Photosynthesis
Primary production supports all food webs, controls the atmosphere, and has produced a series of marine ecosystems cumulatively worth $400 billion - therefore, photosynthesis is a major control over socio-economics. Photosynthesis is the coupling of the conversion of light energy to chemical energy and inorganic carbon to organic carbon. Simple, dissolved organic compounds may be converted in to more complex substances via photo-assimilation.
Rates of primary production are greatest in the coral reefs due to the endosymbiotic relationship between animal and algae - the reefs themselves actually grow in oligotrophic waters. Seagrass beds also have a strong production rate, aided by epiphyte macroalgae that grows on the shoots. The Antarctic Ocean has very strong vertical mixing, causing a nutrient upwelling that significantly boosts productivity.
Light of 400-700nm wavelength powers the essential first stage of photosynthesis, and is absorbed by photosythetic pigments such as chlorophyll a - this substance exhibits peak absorption of light wavelengths of 670 - 695nm. This photosynthetic fixation generates the organic compounds of the sea, and the gross production will be broken down by the respiratory metabolism of the photosynthetic organism, incorporated in to its tissues and fluids, or constitute growth.
Light-dependant reaction:
As solar energy hits a chloroplast, the electrons become excited, and will exit the chloroplast in to the membrane. There they will be taken up by the electron transport chain, and participate in a series of redox (reduction oxidation) reactions, producing energy. The final acceptor for the electron transport chain (ETC) is an NADP molecule, which then becomes reduced, usable for the light-independent reaction. Water is required in this process, and undergoes hydrolysis; the H+ is utilised by the chloroplast, the oxygen molecules are by-products. Light-independent reaction (Calvin Cycle):
This involves constant regeneration, and is activated by RUBSICO (ribulose biphosphate carboxylase/oxygenase), an enzyme that may be bound with carbon or oxygen: for this reason a competitive state exists between the oxygen and carbon dioxide connection. However, with 45x less oxygen in the ocean than in air, the carboxylase function dominates, which is much more efficient. The five ribulose biphosphate carbon molecules are therefore synthesised with two carbon molecules to produce phosphoglycerate. ATP molecules donate one phosphate molecule each to the PG in order to form two biphosphoglycerate chains. Reduced NADP then donates H+ in order to reduce the substance and produce glyceraldehyde-3-phosphate. This is then broken down in to organic compounds, glucose and partly regenerated for continuous use in the cycle. Limiting Factors
Light
Longer wavelengths preferentially excite the photosystem: it is absorbed exponentially with distance in the water column, and much of the solar radiation is in the infrared section of the spectrum, absorbed in the first 2m of a water body. The photocompensation depth describes the point at which there is no net photosynthesis - respiration balances it out. Above this depth, phytoplankton may grow and multiply; belo they must subsist on accumulated reserves from inactive resting bodies, or starve. In high intensity light, an overflow of photo-oxidation and the destructive impact of UV radiation accounts for a decline in photosynthesis. On the water surface, excess light may result in photoinhibition, whereas at depth, light intensity will be a limiting factor.
Turbulence
The compensation depth is a balance point in the metabolic physiology of a phytoplanktonic cell.Wind-induced turbulence may extend to depths of 200m, while the photic zone may be much more shallow. The critical depth describes (in terms of irradiance) the depth where light intensity is equal to the compensation light intensity. Hence, if the depth to which mixing takes place is higher in the water column than the critical depth, then the average light intensity experienced by a photosynthetic organism will be less than the photocompensation depth. Mixing often extends below the critical depth during the winter in high latitudes, causing a negative production until mixing decreases as with the wind velocity, or light intensity increases with the spring/summer seasons.
Nutrients
If the mixing depth is less than the euphotic depth, then nutrients will be the limiting factor to photosynthesis. The primary macronutrients are nitrogen, phosphorous and potassium. Nitrogen is necessary for the formation of amino acids, cell division, and it is directly involved in photosynthesis. Phosphorous promotes early root formation and growth, increases water-use efficiency, and is involved in almost all metabolic processes. Potassium increases photosynthesis, and is essential to protein synthesis; it also activates enzymes, controls their reaction rates, and directs the carbohydrate metabolism. Nutrients are known to be depleted during warm-season thermoclines, and production decreases with it. When nutrients are enriched artificially, such as with effluent runoff in to a semi-enclosed bay, productivity is increased, sometimes to the point of eutrophication.
Measurement
Traditional measurements of primary productivity and biomass often involve destructive sampling or are intrusive to the system. Recently, fluorescence measurements have been taken from chlorophyll a, in order to measure the light energy emitted from the light harvesting pigments. Productivity may also be measured by recording oxygen evolution with an electrode or Winkler titration; carbon -14 uptake may also be measured as a method of determining productivity.
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