Word Wednesday: Primary Productivity

We have chatted before about food chains and food webs and you may recall learning about the process of photosynthesis. When scientists consider ecosystems, you will more often hear the term primary production. What is the link between these terms? Are they the same thing? Why do scientists measure it?

Photosynthesis is the process by which light energy is used to power chemical reactions that convert carbon dioxide (CO₂) to carbohydrates. You may have seen the general chemical equation for photosynthesis:

CO₂ + H₂0 → C₆H₁₂O₆ + O₂ + H₂O

Here light energy, typically supplied by the sun (but those of you with indoor plants may have used a grow light), is used to split a water molecule into 2 hydrogen and 1 oxygen. The energy from that action is then used to create a carbohydrate, in this case, the sugar glucose from carbon dioxide (what you breathe out when you exhale). Scientists also refer to this process as carbon fixation as an inorganic form of carbon has been converted to (fixed into) an organic molecule that contains carbon.

Photosynthesis is the process forming the base of the food chain. (Let’s ignore chemosynthesis for now, we’ll get to that a little later.) Photosynthetic organisms are the only organisms capable of taking an inorganic form of carbon and, using an energy source, making food. Other organisms are dependent on this ‘manufactured’ food, so the amount of photosynthesis determines the amount of food that is available to all other organisms in the food chain or web. The amount of food made over time is called primary production. Put another way, primary production is the amount of carbon fixed over a period of time measured for a specific area. It is primary as it is the base of the food chain and it is production as organic matter is produced from inorganic matter. Primary production is typically reported as grams of carbon per some unit of time, such as g carbon per day per square meter. If we talk about the organisms that eat, that is consume, this primary production, we can use the term secondary production. This is the amount of new flesh (biomass in scientific lingo) created, either by organisms growing larger or heavier, or from making new organisms by reproducing. The higher the consumer is in the food chain, the higher the level of production (for example: tertiary (3rd), 4th or 5th levels).

And what about chemosynthesis? This process is analogous to photosynthesis in that chemosynthetic organisms use an inorganic form of carbon to make organic matter, but in contrast, they use energy stored in chemicals and not solar energy. Chemosynthesis occurs in many habitats, but can only be done by several groups of bacteria. For many of us, chemosynthesis came to our attention after the discovery of the hydrothermal vents. We have learned that in these ecosystems, the amount of primary production by chemosynthesis can be significant enough to form the base of an entire food chain.

Due to the cellular machinery needed, photosynthesis can only be done by some organisms. Certainly, it can be done by all plants, the organisms we call algae or seaweed, and some microplankton but can also be carried out by some bacteria. In the ocean, the most important photosynthesizers are phytoplankton, the microscopic algae found within the sunlit zone of the water column. In coastal aquatic environments, marsh grasses, mangroves and seagrasses are important groups of photosynthesizers. In some environments, microscopic, photosynthetic algae, called microphytobenthos, growing on surfaces, like mudflats and sandflats, are an important primary producer.

ACER scientists are measuring primary production by several groups of photosynthetic organisms as one way to assess impacts of the Deepwater Horizon oil spill on coastal ecosystems. Dr. Just Cebrian of the Wetlands group is measuring primary production by marsh grasses and mangroves in the Chandeleur Islands and in mesocosms exposed to oil. The Microplankton group, lead by Dr. Jeffrey Krause, is looking at rates of photosynthesis by the microplankton and how they are affected by oil and dispersant in the water. The microbes group, lead by Dr. Alison Robertson is investigating primary production by the microphytobenthos. Thus, ACER research groups are investigating the impacts of the spill on the taxonomic diversity, but they are also examining impacts on ecosystem function. Primary production is one aspect of this ecosystem function.

Come back for our next Word Wednesday to learn more about food webs and control of production in our next blog on bottom-up and top-down control of production.