Photosynthesis and Climate Change

A blog about how plants will respond to climate change

by David Gallagher

What is climate change?

Climate change describes the unprecedented rate of global warming that is altering the climate of Earth. Carbon dioxide (CO₂ )is the principle gas driving climate change. Atmospheric CO₂  is increasing due to the human activities of burning fossil fuels, deforestation, and poor agricultural practices. Carbon dioxide levels in the atmosphere have increased by 40% since the Industrial Revolution, contributing to global heat retention and elevated atmospheric temperatures, as well as an array of other ecological impacts^. For example, there is a cumulative loss of glacier mass that is rapidly occurring. The Himalayan mountain ranges are comprised of an extensive glacier network that feeds rivers that comprise the largest river run-off system in the world. Changes in water run-off would influence water resources, agriculture, infrastructure, livelihoods, biodiversity and cultures and would affect the lives of about 40% of the world’s population. Additionally, extreme weather events such as heat waves, wildfires, storms and flash floods are expected to increase and have been linked to climate change. From 1987 to 1998, the average number of climate-related disasters was 195. From 2000 to 2006, the average was 365, representing an increase of 87%. Atmospheric CO₂ is still increasing exponentially and will likely double (to 700 parts per million (ppm)) within the next century. Collectively, these impacts are serious threats to global ecosystems and ultimately, human welfare. Although the mitigation of increasing atmospheric CO₂ levels is a pressing issue for policy makers at the national and international levels, many of the proposed solutions have prohibitive costs and uncertain or already documented detrimental ecological impacts. Understanding how plants respond to elevated CO₂ levels could help guide proposed solutions for climate change, from reforestation strategies to agricultural practices.

The cumulative loss of glacier mass by global region. The data are reported in sea-level equivalent (SLE) rise in millimeters (mm)

Atmospheric CO₂ concentration for the past 10,000 years. Notice the start of the exponential increase since the beginning of the 19th century (start of the Industrial Revolution)

What do photosynthesis and climate change have in common?

Plants need CO₂ and the atmospheric levels of CO₂ are increasing. Plants covert CO₂  into complex carbohydrates through the process of photosynthesis. Photosynthesis is composed of two sets of reactions: the light reactions that convert light energy into chemical energy in the form of the high energy molecules ATP and NADPH; the Calvin cycle then uses the high energy molecules produced in the light reactions to convert (fix) CO₂ into the carbohydrate sucrose (among other products). These complex carbohydrates enter into the food chain through herbivores (plant eaters, like a cow). Herbivores are then consumed by carnivores (meat eaters, like humans). Photosynthesis is the route by which energy from the sun enters the biosphere and drives the process of life. Without photosynthesis life on this planet would come to an end. Photosynthesis is the major pathway by which CO₂ is removed from the atmosphere. Approximately 50% of carbon that is emitted into the atmosphere is currently removed by plants through photosynthesis. Plants are the basis of what is called the carbon cycle and regulate the amount of carbon (in the form of CO₂ ) in the atmosphere. It can easily be argued that measuring the effects of increased CO₂ levels on the process of photosynthesis is important in understanding how climate change will impact humanity.

The process of photosynthesis in a chloroplast of a plant. Photosynthesis is the conversion of light energy into chemical energy

What is the carbon cycle?

Carbon like many elements, gets cycled through ecosystems. Think of CO₂ and carbon as the same thing. The amount of carbon that is available for storage (removed from the atmosphere) in a ecosystem is called net ecosystem production (NEP), which is a fundamental property of ecosystems. Ecosystems can either be autotrophic (carbon storage takes place), as in a typical forest or grassland or heterotrophic (carbon is entering the atmosphere) as in cities and many lakes and rivers. In particular, plant assemblages like tropical rainforests and large grasslands (like the Great Plains) have high rates of net primary productivity (NPP) and is the amount of CO₂ converted into plant biomass (gross primary productivity or GPP)  less the amount of CO₂  used by the plant for respiration and can directly affect the magnitude and direction of global climate change via the cycling of carbon through the ecosystem and consequently change the amount of carbon stored in the soil and as biomass. The concept of a source-sink relationship between CO₂ and soil is useful because if you think of the atmosphere as the source of CO₂  and the soil as the sink of CO₂ ,  then you can imagine that you want ecosystems to favor the sinking (storage) of CO₂  so that it does not stay in the atmosphere and drive climate change. Any change to photosynthetic rate can change the amount of CO₂ stored in the soil and as biomass.

The Fate of CO₂ in an ecosystem. The total ecosystem respiration (R_e ) is the sum of autotrophic (plants) respiration ( R_a) and heterotrophic (non-plants or consumers) respiration( R_h). Net ecosystem production (NEP) is the amount of carbon available for storage in the ecosystem (indicated in the grey shaded area). Carbon can be stored as biomass or in the soil.

What does this mean for photosynthesis?

