Primary production

Primary production is the production of organic compounds from inorganic materials principally through the process of photosynthesis (though chemosynthesis also plays a role). The organisms responsible for primary production are known as primary producers or autotrophs, and form the base of the food chain. In terrestrial ecosystems, production is typically driven by plants, while in oceanic ecosystems, algae are generally responsible.

Overview

At the most fundamental level, primary production is the capture and storage of energy by living organisms. The source of this energy is mostly electromagnetic radiation (as photons) from the Sun. A much smaller fraction of production is driven by organisms utilising chemical energy.

Regardless of its source, this energy is used to synthesise complex organic molecules from simpler inorganic compounds such as carbon dioxide (CO₂) and water (H₂O). The following two equations are simplified representations of photosynthesis (top) and (one form of) chemosynthesis (bottom) :

CO₂ + H₂O + light

\rightarrow

CH₂O + O₂

CO₂ + O₂ + 4 H₂S

\rightarrow

CH₂O + 4 S + 3 H₂O

In both cases, the end point is reduced carbohydrate (CH₂O), typically molecules such as glucose or other sugars. These relatively simple molecules may be then used to synthesise further more complicated molecules such as proteins and DNA, or be respired to perform work. Consumption of primary producers by heterotrophic organisms, such as animals, transfers these organic molecules (and the energy stored within them) up the food chain, fuelling all of the Earth's living systems.

Terrestrial production

On the land, almost all primary production is performed by higher plants, although a small fraction is the result of activity by algae. Higher plants include familiar forms such as trees, flowers and grasses, ubiquitous in the modern environment, and also ferns and mosses, groups that have played larger roles earlier in Earth's history.

Primary production on land is a function of many factors, but principally local temperature and hydrology. While plants are able to colonise most of the Earth's surface, plant biomass is strongly curtailed in regions such as deserts that are too hot or too cold, or where water availability is limited (which often coincides with temperature extremes).

Water is "consumed" in plants by the processes of photosynthesis (see above) and transpiration. The latter process (which is responsible for about 90% of water use) is driven by the evaporation of water from the leaves of plants. It allows plants to transport water and mineral nutrients from the soil to growth regions, and also cools a plant down. It can be regulated by structures known as stomata, but these also regulate the supply of carbon dioxide from the atmosphere, so that decreasing water loss also decreases carbon dioxide gain. Crassulacean acid metabolism (CAM) and C4 plants use physiological and anatomical workarounds to increase their water-use efficiency and allow increased primary production to take place under conditions that would limit "normal" C3 plants (the majority).

Oceanic production

In a reversal of the pattern on land, in the oceans, almost all primary production is performed by algae, with a small fraction contributed by vascular plants and other groups. Algae encompass a diverse range of organisms, ranging from single floating cells to attached seaweeds. They include photoautotrophs from a variety of groups: prokaryotic bacteria (both eubacteria and archaea); and three eukaryote categories the green, brown and red algae. Vascular plants are represented in the ocean by groups such as the seagrasses.

In another departure from the situation on land, the majority of primary production in the ocean is performed by microscopic organisms, the phytoplankton. Larger autotrophs, such as the seagrasses and macroalgal seaweeds are generally confined to the littoral zone and adjacent shallow waters, where they can attach to the underlying substrate but still be within the photic zone. There are exceptions, such as Sargassum, but the vast majority of free-floating production takes place within microscopic organisms.

The factors limiting primary production in the ocean are also very different from those on land. The availability of water, obviously, is not an issue (though its salinity can be). Similarly, temperature, while affecting metabolic rates (see Q₁₀), ranges less widely in the ocean than on land because the heat capacity of seawater buffers temperature changes, and the formation of sea ice insulates it at lower temperatures. However, the availability of light, the source of energy for photosynthesis, and nutrients, the building blocks for new growth, play crucial roles in regulating primary production in the ocean.

The sunlit zone of the ocean is called the photic zone (or euphotic zone). This is a relatively thin layer (10-100 m) near the ocean's surface where there is sufficient light for photosynthesis to occur. For practical purposes, the thickness of the photic zone is typically defined by the depth at which light reaches 1% of its surface value. Light is attenuated down the water column by its absorption or scattering by the water itself, and by dissolved or particulate material within it (for instance, phytoplankton).

Net photosynthesis in the water column is determined by the interaction between the photic zone and the mixed layer. Turbulent mixing by wind energy at the ocean's surface homogenises the water column vertically until the turbulence dissipates (creating the aforementioned mixed layer). The deeper the mixed layer, the lower the average amount of light intercepted by phytoplankton within it. This can vary from being shallower than the photic zone, to being much deeper than the photic zone. When it is much deeper than the photic zone, this results in phytoplankton spending too much time in the dark for growth to occur. The maximum depth of the mixed layer in which growth can occur in the phytoplankton population is called the critical depth. Assuming there are adequate nutrients available, net primary production occurs when the mixed layer is shallower than the critical depth. Depending on the region of the ocean, primary production due to phytoplankton can change on a variety of time scales. In places like the North Atlantic, with a highly seasonal wind field, the mixed layer in winter can far exceed the critical depth. Consequently, primary production in the North Atlantic is highly seasonal, varying with both incident light at the water's surface (reduced in winter) and the degree of mixing (increased in winter). In tropical ocean regions, such as the gyre in the middle of the Atlantic Ocean, the mixing may occur more episodically, such as after a large hurricane.

