Photosynthesis is the synthesis of nutrients from carbon dioxide (CO2) and water driven by sunlight. pH is a measure of hydrogen ion concentration in a solution. It might not seem so obvious at first, but there are actually certain circumstances where a change in pH can tell you something important about photosynthesis.
Take Figure 1 as an example. In this figure, we have 2 sets of measurements of pH with one set of measurement made in sunlight (red line) and the second set of measurements made in the dark (blue line). The measurements commence at a distance of 1mm above a coral species, and are continued at incremental steps of 100μm down towards the coral tissue, through the coral tissue and then ceases just above the surface of the coral skeleton.
In sunlight, it is observed that pH decreases (the water becomes more alkaline) as the pH sensors nears and enters the photosynthetically active coral tissue. In the dark, however, pH increases (the water becomes less alkaline or more acidic).
Photosynthetically active tissues uptake CO2 and release oxygen (O2), whereas in the dark the opposite occurs. The tissue, in the dark, is undergoing respiration so it uptakes O2 and releases CO2.
CO2 dissolves in water to form carbonic acid that, in turn, dissociates into biocarbonate and hydrogen ions. As we know, pH is a measure of hydrogen ion concentration in a solution. Therefore, as CO2 reacts in water there will be a related increase or decrease in pH due to the increase or decrease in hydrogen ion concentration. By measuring pH in water near photosynthesizing organisms, we can indirectly assess whether photosynthesis is taking place.
Figure 2 demonstrates the relationship between pH, O2 and photosynthesis. The data presented in Figure 2 has been adapted from Kuhl et al (1995) and their profile measurement above the coral species, Acropora, measured in sunlight. Similar to the data presented in Figure 1, measurements commenced at a small distance above the coral and moved at incremental steps towards, and through, the coral tissue and ceased just above the coral skeleton.
The red line in Figure 2 is pH, the blue line is O2, and the orange line is the rate of gross photosynthesis. As the sensors pierce the coral tissue, there is a change in pH, O2 and the rate of gross photosynthesis. As the sensors penetrate further into the coral tissue, there are less active photosynthetic tissue and the rate of gross photosynthesis decreases. There is also a further change in pH and O2.
This type of data is useful in demonstrating photosynthesis in underwater environments. Unfortunately, changes in pH cannot be used to calculate gross or net photosynthesis, but it can be used to show the spatial and temporal patterns of photosynthesis.
instruments to measure photosynthesis
Photosynthesising organs “inhale” CO2 and “exhale” oxygen (O2). Therefore, the more common methods of measuring the rate of photosynthesis measure changes in either CO2 or O2. In terrestrial plants, measuring changes in CO2 can be achieved with a photosynthesis meter, such as the Phyto-Sensor PTM-48A Photosynthesis Meter. A respiration system, such as the Unisense MicroRespiration System, can be used to measure O2 as photosynthesis or respiration.
Under water, scientists can also measure O2 to detail the rate of photosynthesis in species such as seagrass, algae and coral (zooxanthellae). Pedersen et al (2013) extensively review a number of techniques and methodologies on how to measure O2 under water for photosynthesis.
The accurate measurement of CO2 under water is difficult. The range of dissolved CO2 sensors manufactured by companies such as Pro-Oceanus and Franatech cannot, unfortunately, be used to measure photosynthesis. Although these sensors are very accurate at measuring dissolved CO2, their response time is too slow to measure rapid changes in CO2 that occurs with photosynthesis. For example, a T90 response time can be in the order of 3 to 5 minutes whereas photosynthesis can change in the order of seconds.
Therefore, scientists rely on pH microelectrodes to infer changes in CO2 caused by photosynthesis. pH microelectrodes have a very rapid response time and are extremely small in size (approximately 10μm to 500μm in diameter). The pH microelectrodes can be deployed in either the lab or field for CO2 experiments.
Kuhl et al 1995. Microenvironment and photosynthesis of zooxanthellae in scleractinian corals studied with microsensors for 02, pH and light. Marine Ecology Progress Series, 117. 159-172.
Pedersen et al 2013. Underwater Photosynthesis of Submerged Plants – Recent Advances and Methods. Frontiers in Plant Science, 4, 140. DOI: 10.3389/fpls.2013.00140
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