Phytoplankton & Primary Productivity

What Phytoplankton Actually Are

"Phytoplankton" sounds like one type of organism. It's not — it's a category that lumps together every photosynthetic organism that drifts in the ocean. They span four major groups:

Cyanobacteria — Bacteria that photosynthesize. Includes Prochlorococcus and Synechococcus, the two most abundant photosynthesizers on Earth. Cyanobacteria are also the original source of atmospheric oxygen — they evolved photosynthesis 2.5 billion years ago and slowly oxygenated the planet in what's called the Great Oxidation Event.

Diatoms — Single-celled algae with intricate, glass-like silica shells. Hauntingly beautiful under a microscope. Diatoms alone account for an estimated 20% of all photosynthesis on Earth.

Dinoflagellates — Single-celled organisms with two whip-like tails (flagella) for swimming. Some species cause harmful algal blooms (HABs), the toxic "red tides" that close beaches and kill fish.

Coccolithophores — Algae covered in tiny calcium carbonate plates. When they die, their plates sink and eventually form chalk and limestone deposits. The White Cliffs of Dover are made almost entirely of dead coccolithophore plates.

All four groups perform the same fundamental job: take dissolved carbon dioxide and water, use sunlight to power the chemistry, and produce sugars and oxygen. They are the producers F2 introduced, and they are the foundation of every marine food web on Earth.

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Marine Photosynthesis

The basic photosynthesis equation hasn't changed in 2 billion years:

> 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

Carbon dioxide plus water, powered by light, produces glucose plus oxygen.

In the ocean, this happens almost exclusively in the epipelagic zone (MB1) — the upper 200 meters where light penetrates. Below that, there isn't enough light to drive the reactions. The depth at which photosynthesis can still happen at all is called the compensation depth.

What's different about marine photosynthesis compared to plants on land:

Light is limiting in a different way. On land, plants compete for sunlight by growing tall. In the ocean, phytoplankton just float — but only the ones near the surface get enough light. Most marine photosynthesis happens in the top 50–100 meters.

Nutrients are often limiting. Phytoplankton need nitrogen, phosphorus, and trace metals (especially iron) to grow. In huge regions of the open ocean, these nutrients are scarce — and phytoplankton populations are sparse as a result. The richest ocean regions are where nutrients get delivered by upwelling (cold, nutrient-rich deep water rising to the surface) or river runoff.

Different pigments capture different light. Different phytoplankton groups use different photosynthetic pigments tuned to the wavelengths of light that reach their depth. Surface dwellers use green pigments (like terrestrial plants). Deeper organisms use red or yellow pigments to capture the blue-green light that penetrates further.

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The Biological Carbon Pump

Here's something most people don't realize: the ocean is the single largest carbon sink on the planet, and phytoplankton are the reason.

When phytoplankton photosynthesize, they pull dissolved CO₂ out of seawater and convert it into organic carbon (sugars, proteins, lipids — their own bodies). When they die, some of that organic carbon sinks to the deep ocean — either as dead cells, as fecal matter from animals that ate them, or as aggregations called marine snow that drift slowly downward.

Once that carbon reaches the deep ocean, it can stay there for hundreds or thousands of years before circulation eventually returns it to the surface. This process is called the biological carbon pump, and it's been operating for hundreds of millions of years.

The numbers are enormous. The biological carbon pump moves roughly 10–15 billion tons of carbon per year from the surface to the deep ocean. For comparison, human fossil fuel emissions are about 35 billion tons of CO₂ per year. Without the ocean's biological carbon pump, atmospheric CO₂ would be significantly higher than it is today, and climate change would be far worse.

Climate change is also affecting the pump itself. Warmer surface water is less dense and mixes less with the cold deep water below, which reduces the nutrient supply phytoplankton need. Acidification (covered in MB5) makes it harder for coccolithophores to build their calcium carbonate shells. The pump is weakening even as the need for it grows.

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Measuring Primary Productivity

How do scientists measure something they can't really see — the global productivity of phytoplankton across an entire ocean?

Satellite remote sensing is the modern answer. Satellites like NASA's MODIS and PACE measure the color of the ocean from space. Chlorophyll, the pigment phytoplankton use to capture light, has a distinctive reflectance signature. The more chlorophyll in a patch of ocean, the greener it appears from space. By measuring color globally and continuously, scientists can map phytoplankton populations in near-real-time across the entire planet.

The maps are striking. Some regions of the ocean — like the equatorial Pacific and the Southern Ocean — have very low chlorophyll despite plenty of sunlight, because they're starved for iron and other micronutrients. Other regions — coastal upwelling zones off Peru, California, and West Africa — light up green from space because nutrients are abundant and phytoplankton thrive.

Older methods are still used too: water samples, cell counts under a microscope, measurements of dissolved oxygen production, and incorporation of radioactive carbon (carbon-14) into phytoplankton biomass to track photosynthesis rates directly.

Putting all these methods together, scientists estimate global ocean primary productivity at roughly 50 billion tons of carbon fixed per year — comparable to all terrestrial primary productivity combined. The ocean isn't just half of Earth's photosynthesis. It is half of Earth's biology.

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Wait, Actually...

Prochlorococcus — the most abundant photosynthetic organism on Earth — has been hiding in plain sight for billions of years.

It wasn't discovered until 1986, when oceanographer Sallie "Penny" Chisholm found it by flow cytometry. The reason it had been missed: it's smaller than a typical bacterium, has no obvious distinguishing features under a regular microscope, and is so common that scientists had assumed early samples were contamination or noise.

Current estimates put the total Prochlorococcus population at around 10^27 cells — that's one octillion, or roughly 10,000,000,000,000,000,000,000,000,000 individual organisms. There are about 100 trillion Prochlorococcus cells in the ocean for every star in the observable universe. They've been doing photosynthesis longer than any other surviving lineage of life. And we didn't know they existed until the year of the Chernobyl disaster.

This is the kind of thing that should keep you humble about how much biology we still don't know.

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