Ochrophyte

Ochrophytes, also called heterokontophytes or stramenochromes, are a group of algae. They are a special kind of eukaryotes, which means they have a cell nucleus. These algae have two different-sized flagella, and one of them has special hairs called mastigonemes.

What makes ochrophytes unique are their photosynthetic organelles or plastids. These are wrapped in four membranes and have parts called thylakoids stacked in groups of three. They use chlorophyll a and c for capturing energy from sunlight, along with other colors like β-carotene and xanthophylls.

Ochrophytes are very diverse and include important algae like brown algae and diatoms. They can be called the phylum Ochrophyta, Heterokontophyta, or they are sometimes grouped as a subphylum called Ochrophytina within the phylum Gyrista. Their plastids originally came from red algal ancestors.

Etymology

Throughout history, scientists have used different names to describe a special group of green and yellow algae. The most common name today is Ochrophyta, named after a golden alga called Ochromonas. This name was created by a scientist named Thomas Cavalier-Smith in 1986. Later, he changed the name a bit, but most scientists still prefer to use Ochrophyta.

Another name used by algae experts is Heterokontophyta. This name comes from old Greek words meaning "different pole" and was first used in 1899. It refers to the special moving parts, called flagella, that these algae have. Over time, this name was expanded to include more types of algae.

Characteristics

Ochrophytes are a group of tiny living things made up of cells. These cells can be simple and single, or they can be grouped together in many ways. Some brown algae, for example, grow big and have many different parts. All ochrophytes share a special feature in their energy-making parts called mitochondria.

What makes ochrophytes special are their tiny hair-like parts called flagella and their green parts called chloroplasts, which help them make food from sunlight. Their chloroplasts have four layers and special green pigments that help them catch sunlight.

Diversity

Ochrophytes are a large group of algae with many different types. A 2024 survey found 23,314 described species, but there may be over 100,000 in total, with most being diatoms. They are divided into several groups, or classes, including:

• Bolidophyceae – 18 species of marine algae that can be either swimming or have silica "shields".
• Chrysophyceae – Known as golden algae, with 1,274 species found in freshwater or on land. They can be single-celled, colonial, or have different shapes.
• Diatomeae – The most species-rich group, with 14,684 described species. They live in freshwater, marine, and land environments, and are known for their silica coverings called frustules.
• Dictyochophyceae – 217 species of algae that can be naked or have organic scales, and some grow basket-like silica skeletons.
• Eustigmatophyceae – 218 species of algae with cell walls, found in freshwater and terrestrial habitats, lacking some common pigments.
• Olisthodiscophyceae – Two marine species that are flat and move close to surfaces.
• Pelagophyceae – 31 species of marine algae with a dense, perforated covering.
• Phaeophyceae – Known as brown algae, with 2,124 marine species. They are multicellular and have complex structures.
• Phaeothamniophyceae – 31 species of algae with cell walls and a gelatinous covering.
• Picophagea – One species of tiny marine algae.
• Pinguiophyceae – Five species of marine algae with high amounts of fatty acids.
• Raphidophyceae – 58 species of algae with various surface structures.
• Synchromophyceae – Five species of marine algae that can join together and some lack color.
• Xanthophyceae – 616 species of yellow-green algae, some of which are large and filamentous.

Reproduction

Ochrophytes can reproduce without parents through breaking apart, creating small pieces, dividing cells, or making spores. They can also reproduce with parents by making special cells called gametes in three ways: having cells that look the same, having cells that look different in size, or having one large cell and one small cell.

Ecology

Ochrophytes are found almost everywhere, from the ocean to rivers and soil. Some types live mainly in the sea, like brown algae and golden algae, while others are more common in freshwater or soil. Diatoms, a type of ochrophyte, are very common in the ocean.

Some ochrophytes can live in both water and by eating other tiny organisms. In rivers, golden algae and yellow-green algae grow attached to rocks or float freely. Diatoms in rivers have special ways to stick to surfaces so they don’t get washed away.

Some ochrophytes can harm fish. For example, certain types can produce substances that hurt fish or have sharp parts that block fish gills. One type can make a toxin that can be harmful if eaten by people.

Evolution

External

Ochrophytes are a diverse group within a larger set of organisms called Stramenopila. This group includes many different types of simple, single-celled organisms. Ochrophytes came from an important event where a red alga became part of a cell, helping it make food through sunlight.

Scientists think ochrophytes began to evolve between 874 and 543 million years ago. Early fossils, like ones from a type called xanthophyte that are about a billion years old, show that ochrophytes were already around by 1000 million years ago. Other early examples include types that lived between 750 and 550 million years ago.

Internal

The relationships between different groups of ochrophytes are still being studied, but three main groups are commonly accepted. One group includes brown algae, which became very diverse around 310 million years ago. Other groups include golden algae and diatoms, which are very small but numerous in water.

Recently, scientists discovered a new group of algae in 2021. Some simple organisms without chloroplasts might be closely related to ochrophytes, suggesting their ancestors might have started to develop chloroplasts before losing them later.

History of knowledge

Pre-Linnean

People have known about brown algae, like kelp and other seaweeds, for a very long time. Records of these plants go back to early China (around 3000 BC), Japan (around 500 BC), and Greece (300 BC, such as Theophrastus). They were probably used for food, dyes, and medicine long before we had written records. Brown algae might have even helped people travel along coastlines, especially from East Asia to the Americas, by providing plenty of resources. This idea is called the kelp highway hypothesis. Other tiny algae were not written about because they are too small to see without a microscope.

In the late 1600s, Antony van Leeuwenhoek was the first to see tiny living things, but he did not see these small algae. The first clear pictures of diatoms, a type of algae, were made in England in 1703, long before scientists gave them a formal name.

Discovery period (1753–1882)

The first official description of these algae was made by Carl Linnaeus in 1753 for a brown alga called Fucus. Over the next years, scientists began describing single-celled chrysophytes and diatoms for the first time. During this time, scientists thought brown algae were plants, while tiny algae were animals, calling them infusoria. Other types of algae, like xanthophytes, were first described in 1801, and raphidophytes were found in 1865.

Important books were written during this time, like one in 1813 by Jean Vincent Félix Lamouroux, who used color to sort algae. A big step happened in 1838 when Christian Gottfried Ehrenberg wrote about many of these algae after looking at them with a microscope. But scientists still thought these different algae were not closely related.

First synthesis period (1882–1914)

In 1882, a scientist named M.J. Rostafinski suggested that diatoms, golden algae, and brown algae might be related. Other scientists like Carl Correns, Georg Klebs, and Ernst Lemmermann added to this idea. In 1900, Frederick Blackman proposed that complex plants came from simple moving algae, happening separately in green plants and in the algae that would become brown algae and diatoms. In 1914, Adolf Pascher wrote a summary but did not fully agree that all these algae were related. Instead, he split them into two groups.

Floristic period (1914–1950)

During this time, scientists stopped talking much about how these algae might be related because the tools they had were not good enough to show clear connections. However, they described many new species.

In the 20th century, new ways to study cells and DNA helped scientists discover many new groups of algae. The first full mapping of genes for an ochrophyte, from Thalassiosira pseudonana, began in 2002.