 It should be clear that a change in efficiency of photosynthesis can have dramatic effects on atmospheric CO₂  levels. Land plants utilize one of three modes of photosynthesis: C3, C4 (so called because the CO₂ is initially incorporated into either 3-carbon or 4-carbon compounds) and CAM, a special case of C4. C3 photosynthesis is the most prevalent photosynthetic pathway and is found in about 90% of all known land plants, including important crops like barley, wheat, tomatoes and cotton and most species of tree. This form of photosynthesis is the most efficient in climates not exposed to temperature extremes or drought. C4 and CAM photosynthetic pathways have evolved from the C3 pathway as adaptations to hot, arid conditions, as they result in more efficient uptake of CO₂ and more efficient use of water. While only about 3% of all known plants (7,000-8,000 species) use the C4 pathway (including the key crop corn), they are common components of the tropical and subtropical grassland, savannah and marsh habitats, and collectively account for 20-25% of global primary productivity. About 20,000 species utilize the CAM pathway (primarily cacti). Most grow in arid ecosystems and collectively contribute relatively little to global net primary productivity but they are ecologically important in areas where relatively few plant species grow. The main difference between all three pathways is that in C3 plants, Rubisco, the enzyme that converts CO₂ into a three carbon molecule that is a precursor to sucrose (Calvin cycle) is directly involved in the initial uptake of CO₂. In C4 plants, a different enzyme (PEP carboxylase) binds to CO₂  and coverts it to malic acid. Malic acid is then transported into a specialized cell and reconverted to CO₂. The specialized cell, called a bundle sheath cell, contains Rubisco and is where the Calvin cycle then takes place. C4 photosynthesis is more efficient because it only allows Rubisco to bind to CO_2. The reason C4 plants have this extra energy-consuming step is because Rubisco also binds to oxygen (O₂ ) in a process called photorespiration. 

The C4 and CAM photosynthetic pathways. In the C4 pathway notice that the Calvin cycle takes place in a specialized cell and that CO₂ is converted into an organic acid first then transported into the specialized cell.  In the CAM pathway, notice that CO₂ is taken in at night and stored as an organic acid and then during the day it is released into the Calvin cycle.

What is photorespiration?

Photorespiration is an alternative but less efficient pathway in which Rubsico binds to O₂ instead CO₂ . Photorespiration occurs frequently in C3 plants because O₂ and CO₂ compete with one another and each have an equal chance of binding to Rubisco. The oxygenation reaction (where O₂ binds) forms phosphoglycolate, which represents carbon lost from the photosynthetic pathway and it also inhibits photosynthesis if it is allowed to accumulate in the plant. Photorespiration reduces the rate of photosynthesis in plants by diverting energy from photosynthetic reactions to photorespiratory reactions. It is less efficient then C4 photosynthesis.

Elevated levels of CO₂ favor C3 photosynthesis.

Photosynthesis by terrestrial vegetation accounts for about half of the carbon that annually cycles (the carbon cycle) between the earth’s surface and the atmosphere. Most C3 land plants respond to elevated CO₂ by increased photosynthetic capacity. The ability of plants to produce additional biomass because of elevated CO₂ levels is one of the reasons that terrestrial plants have become increasingly greater carbon sinks over the past 50 years, keeping CO₂ build-up in the atmosphere at 40-50% of what it would otherwise be due to emissions by human activities. C4 plants respond similarly to C3 plants but to a substantially lesser degree because these photosynthetic pathways already function to minimize photorespiration. In summary, elevated CO₂ levels favor C3 photosynthesis.

The change in net photosynthetic rate for a C3 versus C4 plant under ambient and high CO₂ concentrations. Notice how the C3 plant responds to an increase in CO₂ concentration across ecological significant temperatures.

If C3 plants respond to higher CO₂ concentrations, do we need to really worry about additional CO₂ emissions?

If it were only that simple. Some photosynthetic-climate change models show an  increase in global plant productivity and an increase in global plant biomass and consequently an increase in the removal of CO₂ from the atmosphere. Other studies suggest that plants will eventual acclimatize to higher CO₂ levels. It has been shown that long term exposure to elevated CO₂ leads to the accumulation of carbohydrates in the photosynthetic tissues of the plant and this in turn leads to a reduction in photosynthetic rates. Additionally, although increased CO₂ makes C3 plants grow larger initially, plants growing larger and faster need more nutrients, such as nitrogen (N) and water. These resources may not be available depending on how climate change will impact a particular ecosystem. As well as this down-regulation of photosynthetic capacity, plants that respond to elevated CO₂ also produce tissue with lower nutrient concentrations (reduced leaf N content). This has clear implications for herbivores and ultimately humans. The implications of nutrient poor agricultural crops could have serious implications for food security for a growing world population.

A Final Word

Plants grow and develop in response to a range of abiotic factors. The most important abiotic factors are the availability of CO2, water and mineral nutrients, temperature and light. It follows that the distribution of different plant assemblages is influenced by the climate, since climate determines temperature and precipitation. One of the most pressing questions is how climate change will effect terrestrial vegetation.

“All flesh is grass” -Isaiah

Calochortus obispoensis (San Luis Obispo star tulip) is endemic to San Luis Obispo County and is in the lily family. Photographed in West Cuesta.

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