Compensation point

As defined above, the light compensation point Ic is when no net carbon assimilation occurs. At this point, the organism is neither consuming nor building biomass. The net gaseous exchange is also zero at this point. Ic is a practical value that can be reached during early mornings and early evenings. Respiration is relatively constant with regard to light, whereas photosynthesis depends on the intensity of sunlight. Depth 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. == CO2 compensation point ==

The CO2 compensation point (Γ) is the CO2 concentration at which the rate of photosynthesis exactly matches the rate of respiration. There is a significant difference in Γ between plants and plants: on land, the typical value for Γ in a plant ranges from 40–100 μmol/mol, while in plants the values are lower at 3–10 μmol/mol. Plants with a weaker CCM, such as C2 photosynthesis, may display an intermediate value at 25 μmol/mol. The μmol/mol unit may alternatively be expressed as the partial pressure of CO2 in pascals; for atmospheric conditions, 1μmol/mol = 1 ppm ≈ 0.1 Pa. For modeling of photosynthesis, the more important variable is the CO2 compensation point in the absence of mitochondrial respiration, also known as the CO2 photocompensation point (Γ*), the biochemical CO2 compensation point of Rubisco. It may be measured by whole-leaf isotopic gas exchange, or be estimated in the Laisk method using an intermediate "apparent" value of C* with correction. C* approximates Γ* in the absence of carbon refixation, i.e. carbon fixation from photorespiration products. In plants, both values are lower than their counterparts. In C2 plants that operate by refixation, only C* is significantly lower. As it is not yet common to routinely change the concentration of air, the concentration points are largely theoretical derived from modeling and extrapolation, though they do hold up well in these applications. Both Γ and Γ* are linearly related to the partial pressure of oxygen (p(O2)) due to the side reaction of Rubisco. Γ is also related to temperature due to the temperature-dependence of respiration rates. It is also related to irradiation, as light is required to produce RuBP (ribulose-1,5-bisphosphate), the electron acceptor for Rubisco. At normal irradiation, there would almost always be enough RuBP; but at low irradiation, lack of RuBP decreases the photosynthetic activity and therefore affects Γ. ==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 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. ==See also==