Then, in a paper that just appeared a few issues ago in the journal Nature, we discovered novel picoeucaryotic microorganisms in the marine plankton, and, of course, this leads to the key question, "are there additional key discoveries yet to be made?" This reminds me of the old Arlo Guthrie song about the Nixon-Watergate era. "If he didn't know about that one, then what else don't he know?" Well, this brief history of oceanic microbes tells us a little bit about our level of ignorance of biological oceanography.

imageWhy is it so important to know about microorganisms? Well, it turns out that microbes have a fundamental role in global ecology. They control the production and consumption of organic matter, as Dr. Chisholm just alluded to; they also control oxygen levels, pH and redox levels in the sea. Most of the chemical reactions in the ocean are poised by marine microorganisms. They also produce and consume greenhouse gases, and they make available, or unavailable, various forms of nitrogen.

imageNow let's talk about the ocean's carbon cycle. The carbon cycle is an intersection between physical and biological processes, between gases like CO2 and organic matter, both in the dissolved and particulate form.

We have a large and ever-increasing reservoir of CO2 in the atmosphere. This reservoir is in equilibrium with the surface ocean under the control of gas laws and habitat characteristics (e.g., solubilities, wind speed, etc.). Major circulation processes redistribute inorganic carbon around the world's oceans.

Intersecting this inorganic, physical system is the organic carbon system, which I'd like to focus on today, the so-called biological pump. This complex system begins with the production of organic matter via the process of photosynthesis, and the removal of the unused organic matter in the form of dissolved and particulate carbon into the ocean's interior where it is sequestered for periods ranging from decades to centuries.


image R. Murnane and J. Sarmiento have established some fairly simple models of the physical and the biological carbon pumps, and they compared the model outputs to actual ocean data from the Pacific Ocean. From this exercise they concluded that if there was only a solubility pump, as might occur in a hypothetical ocean with no living organisms, we would eventually produce a profile of dissolved inorganic carbon in the sea that is very different from that observed in the real world. Only when a biological pump was included were they able to reproduce the very steep gradient in inorganic carbon as a function of depth that is characteristic of the world ocean.

imageThere are several important characteristics and controls of the biological pump, and I'd like to focus on a couple of these in the time remaining. We have a reciprocating constant Redfield Ratio dissolved and particulate organic matter pump. This pump simply exchanges nutrients and organic matter at constant Redfield Ratios, which has important implications for carbon sequestration in the sea.

Second, we have a stochastic, event-driven diatom aggregation pump, which responds quickly and efficiently to fertilization events in the ocean. Stochastic natural and deliberate discharge of iron or plant nutrients into the ocean will sustain the rapid growth of select groups of plankton and provide an opportunity for pulsed carbon export.

Then we have the vertically migrating metazoan plankton pump. These are large organisms, well, large from a microbiological perspective, that move up and down in the water column in response to light. These daily migrations transport materials in both directions, generally with a net downward flux.

Finally, we have the nitrogen-primed prokaryote pump. This pump is driven by nitrogen (N2) fixation, which is a process whereby certain prokaryotic microorganisms can use the unlimited supply of N2 gas dissolved in seawater as a nitrogen source to drive their metabolism and growth. This relieves the ecosystem of fixed N limitation and can lead to efficient carbon export. N2 fixation in the sea may be controlled by Fe availability, so pulsed additions of Fe to N-limited habitats should promote N2 fixation and subsequent carbon export.

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