Cooperative Institute for Research in Environmental Sciences

Ocean Interactions
Ocean and Atmosphere connecting Scientists, Teachers & Students

Phytoplankton and the ocean carbon cycle

This week's science focus is on phytoplankton and the ocean carbon cycle. Scientists from NASA are on board the R/V Ronald H. Brown to study how iron limits the growth of phytoplankton, and to determine a method for measuring carbon content in the ocean using satellites.

Ocean Carbon Cycle

The Earth's carbon cycle can be split into the ocean carbon cycle and the atmosphere carbon cycle. The carbon cycle in the ocean has an organic component and an inorganic component. The inorganic part acts as a pump of CO2 in and out of the ocean depending on the temperature of the water. The solubility of CO2 is larger in cold water than in warm water. So in areas of cold water, like the polar oceans, CO2 is being pumped from the atmosphere into the ocean, while in the warmer regions CO2 is being expelled into the atmosphere from the oceans. Did you know that the equatorial Pacific is the biggest single natural source of CO2 to the atmosphere? This is because cold water is brought to the surface in the eastern part of the equatorial Pacific ("upwelling") and then warmed by the sun, causing the CO2 to be released into the atmosphere. The organic part of the carbon cycle also acts as a pump of carbon to and from the atmosphere. The carbon in the water takes the form of CO2, bicarbonate, and carbonate. Plants like phytoplankton use the CO2 and bicarbonate to make food and energy through a process called photosynthesis. Carbon is used during photosynthesis and then released during respiration. These two processes produce a net of 50 gigatons of carbon, most of which goes back into CO2 and then is release to the atmosphere. Because a large portion of the carbon in the ocean is in the form of biomass (plants like phytoplankton), scientists are interested in studying various aspects of the biomass, such as phytoplankton. Global warming is a concern because it may change the CO2 pump and thus may affect the amount of biomass in the ocean. In order to understand how these effects may manifest themselves, scientists need to understand what limits phytoplankton growth.

Iron and How it Limits Phytoplankton Growth

Phytoplankton is a single cell plant and grows in the upper layers of the world's oceans because of its need for light. It uses light through a process called photosynthesis to create food and energy for itself. Recently, scientists like Dr. Mike Behrenfeld from NASA have found that phytoplankton do not grow as well in ocean regions that are low in iron. Why do phytoplankton need iron? Well, all living things need macronutrients and micronutrients in order to survive. Iron is a micronutrient (think of it as a vitamin supplement). We, for example, need iron to help our blood carry oxygen. Iron is important to phytoplankton because it acts as an electron carrier and a catalyst during photosynthesis. When there isn't enough iron around, then the phytoplankton can't grow. For example, if the water has 10 parts CO2, 10 parts oxygen, 5 parts nitrogen and 1 part iron, and the phytoplankton need 2 parts iron for photosynthesis, then the phytoplankton can't do photosynthesis and thus can't grow. If you think of it in terms of natural resources, iron is the first resource to run out.

Dr. Behrenfeld is using an instrument called a Fast Repetition Fluorometer (FRF) to study the phytoplankton. The FRF shines blue light at the plants and measures the red light given off (called fluorescence). The instruments measures the amount of light that is absorbed from one flash of light and the amount of light absorbed by lots of flashes. The difference between the two is the change in fluorescence and it tells how much energy is used to photosynthesize. Healthy cells will use more energy and fluoresce less than unhealthy cells. The measurements of healthy cells (iron rich) can then be compared to unhealthy cells (iron poor).

Measuring Carbon from Satellite

Scientists are currently focused on what kind of measurements can be made from satellite. Why use a satellite?. Because the oceans are so big, it is difficult (and very expensive!) to collect data that is representative of the entire ocean. Satellites can provide more global coverage for data collection than any combination of field experiments. The types of measurements made by satellite are, however, limited to optical signatures. So scientists need to be develop ways to derive their data from optical measurements. The next question is why are scientists like Dr. Behrenfeld trying to measure carbon content of the ocean from space? The amount of photosynthesis that is going on can be calculated by multiplying the growth rate by the carbon biomass. The growth rate is related to the amount of light used and the amount of chorophyl. The amount of chlorophyll can be measured from satellite from the color of the oceans. The greener the ocean is, the more chlorophyll is present. The amount of light used cannot be directly measured and is determined empirically. That leaves measurement of carbon.

Dr. Behrenfeld from NASA, with the assistance of Kirby Worthington, is determining a method from which the amount of carbon can be determined from satellite measurements. Concentration of biomass can be measured using light scattering techniques. An instrument called a beam transmisometers shines light through a sample of water. A detector at the opposite end of the light source measures the light coming through the sample. When the light passes through the water sample, some of it is absorbed and some of it is scattered toward the detector (called forward scattering). The difference between the amount of light sent to the sample and the amount detected can be used to determine how much biomass is in the sample. However, satellite can only measure the light that is sent back to their detectors, which is back scattered. Larger particle sizes can be measured from forward scattering than from back scattering. Dr. Behrenfeld is devising a way to determine the amount of forward scattering from back scattered measurements, from which he can then determine particle concentrations and hence carbon biomass.

Kirby Worthington (left) and Dr. Mike Behrenfeld (right) from NASA in their ship lab.