Diatoms—Tiny Plants in Glasshouses

Jeremy L. asks: What do diatoms look like? How big are they and what colors? What are they doing to the ecosystem?

Thank you for the great questions, Jeremy! We have been talking about diatoms often on this blog and about our work with them, but we haven’t fully explored what they’re like yet.

What do they look like?

Up to 12,000 species of diatoms have been identified so far—though scientists estimate there may be many more extant (living now) species—and they come in an extraordinary array of shapes.

One of our favorite diatoms—Eucampia antarctica, which you can see in the center of the image below—reminds us of a tiny pair of pants! In fact, if you look at this diatom from another angle, it’s actually triangular—it’s like a pair of tiny pants with three legs, not two. When it’s alive (rather than a fossil, as it is in the image), it has two identical sides, so it looks like two three-legged pants, one upside-down on top of the other. This is an example of a centric diatom—a diatom that is radially symmetrical.

A photomicrograph of the frustules of Eucampia antarctica and some other diatoms obtained from diatomaceous mud (mud from the deep-sea floor containing diatom fossils) we have collected on our cruise.

As well as centric diatoms, there are pennate diatoms. Pennate diatoms are bilaterally symmetrical—just like people and most animals. (You could slice them in two along their center and have two identical halves.)

Some diatoms exist as single cells. Others form chains.

How big are they?

Just as there are many species of diatoms, there is wide variation in the sizes of species. (There are also differences in sizes of diatoms within a species due to their unusual reproductive habits, which I look forward to writing about in a later blog post!) Most are miniscule—only a few species can be seen without the aid of a microscope. They range from 1 µm up to 2 mm in size.

What colors are they?

Because diatoms’ skeletons are made of silica (which is, essentially, glass), they are clear. Diatoms are sometimes known as brown algae because, although their skeletons are clear, the material inside them is not. Diatoms’ golden brown to greenish color comes from their chloroplasts, the structures containing chlorophyll and other pigments. Chlorophyll is the green pigment in most plants that enables photosynthesis. The golden-brown color comes from another photosynthetic pigment called fucoxanthin. You can clearly see the chloroplasts in the diatom below.

If you look for pictures of diatoms on the Internet, you may see detailed diatoms that are very brightly colored, but many of these images are electron scanning microscope images that have been color-enhanced.

What do they do in the ecosystem?

Diatoms have an immensely important role on Earth, and they have for millions of years. The earliest known diatom fossils date from the Jurassic Period (185 million years ago), although there is some evidence they may have originated in the Triassic Period (nearly 250 million years ago). They became very important in the oceans around 40 million years ago, just as we started to grow ice on Antarctica, and today we think they play a key role in maintaining our current climate.

Diatoms can be found in both marine (ocean) and aquatic (fresh water) environments. They can even be found in soil and on damp surfaces such as rocks and tree bark.

Firstly, they are part of the base of the food chain in many environments. Diatoms are eaten by small organisms, which are eaten by other organisms, and so on up the food chain. Scientists estimate that 20% of global primary production (the synthesis of organic compounds from atmospheric or aqueous carbon dioxide) is by diatoms.

As they photosynthesize, diatoms take up carbon dioxide. Because they have such a large role in primary production, they also have a huge role in regulating the amount of carbon dioxide in Earth’s atmosphere. During photosynthesis, diatoms produce oxygen as a byproduct.

Their success—their ability to grow and reproduce and therefore take up carbon dioxide, produce organic compounds, and produce oxygen—is affected by the availability of nutrients in the water in which they live. Diatoms need nutrients such as nitrogen, silicic acid, and iron, to name a few. Everything needs nitrogen and iron, but diatoms are special in their requirement for the element silicon. (We are studying the relationships of diatoms to nitrogen and silica on our cruise.)

Diatoms need silica to build their glass skeletons—to grow and reproduce. But because their silica skeletons are relatively heavy, diatoms sink easily. They must rely on turbulent mixing near the ocean’s surface to stay in sunlit waters, where they can photosynthesize, grow, and reproduce. Diatoms thrive in the Southern Ocean due to strong currents that cause the upwelling (rising) of nutrient-rich water from deep in the ocean. Heather has elegantly explained this process here.

As they take up carbon dioxide from the ocean, they allow the water to absorb more carbon dioxide. When they die, diatoms gradually sink through the ocean and are either eaten by other organisms or eventually accumulate on the sea floor. Diatoms act as a carbon sink, and those that sink to the sea floor effectively lock carbon away from the atmosphere for long periods.

Because the amount of carbon dioxide in the atmosphere can affect how warm or cool it is, the success—or lack of success—in diatom production and reproduction can affect global climate.

Minuscule, intricate, and jewel-like, diatoms drift on the ocean currents, using water, carbon dioxide, and nutrients to convert light energy into simple sugars and oxygen during photosynthesis, as they have done for many millions of years. It’s safe to guess that without them, our planet would be unrecognizable.