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The (light) compensation point is the light intensity on the light curve where the rate of photosynthesis exactly matches the rate of cellular respiration. At this point, the uptake of CO2 through photosynthetic pathways is equal to the respiratory release of carbon dioxide, and the uptake of O2 by respiration is equal to the photosynthetic release of oxygen.
In assimilation terms, at the compensation point, the net carbon dioxide assimilation is zero. Leaves release CO2 by photorespiration and cellular respiration, but CO2 is also converted into carbohydrate by photosynthesis. Assimilation is therefore the difference in the rate of these processes. At a normal partial pressure of CO2 (0.343 hPa in 1980), there is an irradiation at which the net assimilation of CO2 is zero. For instance, in the early morning and late evenings, the compensation point may be reached as photosynthetic activity decreases and respiration increases. Therefore, the partial pressure of CO2 at the compensation point, also known as gamma, is a function of irradiation. The irradiation dependence of the compensation point is explained by the RuBP (ribulose-1,5-bisphosphate) concentration. When the acceptor RuBP is in saturated concentration, gamma is independent of irradiation. However at low irradiation, only a small fraction of the sites on RuBP carboxylase-oxygenase (RuBisCO) have the electron acceptor RuBP. This decreases the photosynthetic activity and therefore affects gamma. The intracellular concentration of CO2 affects the rates of photosynthesis and photorespiration. Higher CO2 concentrations favour photosynthesis whereas low CO2 concentrations favor photorespiration.
The compensation point is reached during early mornings and late evenings. Respiration is relatively constant, whereas photosynthesis depends on the intensity of sunlight. When the rate of photosynthesis equals the rate of respiration or photorespiration, the compensation point occurs.
At the compensation point, the rate of photosynthesis is equal to the rate of respiration. Products of photosynthesis are used up in respiration so that the organism is neither consuming nor building biomass. The net gaseous exchange is also zero at this point.
For aquatic plants where the level of light at any given depth is roughly constant for most of the day, the compensation point is the depth at which light penetrating the water creates the same balanced effect.
The marine environment
Respiration occurs by both plants and animals throughout the water column, resulting in the destruction, or usage, of organic matter, but photosynthesis can only take place via photosynthetic algae in the presence of light, nutrients and CO2. In well-mixed water columns plankton are evenly distributed, but a net production only occurs above the compensation depth. Below the compensation depth there is a net loss of organic matter. The total population of photosynthetic organisms cannot increase if the loss exceeds the net production.
The compensation depth between photosynthesis and respiration of phytoplankton in the ocean must be dependent on some factors: the illumination at the surface, the transparency of the water, the biological character of the plankton present, and the temperature. The compensation point was found nearer to the surface as you move closer to the coast. It is also lower in the winter seasons in the Baltic Sea according to a study that examined the compensation point of multiple photosynthetic species. The blue portion of the visible spectrum, between 455 and 495 nanometers, dominates light at the compensation depth.
A concern regarding the concept of the compensation point is it assumes that phytoplankton remain at a fixed depth throughout a 24-hour period (time frame in which compensation depth is measured), but phytoplankton experience displacement due to isopycnals moving them tens of meters.
- ESRL / Mauna Loa CO2 annual mean data, , 
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