Diatoms

Diatoms are microscopic photosynthetic algae with silica cell walls called frustules. In General Biology I, you study them as major phytoplankton that shape aquatic food webs, oxygen production, and carbon cycling.

What are diatoms?

Diatoms are a group of microscopic, photosynthetic algae in General Biology I that often show up as major members of phytoplankton. They live in freshwater and marine environments, and they make food by photosynthesis just like plants do, using light energy and carbon dioxide to build sugars.

What makes diatoms stand out is their cell wall. Instead of a flexible outer covering, each diatom is surrounded by a glassy shell made of silica called a frustule. This frustule has two overlapping halves that fit together a bit like a petri dish lid and base. Under a microscope, the patterns can look almost like lace or etched glass, which is why diatoms are often easy to spot in lab images.

Their structure matters for more than appearance. Silica frustules are durable, so when diatoms die, their shells can sink and accumulate in sediments. That makes diatoms a useful record of past water conditions in environmental studies, since the species found in older sediment layers can reflect changes in salinity, nutrient levels, temperature, or pollution.

Diatoms also sit near the base of aquatic food webs. Tiny zooplankton and other organisms eat them, and larger animals depend on that energy indirectly. Because they photosynthesize on such a huge scale, diatoms contribute a major share of global oxygen production and help move carbon from surface waters into deeper ocean storage.

One detail that often comes up in biology is how they reproduce. Diatoms usually divide asexually by binary fission, but because each daughter cell inherits one half of the frustule, repeated division can make cells gradually smaller. That size reduction is part of why diatom life cycles are more interesting than a simple “copy and split” story, and it connects structure to reproduction in a way biology classes like to test.

Why diatoms matter in General Biology I

Diatoms show up in General Biology I whenever a topic connects photosynthesis, aquatic food webs, or the carbon cycle. They are one of the clearest examples of how a tiny protist can affect an entire ecosystem, from primary production at the surface to carbon storage in deep sediment.

They also give you a concrete way to think about form and function. The silica frustule is not just a cool shell, it affects buoyancy, protection, fossil preservation, and how scientists identify species in samples. That makes diatoms useful in ecology labs, where you might examine a microscope slide, compare species in different water samples, or interpret what a sediment core says about past aquatic conditions.

Diatoms also connect to broader environmental questions. If a lake or coastal system gets nutrient pollution, diatom communities can shift, and those shifts change the whole food web. In other words, diatoms are one of the first places you can see the biological effects of water chemistry and ecosystem change.

If your course covers protists, aquatic biomes, or photosynthesis, diatoms are one of the best examples to keep in mind because they link cell biology to ecosystem-level patterns.

How diatoms connect across the course

Phytoplankton

Diatoms are one of the biggest groups of phytoplankton, meaning they are photosynthetic organisms drifting in the water column. When you see a question about the base of an aquatic food web, diatoms are often part of the answer because they convert light energy into organic matter that supports zooplankton, fish, and larger predators.

Frustule

The frustule is the silica shell that defines diatoms. This structure is what makes them so easy to identify under a microscope and why they fossilize so well in sediments. If a lab image shows a patterned glass-like wall, you are probably looking at a frustule, not just any algae cell wall.

Eutrophication

Nutrient runoff can change which phytoplankton dominate a water body, and diatoms often respond quickly when nitrogen and phosphorus levels rise. In ecology questions, you may need to trace how eutrophication shifts algal growth, changes oxygen levels, and affects the organisms that depend on the original community.

Aquatic Biomes

Diatoms are a strong example of how abiotic conditions shape life in aquatic biomes. Light availability, nutrient concentration, mixing, and salinity all influence where different diatom species thrive. That makes them useful when you are comparing freshwater and marine systems or explaining why productivity varies across water habitats.

Are diatoms on the General Biology I exam?

A quiz question might show a microscope image and ask you to identify the organism by its silica frustule. Another common task is tracing how diatoms fit into an aquatic food web or explaining why they are considered primary producers. In lab reports, you may need to describe how diatom abundance changes with nutrient levels, salinity, or water depth. If a short-answer prompt asks about carbon cycling, diatoms are a good example because they fix carbon during photosynthesis and can transport it to sediments after they die. For a classification question, you should connect them to protists, phytoplankton, and photosynthetic eukaryotes.

Diatoms vs Phytoplankton

Phytoplankton is the broader group of photosynthetic organisms drifting in water, while diatoms are one specific kind of phytoplankton. So every diatom in this context counts as phytoplankton, but not all phytoplankton are diatoms. If a question asks for the exact organism with a silica frustule, diatoms are the better answer.

Key things to remember about diatoms

• Diatoms are microscopic photosynthetic algae that are major members of phytoplankton in freshwater and marine ecosystems.

• Their defining feature is a silica-based cell wall called a frustule, which has a patterned, glass-like appearance and preserves well in sediments.

• Diatoms drive primary production, feed aquatic food webs, and contribute heavily to oxygen production and carbon cycling.

• They usually reproduce by binary fission, and repeated division can lead to smaller daughter cells over time.

• Because different species prefer different water conditions, diatoms can be used as indicators of environmental change.

Frequently asked questions about diatoms

What is diatoms in General Biology I?

Diatoms are microscopic photosynthetic protists with silica cell walls called frustules. In General Biology I, they show up as major phytoplankton that support aquatic food webs and help move carbon through ecosystems.

Are diatoms plants or protists?

Diatoms are protists, not plants, even though they photosynthesize. They behave like algae in aquatic ecosystems, but they are classified with the protists because they are eukaryotic organisms that do not fit neatly into the plant kingdom.

Why do diatoms have silica shells?

Their silica frustules give them protection and structural support, and they also help these organisms leave a fossil record in sediments. That shell is one reason diatoms are so useful in environmental and ecological studies.

How are diatoms used in biology labs?

You might identify diatoms under a microscope by their glassy, patterned frustules. They are also common in ecology labs and environmental sampling because shifts in diatom species can reveal changes in water quality, nutrients, or salinity.