Diatoms under the microscope
We examine the tiny plants that are the foundation of Earth’s ecosystem
YOU might have heard of plankton – tiny plants (phytoplankton) and microscopic animals (zooplankton) suspended in oceans, lakes and other bodies of water. Plankton is crucial to life on Earth as it’s the first link in the food chain. The most common kind of phytoplankton is diatoms – there are countless diatoms in the world’s oceans. The word diatom comes from the Greek word “diatomos” and literally means “cut in two”. These words might not make sense yet – but hold on, we’ll explain!
WHAT ARE PHYTOPLANKTON?
They’re microscopic, floating plants found mainly in Earth’s oceans. Phytoplankton are the first link in the food chain as these tiny plants are food for zooplankton and tiny fish. Larger fish eat the zooplankton and the tiny fish, and humans catch and eat the larger fish. In this way phytoplankton’s energy eventually becomes our energy.
WHAT DO DIATOMS LOOK LIKE?
Diatoms are a kind of yellow algae and are unicellular (each diatom consists of a single cell). Like other algae, diatoms don’t have leaves, a stem, roots or flowers, but each unicellular diatom does contain chlorophyll. This means a diatom can produce oxygen and energy.
That tiny cell’s wall is made of silica, a glass-like substance that protects the cell like a shell. The diatom itself is inside this cell wall. Like a hat box or Petri dish, the shell has two parts: the larger one fits over the other like a lid and the halves are sym-
metrical – hence the description “cut in two”.
One part has a hole through which the diatom absorbs substances from its environment and excretes waste.
There are many kinds of diatoms. Some are shaped like pill boxes, needles or Frisbees, while others are oval or rectangular. They can also be linked like a chain. They vary in size from less than one micron to more than 1 000 micron (a micron is one micrometre – a millionth of a metre or 0,01mm), but most are between 10 and 100 micron. It’s been calculated that it would take about 25 million diatoms to fill a teaspoon.
WHY DIATOMS MATTER
Scientists are interested in algae – and especially diatoms – because they’re a good indication of how healthy an ecosystem is.
The plants and animals living together in a certain area form what we call an ecosystem. Scientists study the diatoms found in a certain area and from this they’re able to tell if, for example, the area is prone to acid rain, if there are heavy metals in the water, and whether the water is healthy and productive.
Diatoms’ hard silica walls ensure their fossilised remains stay intact and as they die they pile up on the ocean floor. Thousands of years later these remains have formed layers and, by studying them, scientists are able to tell what the ecosystem in a specific area was like long ago, and how it’s changed since then.
THE LIFE OF PHYTOPLANKTON
These microscopic plants live near the water’s surface because, like all green plants, they need sunlight. They also need water and nutrients to survive. Phytoplankton use water and carbon dioxide to grow but vitamins and minerals such as iron also help to keep them healthy. When the ocean’s surface is cold, nutrients rise from the deep towards the surface and are absorbed by diatoms and other phytoplankton. But when the ocean’s surface is warm (for instance, when the El Niño climate pattern occurs), fewer nutrients rise to the surface and the phytoplankton start to die. This causes enormous problems as phytoplankton are the first link in the food chain, so if there are fewer phytoplankton it has an impact on the whole chain.
THE PLANET’S CLIMATE
About 90% of aquatic plants are phytoplankton. Plants, including phytoplankton, use carbon dioxide to survive. The more phytoplankton there are, the more carbon dioxide is removed from the air. If there’s less carbon dioxide in the air, Earth’s temperature drops. So less phytoplankton in the world’s oceans means the planet’s average temperature rises.
This can have many negative consequences – for instance, it can cause the ice at the poles to melt more rapidly, which in turn causes ocean levels to rise, threatening to flood coastal cities and to submerge islands.
Air pollution also affects Earth’s ozone layer. The ozone is a thin layer of gas in our planet’s atmosphere that shields us from the sun’s most harmful rays. Scientists believe that holes in the ozone layer allow more of these harmful rays through at both the North and South Poles. This is causing many kinds of plankton, including phytoplankton, to die because they’re sensitive to those harmful rays.