Photosynthesis nutrient limitation

For example, in the carbon cycle consider the balance between terrestrial photosynthesis and respiration-decay. If the respiration and decay flux to the atmosphere were doubled (perhaps by a temperature increase) from about 5200 x 1012 to 10,400 x 1012 moles y-l, and photosynthesis remained constant, the CO2 content of the atmosphere would be doubled in about 12 years. If the reverse occurred, and photosynthesis were doubled, while respiration and decay remained constant, the CO2 content of the atmosphere would be halved in about the same time. An effective and rapid feedback mechanism is necessary to prevent such excursions, although they have occurred in the geologic past. On a short time scale (hundreds of years or less), the feedbacks involve the ocean and terrestrial biota. As was shown in Chapter 4, an increase in atmospheric CO2 leads to an increase in the uptake of CO2 in the ocean. Also, an initial increase in atmospheric CO2 could lead to fertilization of those terrestrial plants which are not nutrient limited, provided there is sufficient water, removal of CO2, and growth of the terrestrial biosphere. Thus, both of the aforementioned processes are feedback mechanisms that can operate in a positive or negative sense. An increased rate of photosynthesis would deplete atmospheric CO2, which would in turn decrease photosynthesis and increase the oceanic evasion rate of CO2, leading to a rise in atmospheric CO2 content. More will be said later about feedback mechanisms in the carbon system. 

B. R. Forsberg (unpublished data) provides further evidence for nutrient limitation (Table 14.2). Light saturated rates of photosynthesis (Pmax) were found to vary as a linear function of either total nitrogen (TN) or total phosphorus (TN) concentration. TP was the main source of variation in Pmax in black- and Clearwater lakes at both high and low water. This indicates a consistent pattern of P-limitation in these systems, which was linked to the high TN to TP ratios. In whitewater lakes, Pmax varied as a function of TP at high water and TN at low water. This reflects a shift from P-limitation to N-limitation and was accompanied by a decrease in average TN to TP ratios. 

Extracellular release is now well established as a part of the primary production. Rapidly growing phytoplankton releases 2-10% (PER) in most cases, increasing to 10-60% in the stationary phase of growth mainly because of a lower photosynthetic rate. There is increasing evidence that the absolute rate of release is highest in the exponential phase of growth. Nutrient limitation has been shown to increase the relative rate of release and nntrient ratios, i. e. N/P, also seems to be of importance. Extracellular release is relatively imaffected by irra-diance but PER is correlated to the relative inhibition of photosynthesis at high irradiances. 

Since photosynthesis is integrated into virtually all metabolie pathways in photoautrophs, through either the requirement of carbon skeletons, reducing power or ATP, nutrient limitations may induce a feedback... 

There is further scope for these types of experiments. One approach likely to be of value is to examine the interactive effects of a combination of stressors. For example, one might observe the interaction of a nutrient limitation with the exposure of cells to ultraviolet radiation (UVR). Because UVR inhibits photosynthesis we might expect to see different responses to the effects of nutrient limitation, for instance, on the rates of synthesis and relative concentration of macromolecules. 

The possible effects of increased atmospheric CO2 on photosynthesis are reviewed by Goud-riaan and Ajtay (1979) and Rosenberg (1981). Increasing CO2 in a controlled environment (i.e., greenhouse) increases the assimilation rate of some plants, however, the anthropogenic fertilization of the atmosphere with CO2 is probably unable to induce much of this effect since most plants in natural ecosystems are growth limited by other environmental factors, notably light, temperature, water, and nutrients. 

C02 is not the only factor involved in photosynthesis, so that for its use, other factors must be at levels that do not limit the process. Light, temperature, amount of available nutrients and the relative humidity are other environmental factors affecting photosynthetic activity. 

The rate of photosynthesis at depth z depends on water temperature Tw, concentration of nutrient salts n, and phytoplankton biomass p, as well as on other factors, which are not considered here. To express this dependence, various equations are used, which reflect the limiting role of elements E, n, and p. Considering that dp Ip <)z > 0 at n > 0 as dp/pdz —> const with increasing n, let us take the following function as the basic one to describe photosynthesis intensity at depth z ... 

In Equation 8.27, Vmax and, to some extent, Kcch depend on the photosynthetic photon flux (PPF), temperature, and nutrient status. For instance, Vmax is zero in the dark because photosynthesis ceases then, and it is directly proportional to PPF up to about 50 jimol m-2 s-1. If we continually increase the PPF, Fmax can reach an upper limit, its value for light saturation. This usually occurs at about 600 junol m-2 s-1 for most C3 plants, whereas photosynthesis for C4 plants is generally not light saturated even at full sunlight, 2000 pmol m-2 s-1 (see Chapter 6, Section 6.3D for comments on C3 and C4 plants also see Fig. 8-20 for responses of leaves of C3 plants and a C4 plant to PPF). Photosynthesis is maximal at certain temperatures, often from 30°C to 40° C. We note that Vmax increases as the leaf temperature is raised to the optimum and then decreases with a further increase in temperature. 

Littler, M. M., Littler, D. S., and Lapointe, B. E. (1988). A comparison of nutrient and light-limited photosynthesis in psammophytic versus epilithic forms of Halimeda caulerpales Halimedaceae from the Bahamas. Coral Reefs 6, 219—226. 

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