Whether you’re swimming with big animals, macro critters, or in coral gardens, you’re also diving into an entire world of microscopic plants called phytoplankton.

Whether you’re swimming with big animals, macro critters, or in coral gardens, you’re also diving into an entire world of microscopic plants called phytoplankton. The tiniest of marine algae, phytoplankton form the foundation of life in our oceans. They provide food for a range of animals, from the smallest single-celled organisms to the largest whales.

Phytoplankton use sunlight to synthesize food from carbon dioxide and water, releasing oxygen back into the environment via a process called photosynthesis. Most phytoplankton float in the upper ocean layer where you dive and sunlight penetrates the water. Two common types of phytoplankton exist as diatoms and dinoflagellates.


Effusive in his praise, senior principal research scientist David G. Mann of the Royal Botanic Garden Edinburgh names diatoms “one of the largest and ecologically most significant groups of organisms on Earth.” These small, single-celled organisms generate far more oxygen via photosynthesis than all of the vast tropical rainforests in the world combined. In fact, diatoms gift you every fifth lungful of the oxygen you breathe.

The life cycle of most diatoms is unique. Daughter cells form from cell division within the parent constrained in size by the parent’s rigid silica walls. Diatoms then become progressively smaller over time with each subsequent cell division. When the size of an individual diatom becomes too small, it produces a specialized cell called an auxospore, used to restore the diatom’s size back to normal. If this doesn’t occur (e.g., due to unsuitable environmental conditions), the diatom will continue to divide, becoming smaller and smaller until it dies.

While invisible to the naked eye, diatoms become easily recognizable under the microscope as viewed below due to their regular geometrical shapes and silica shells, which form a glass-like external barrier.


A colony of diatoms sliding back and forth across each other like a folding carpenter’s ruler. Video courtesy of Merry Passage.



Approximately half of all dinoflagellates are photosynthetic, fitting into the world as important oxygen producers according to the Scripps Institution of Oceanography. The other half of dinoflagellates are heterotrophic, meaning they cannot manufacture their own food and, instead, must eat diatoms and other plankton to survive.

If you’ve ever been diving at night accompanied by a blue luminescence or been warned against eating shellfish due to red tide, you’ve chanced upon dinoflagellates both beautiful and deadly. Dinoflagellates are the primary source of the bioluminescence you see during nighttime diving. The agitation of these organisms results in a beautiful glowing effect like a swirl of miniature stars as you swim through them. However, dinoflagellates actually use these bioluminescent flashes to cause a startle response or reflex in predators which decreases the number of dinoflagellates they consume.

Also bioluminescent in some cases, red tides occur when a particular species of dinoflagellates “blooms,” amassing in such numbers that it discolors the water with a red or brown tinge. Some red tides become toxic with the dinoflagellates producing a chemical that acts as a biotoxin. When fish or shellfish ingest these dinoflagellates, the chemicals accumulate in their tissues. Concentrations can be high enough to cause toxicity in the people who eat them. This results in illnesses such as diarrhetic shellfish poisoning, ciguatera, or paralytic shellfish poisoning. Depending upon the type and amount of toxin ingested, victims fall ill with symptoms ranging from minor stomach upset to incapacitating diarrhea to paralysis or even death.

As detailed below, dinoflagellates swim by means of two flagella — movable strands that propel these single-celled organisms through the water.

Dinoflagellate movement. Video courtesy of Merry Passage.

The next time you dive, remember that, even though you can’t see them with the naked eye, you’re also swimming in an amazing world of microscopic plants.

Title Photo courtesy of Merry Passage.

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