture and storage
Using microalgae to absorb carbon dioxide is a new technology that is not only very efficient but also produces valuable products. 

Using microalgae to remove carbon dioxide from the atmosphere


Microalgae remove carbon dioxide from the atmosphere by the natural process of photosynthesis that uses solar energy (or artificial sources of illumination) to drive the process. 

Solar energy 


Approximately 1.8 x 1014 kW of solar energy is intercepted annually by the Earth. About 60% of this amount or 1.08 x 1014 kW reaches the surface of the Earth. The rest is reflected back into space or absorbed by the atmosphere. The total annual solar radiation intercepted by the Earth is more than 7,500 times the world’s total annual primary energy consumption. This number is decreasing as human energy consumption increases.

Photosynthesis


Photosynthesis is the process by which green plants and algae absorb a small fraction of solar energy to synthesise nutrients and biomass from carbon dioxide and water. Photosynthesis in plants involves the green pigment chlorophyll and produces oxygen as a by-product. Photosynthesis is represented by this equation:

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Advantages of using microalgae to absorb carbon dioxide

There a number of reasons why microalgae are an attractive option for absorbing carbon dioxide:

  • They grow very rapidly, doubling their numbers every few hours

  • They store energy as oils and carbohydrates which can be the basis of a wide range of valuable products

  • They can use carbon dioxide in the flue gases produced by power plants and anaerobic digestion facilities preventing release to the atmosphere

  • They do not compete with agriculture in that they can be grown in ponds (see later) situated on soils unsuitable for the growth of crops

  • The microalgal biomass produced can be used directly as human food, animal feed, feed for aquaculture, fertiliser, or can be extracted to produce biofuel, biodegradable plastics, lubricants, and a wide range of pharmaceuticals, cosmetics and nutrachemicals

  • After extraction, the residual biomass can be used as fuel for industrial boilers or as a feedstock for anaerobic digestion

Growing microalgae commercially


This is a large topic and will be dealt with in more detail at a later date. In summary, there are two main ways of growing microalgae on a large scale:

Raceway ponds

– These are shallow artificial outdoor ponds in which sunlight is used as the source of energy. The pond is filled with water and nutrients and inoculated with a selected strain of microalgae. Often paddle wheels are used to circulate the growing microalgae around the pond.

A fraction of the water containing the microalgae is generally harvested each day and the microalgae concentrated by sedimentation, centrifugation or filtration. The resultant biomass can then either be used directly, or dried to a powder. 

Raceway pond for microalgae

Photobioreactors

-  These are closed systems of various designs, made from glass, plastic or metal, in the form of vertical tubes or bags, horizontal tubes, or panels. The photobioreactors may be used out of doors or within a building. They are more expensive to construct than open ponds but offer the advantage of greater control cover the growth conditions, by the use of artificial lighting and temperature control, and greater protection from contamination with unwanted micro-organisms such as rotifers that can use the microalgae as feed.

Horizontal tube photobioreactor for microalgae

Commercially grown microalgae


The number of species of microalgae growing naturally in the world is unknown, but estimates range from 72,000 to several hundred thousand.  To date, a very small number of microalgae have been grown commercially. Photographs of many of these are shown in the Microalgae page.
It is clear that there are many species of microalgae that have not yet been tested or cultivated that may have very useful characteristics and produce valuable products.
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Example of microalgae