Ocean Wanderers

The ocean makes life on land possible. From moderating the Earth’s climate to producing much of the oxygen that animals need (in the sea and out), the ocean helps to keep our planet healthy and habitable. And surprisingly, much of the groundwork for nurturing our planet’s health is done by microbes that thrive by the trillions under the waves.

These microbes are called phytoplankton, and are the base of the marine food web, which means that all other marine life depends on them. They are often likened to plants, but are not. These microscopic organisms that live suspended near the ocean’s surface are so much more.

Too small to see individually without a microscope, when phytoplankton bloom – when millions upon millions grow, which can happen in just days or weeks – together they can change the ocean’s colour as seen from space. Phytoplankton are so numerous, in fact, that they contribute half of the planet’s primary productivity and therefore produce half of the oxygen that sustains animal life.

Amazingly, phytoplanktons’ whole structure performs photosynthesis: unlike a tree that builds roots and a trunk, phytoplankton act like the leaves. Though tiny, because they are so abundant, they take up CO2 and give off oxygen with incredible speed and efficiency. Phytoplankton live short lives compared to plants – only a few days – before dying and sinking to the ocean depths. But more take their place in a constant process of renewal that allows for phytoplankton’s outsized impact on carbon capture and oxygen creation. And, in moving CO2 away from the atmosphere and delivering it to the deep ocean, a process called “marine snow”, phytoplankton act as a form of carbon sink: they contribute to buffering our planet from the effects of climate change.

Phytoplankton are hugely varied. They can be single-celled forms of algae (the kind that can change ocean colour), as well as photosynthesizing bacteria, which are the most abundant organisms in the ocean, and so small that each tablespoon of ocean water contains hundreds of thousands. Bacteria were the first photosynthesizing organisms on Earth and evolved in the ocean 3.5 billion years ago. There even exist carnivorous phytoplankton: these phytoplankton photosynthesize and also have flagella, like hairs, which they can move to propel themselves to seek out bacteria to eat.

Phytoplankton take advantage of ocean currents in the same way that dandelion seeds or maple keys take advantage of the wind to be blown to new areas to grow. The ‘plankton’ part in phytoplankton comes from the Greek word for ‘wanderer’, because phytoplankton flow with the ocean’s currents. And like dandelions blooming in the warmth and light of the spring, phytoplankton bloom in the right conditions for them. They bloom when currents bring them near the ocean’s surface (where there is sunlight), and when the water’s at just the right temperature and contains the nutrients they need to grow. To help them get to places with their ideal growing conditions, phytoplankton have several special adaptations related to their small size: they can have a large surface area relative to their volume to help them float and access the sun’s energy, and spikes that distribute their weight over a larger area, giving them even more surface area for photosynthesis and better flotation.

As the base of the marine food web, phytoplankton’s springtime appearance in droves is immediately succeeded by an equally courageous bloom of minute phytoplankton-eating marine animals called zooplankton. Right whales migrate each summer to the Northwest Atlantic to eat zooplankton called copepods. The whales skim the surface of the ocean, filtering the water for their tiny food. Over the summer they can eat enough copepods to be able to fast for much of their winter spent in the Tropical Atlantic.

Climate change has recently become a concern for the viability of the ocean’s food web. Ocean School Scientific Director Boris Worm explains, “As surface waters warm, nutrient supply from lower waters is diminished and phytoplankton become smaller and less abundant as a result, lowering ecosystem productivity. Climate change mitigation, as agreed under the 2015 Paris accord would help minimise these impacts and help keep ocean life abundant.”

The ocean, and its microscopic life, constitute parts of a complex system that allows life to grow and evolve in the sea and on land. It is up to us to protect our one global ocean that protects and provides for us all, even in ways we cannot see.