Compensation point

The (light) compensation point is the amount of light intensity on the light curve where the rate of photosynthesis exactly matches the rate of respiration. At this point, the uptake of CO2 through photosynthetic pathways is exactly matched to the respiratory release of carbon dioxide, and the uptake of O2 by respiration is exactly matched to the photosynthetic release of oxygen.

In assimilation terms, at compensation point, the net carbon dioxide assimilation is zero. Leaves release CO2 by photorespiration and day 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 (338mbar 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 enzyme (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 photo respiration. At higher carbon dioxide concentrations, the photosynthesis rate is higher, while at low CO2 concentrations, photo respiration is higher.[1]

Timing

The compensation point is reached during early mornings and late evenings. Respiration is relatively constant, whereas photosynthesis depends on the amount of sunlight. When the rate of photosynthesis equals the rate of respiration, the compensation point occurs.

Rate

At the compensation point, the rate of photosynthesis is balanced to the rate of respiration. Products of photosynthesis are used up in respiration so that the plant is neither consuming nor building biomass.

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.

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.[2] In well-mixed water columns plankton organisms 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, therefore the total population of photosynthetic organisms cannot increase if the loss exceeds the net production [3][4]

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.[5] The compensation point was found nearer to the surface as you move closer to the coast [6] 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.[7] The blue portion of the visible spectrum, between 455-495 nanometers, dominates light at the compensation depth.

A concern regarding the concept of 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.[8]

See also

References

  1. ↑ O.L, Lang. Physiological plat ecology II,water relations and carbon assimilation. New York: Springer-Verlag. pp. 556–558.
  2. ↑ Sverdrup, H.U. (1953). "On Conditions of the Vernal Blooming of Phytoplankton". Journal du Conseil. 18.3: 287–295.
  3. ↑ Gran, H.H.; Braarud, T. (1935). "A quantitative study of the phytoplankton in the Bay of Fundy and the Gulf of Maine (including observations on hydrography, chemistry and turbidity).". Journal of the Biological Board of Canada. 1.5: 279–467.
  4. ↑ Sverdrup, H.U. (1953). "On Conditions of the Vernal Blooming of Phytoplankton". Journal du Conseil. 18.3: 287–295.
  5. ↑ Gran, H.H.; Braarud, T. (1935). "A quantitative study of the phytoplankton in the Bay of Fundy and the Gulf of Maine (including observations on hydrography, chemistry and turbidity).". Journal of the Biological Board of Canada. 1.5: 279–467.
  6. ↑ Gran, H.H.; Braarud, T. (1935). "A quantitative study of the phytoplankton in the Bay of Fundy and the Gulf of Maine (including observations on hydrography, chemistry and turbidity).". Journal of the Biological Board of Canada. 1.5: 279–467.
  7. ↑ King, R.J.; Schramm, W. (1976). "Photosynthetic rates of benthic marine algae in relation to light intensity and seasonal variations.". Marine Biology. 37 (3): 215–222.
  8. ↑ Laws, E.A.; Letelier, R.M.; Karl, D.M (2014). "Estimating the compensation irradiance in the ocean: The importance of accounting for non-photosynthetic uptake of inorganic carbon". Deep-Sea Research 1. 93: 35–40.
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