Between mixing events, the phytoplankton are using up the nutrients in the mixed layer. Phytoplankton may also grow below the mixed layer where there are often more nutrients, if there is enough light. However, there is a slight tendency for phytoplankton cells to sink due to gravity. There is also a tendency for faecal pellets to sink, after the phytoplankton are eaten by zooplankton (animal plankton). So over time, the growth of the phytoplankton through photosynthesis takes up the nutrients from the surface layers within the photic zone, and the sinking of phytoplankton cells and feacal pellets then takes some of these nutrients down, out of the photic zone. This has a significant impact on the global carbon cycle. Inorganic nutrients, such as nitrate, phosphate and silicic acid are necessary for phytoplankton to synthesise their cells and cellular machinery. If one nutrient runs out, such as nitrate, primary production goes down dramatically. This effect occurs in summer in the North Atlantic, after the storms abate, and the sun warms the surface layer. Gradually the water within the photic zone stratifies. Stratification strengthens the thermocline, the region where the ocean temperature changes rapidly from the mixed layer temperature to the temperature of the deeper water. The strong thermocline makes it harder for the wind to deepen the mixed layer. The overall effect of the stratification and the lack of wind during the summer means that nutrients are being exported from the surface layer as the phytoplankton sink, but may not be replenished until the mixed layer deepens again in the fall. Other processes, such as upwelling of deep water also bring deep water to the surface and replenish the nutrient supply. Regions where upwelling occurs are usually marked by higher primary production in the ocean.

Another important factor in oceanic primary production for regions far from land is iron. Iron is a micronutrient that is more important for some groups of phytoplankton than others. A major source of iron is desert dust, delivered by the wind, as eolian dust ... so for regions of the ocean that are very far from the nearest desert, the lack of iron can limit the amount of primary production that can occur. These areas are sometimes known as HNLC (High-Nutrient Low-Chlorophyll) zones.

Measurement

Terrestrial

Aquatic

In aquatic systems, primary production is typically measured using one of three main techniques :

variations in oxygen concentration within a sealed bottle (used since 1927)
the Steeman-Neilsen (1952) technique of incorporation of carbon-14 into biomass (where ¹⁴C typically replaces ¹²C in molecules of sodium bicarbonate)
modulated in vivo fluorescence (a modern technique still under development)

The most commonly used technique is that of ¹⁴C incorporation. As ¹⁴C is radioactive (via beta decay), it is relatively straightforward to measure its incorporation in organic material using devices such as scintillation counters.

Depending upon the incubation time chosen, net or gross primary production can be estimated. Gross primary production is best estimated using relatively short incubation times (1 hour or less), since the loss of incorporated ¹⁴C (by respiration and organic material excretion / exudation) will be more limited. Net primary production is the fraction of gross production remaining after these loss processes have consumed some of the fixed carbon.

Loss processes can range between 10-60% of incorporated ¹⁴C according to the incubation period, ambient environmental conditions (especially temperature) and the experimental species used. Aside from those caused by the physiology of the experimental subject itself, potential losses due to the activity of consumers also need to be considered. This is particularly true in experiments making use of natural assemblages of microscopic autotrophs, where it is not possible to isolate them from their consumers.

Global

As primary production in the biosphere is an important part of the carbon cycle, estimating it at the global scale is important in Earth system science. However, quantifying primary production at this scale is difficult because of the range of habitats on Earth, and because of the impact of weather events (availability of sunlight, water) on its variability.

Using satellite-derived estimates of the normalised difference vegetation index (NDVI) for terrestrial habitats and sea-surface chlorophyll for the oceans, Field, Behrenfeld, Randerson & Falkowski (1998) estimated that the total (photoautotrophic) primary production for the Earth was 104.9 Gt of carbon per year. Of this, slightly more than half, 56.4 Gt C (53.8%), was the product of terrestrial organisms, while the remainder, 48.5 Gt, was accounted for by oceanic production.

In areal terms, they estimated that land production was approximately 426 g C m^-2 y^-1 (excluding areas with permanent ice cover), while that for the oceans was 140 g C m^-2 y^-1. Another significant difference between the land and the oceans lies in their standing stocks - while accounting for almost half of total production, oceanic autotrophs only account for about 0.2% of the total biomass.

References

Behrenfeld, M. J. et al. (2001) Biospheric Primary Production During an ENSO Transition. Science 291, 2594-2597.

Field, C. B., Behrenfeld, M. J., Randerson, J. T. and Falkowski, P. (1998) Primary production of the Biosphere: Integrating Terrestrial and Oceanic Components. Science 281, 237-240.

Steeman-Nielsen, E. (1952). The use of radioactive carbon (C14) for measuring organic production in the sea. J. Cons. Int. Explor. Mer. 18, 117-140.

External links

Ecology | Photosynthesis

Gourt Home::Gourt :: Primary production