Photosynthesis in a changing global climate
Climate change can be defined as long-term shifts in temperature and weather patterns. This is inevitable due to solar radiation and weather patterns; the earth’s climate is constantly changing. But, since the 1800s, scientists claim that the main driving force of climate change is human activities. Specifically fossil fuel burning, and CO2 release to the atmosphere.
Climate change affects photosynthesis in many direct and indirect ways. Increasing global temperature increases the rate of photosynthesis in every plant-microbe and algae. In addition, high CO2 levels increase the rate of photosynthesis in C3 because they are more sensitive to atmospheric CO2 concentration.
But the way plants respond to climate changes is dependent on other parameters like mineral availability in soil, humidity, etc. therefore, even for the same temperature and CO2 variation, different plants are expected to respond differently. If a plant is in a water stress environment, the priority for that plant is to conserve water. So, it closes stomata. When the stomata are closed, no new CO2 is coming from the environment. This could reduce photosynthesis and even facilitate photorespiration. Soil nutrient mineralization is also strongly dependent on temperature.
Various studies have shown that the rate of photosynthesis increases by 25% — 75%for the doubling CO2 concentration. When CO2 become excess for a plant, other parameters become a limiting factor. Light is one of them. When light is the limiting factor, the rate depends on the RUBP regeneration phase of the Calvin cycle. In light of excess conditions, the rate is dependent on RUBISCO concentration.
One of the direct results of CO2 accumulation is increasing global temperature. Temperature affects plants and photosynthesis in many different ways. Obeying to the Arrhenius relationship, the rate of photosynthesis increases with temperature until the temperature starts making conformational adjustments to the proteins.
Climate change is affected both temperate and tropical areas of the world. But studies have shown that plants that grow at lower temperatures (temperate and polar) have less responsiveness to CO2 fluctuations compared to plants that grow at higher temperatures. Plants that grow at a lower temperature may even start reducing the rate of photosynthesis with increasing CO2 concentration before plants grow at higher temperatures.
Even though various plants show different variations in temperature and CO2 concentration, these variations can be categorized into C3 and C4 variations.
C4 plants are designed to function in harsh high-temperature environments and adapted to maximize photosynthesis and minimize photorespiration. But considering the effects of temperature, CO2 concentration, and quantum yields of C3 and C4 plants, it is expected C3 plants will benefit more benefited from the predicted CO2 and temperature variation up to 2100.
With increasing CO2 concentrations, although RUBISCO and other dark reaction can keep up with the pace of evolving photosynthetic rates, plants need to evolve more robust Nitrogen and other important nutrient acquiring mechanisms.
Another possible limiting factor with increased CO2 levels is water. Water for a plant is almost all absorbed by the soil. And stomatal transpiration is crucial in water conductance. But experiments show that with elevated CO2 levels there will be a partial closure of stomata in leaves. This will effectively decrease the rate of transpiration. But, on the other hand, more sugar means more energy and raw material to increase root surface area that will increase the water uptake.
Since the industrial evolution, 30% of global CO2 output is recovered by ocean microalgae. So, when talking about climate change microalgae plays a significant role.
Figure 2 The artificial leaf constructed from an amorphous Si triple-junction solar cell
Studies show that in diatom RUBISCOs perform only 28–31% of their maximum capacity. Therefore, increased CO2 levels could be beneficial for them and will effectively increase the primary production. But the way different organisms respond varies. Therefore, with climate change, algal and microbial composition will change over time.
In the ocean, due to the high specific heat capacity of water, temperature fluctuations won’t be drastic as inland. But the increased temperature can change CO2 HCO3- equilibrium and high temperature always release dissolved gasses from a liquid. In addition, equilibrium constants of CO2 HCO3- equilibrium and HCO3- CO32- equilibrium will favor forewarned making more CO32-. CO32- cannot be absorbed by algae or microbes.
Another result of climate change is thinning of the ozone layer. The thin ozone layer lets UV radiation into the earth’s atmosphere and the ocean. UV rays can interfere with the light reaction. Particularly PSII action is interrupted by UV radiation. As a result, carbon concentrating mechanisms (CCMs) can get affected. Another study shows that CO2 and HCO3- transporters can also get interrupted by UV radiation.
Photosynthesis is the primary source of food for all the organisms in the world including humans. Therefore, any alteration to photosynthesis by climate change directly affects global food production. Studies have not shown any considerable increase of yield/fruit by increasing photosynthesis rate(RUBISCO carboxylation). Increasing leaf area has shown some improvements in yield (this is a result of increased mesophyll conductance).
Considering generation time of various plants, annual plants are expected to adapt faster to rapid climate change compared to perennials. But due to habitat loss and fragmentation, plant populations won’t have enough genetic variation to adapt for future changes.
Currently, photosynthesis is exclusive to green plants and algae. But if humans can mimic photosynthesis, it will solve all the implications caused by the excessivegreenhouse effect and climate change. artificial photosynthesis is an active research area. In this method scientists are not interested in making glucose from CO2. Rather they are more focused on making alcohols and other molecules that can replace fossil fuel as the energy source. There are many approaches to this. One is using semiconductors as photocatalysts, and another method is using live bacteria to produce alcohols and other desires molecules. The main focus of researchers is to figure out how to harvest sunlight to split water because sunlight is an inexhaustible source of energy.
The world population is at exponential growth and world energy and food demand follow the same pattern. This only means CO2 release will increase for the foreseeable future. The only mechanism known to mankind that can take CO2 from the atmosphere is photosynthesis. Instead of trying to mimic all the steps in photosynthesis, if the attention is paid only to fundamentals like light harvesting and converting CO2 to a usable form of compounds this can be economically profitable. It is the only viable way to successfully face inevitable climate change.
