Figure 1. Deserts of the world and wind tracks that carry dust.
I'd like to give an example of how research questions can be formulated and pursued from my own experience.
Algae grow in natural waters until they are limited by light or nutrient availability. Algae are the primary producers in lakes and oceans, providing the basis for the food chain. In the oceans, algal growth limitation ultimately controls the yield of fish for human food. Most coastal areas are relatively productive compared to the middle ocean regions because mixing of the water brings nutrients to the surface where they can be used to support algal growth. Many cultures have grown up in coastal areas to take advantage of this resource.
To determine what factors limit algal growth, samples of water are taken and enriched with nutrients (nitrate, ammonium, phosphate, trace metals), exposed to higher light, or combinations of treatments. Through these type of experiments it was often found that the open ocean gyres (areas that spin around the middle of the Pacific and Atlantic Oceans) were often most limited by the availability of nitrogen.
Interestingly, lakes that become nitrogen limited, but have enough phosphate and light, often support nitrogen fixing cyanobacteria (blue-green algae). These algae are able to use light energy to make dissolved N2 gas into amines and then incorportate this into proteins and nucleotides. You might have seen lakes like this in Oregon, such as Klamath and Agency Lakes, Suttle Lake, Lake Oswego, and others. The high concentration of nitrogen-fixing alge form a bloom that can interfere with recreational uses and may degrade the water quality for drinking.
A nagging question in oceanography has been, if the oceans are nitrogen limited (such as suggested by the data) then why isn't there more nitrogen fixation? If nitorgen fixation can allow cyanobacteria to avoid nitrogen limitation in lakes, why not in the ocean?
At the time that I got intereseted in this question the accepted explanation was that nitrogen fixing cyanobacteria can't fix nitrogen effectively in the turbulent ocean water. The only cyanobacteria capable of fixing nitrogen grow in colonies. The colony structure is necessary to protect the biochemical process of nitrogen fixation from oxygen. Mixing stress from winds rips the colonies apart and decreases their ability to fix nitrogen.
I proposed another explanation, which was that in the open ocean, cyanobacteria would be limited by iron. Nitrogen fixation requires high iron nutrition, which if not available will eliminate the potential for nitrogen fixation to occur. Thus, most algae would be limited by nitrogen but the ones that could fix nitrogen would be limited by iron.
Iron atoms are involved in electron transfer reactions in cell biochemistry. Photosynthesis requires substantial iron and nitrogen fixation requires a similarly high cell iron content. Together, these requirements mean that nitrogen fixing cyanobacteria have a very high nutritional requirment for iron.
| Part of my explanation was that the main source of iron to the open ocean is from wind borne dust. This dust is eroded continental material, not what you find under your bed. The main sources of this dust are the desert regions; the Gobi, the Sahara, just about all of Austrailia (Figure 1). |
Figure 1. Deserts of the world and wind tracks that carry dust. |
I proposed to collect nitrogen fixing cyanobacteria from a portion of the ocean that was likely to be iron limited, add iron and then measure their photosynthetic rate and nitrogen fixation rate. As part of the same research, I would also track dust storms at the research site and examine the physiology of the cyanobacteria before and after wind had deposited dust.
The site I chose was Barbados. As the eastern most island in the Windward Islands it is bathed in ocean currents that come right off the middle of the North Tropical Atlantic (Figure 2 and Figure 3). As it turns out, there had been some research on dust deposition also. Some people set up huge collectors in Barbados, hoping to collect cosmic dust. Barbados is over a thousand kilometers down wind of any landmass, and they figured that it would be pristine air. Instead what they got periodically was large amounts of continental dust. Through high tech analysis, they determined that the dust was continental dust being carried by wind storms from the Sahara. Storms on the Sahara inject dust high into the atmosphere and these are carried to Barbados in four to five days.
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| Figure 2. The path of dust from the Sarhara to Barbados. | Figure 3. A satellite image of dust blowing off the Sahara. |
Darwin already knew about this transport of dust. When dust was all over the deck of the Beagle, he asked the crew and they explained that it came from the desert.
Dust storms in the Sahara and transport to the Caribbean seems to have several major events in March and April, so I would routinely set up in Barbados during March and have a student stay on into April to collect more samples )Figure 4). It beats going to the Arctic in Winter.
Figure 4. Seasonal dust impulses to Barbados. The peaks are in spring.
Individual colonies of Trichodesmium were collected by using SCUBA diving techniques. The colonies are about the size of a flake of sawdust (Figure 5). A syringe can be bored out to collect individual colonies (Figure 6). It takes about 90 minutes at 5 meters depth to collect enough colonies to do experiments (Figure 7).
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Figure 5. Different forms of colonies. All can be from about 3 to 10 mm in diameter. |
Figure 6. Collecting an individual colony. |
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Figure 7. Sharks eye view of divers. |
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Iron additions stimulated both photosynthesis and nitrogen fixation. Low levels of added iron increased the rate of both of these processes (figure 5). This shows that iron has an effect, but doesn't prove that cells in the ocean are limited by iron. After a dust storm, cellular paramters that usually depend on iron, are also increased. Certain chemical compounds in cells are higher in iron-limited cells than in iron-replete cells. This trend was consistent with iron limitation and reversal of iron limitation by the dust storm. |
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These results were in no way conclusive. Further research on this problem included the use of cultures and examination of other sites. We even used satellite data to try to make coorelations between dust rich wind and blooms of the cyanobacteria.
I am no longer doing work on Trichodesmium and iron status. My work in that area lead me to work on iron and nitrogen metabolism of other oceanic cyanobacteria. Then, that work lead me to interesting questions on how fast these organisms can adapt to light and nutrient changes. My current interest is concerned with what adaptation strategies will help cyanobacteria in fluctuating light environments.
September 27, 1999
