Algae

Algae characteristics

Algae are mainly photosynthetic eukaryotic organisms that can be both single-celled and multicellular.

Algae are plant-like protists because they have chloroplasts and cell walls and are autotrophs. But they do not have structures that actual plants have and can be motile or moving.

Algae are usually classified based on the origin of their chloroplast. Scientists believe that green and red algae evolved from cyanobacteria while other organisms like Euglenas evolved later from an endosymbiotic relationship with algae. This is because green and red algae have two membranes, while organisms like Euglenas have three.

Furthermore, we can also separate algae by their chloroplast, as shown in the table below:

Algae can reproduce sexually or asexually.

Algae are mainly free-living, but some, like Volvox, can live in colonies.

Algae can range in sizes from picoplankton which is a few micrometers, to kelp which can be over 50 meters long!

Since algae are eukaryotes, they have a nucleus and membrane-bound organelles like mitochondria, ribosomes, vacuoles, endoplasmic reticulum (ER), and Golgi apparatus. All are shown except ribosomes (because they are too small), found either attached to the ER or free in the cytoplasm.

• Mitochondria are organelles in which energy production occurs.
• Ribosomes are organelles in which protein synthesis occurs.
• Vacuoles are organelles that store food or waste for later usage or removal.
• Endoplasmic reticulum (ER) are organelles that produce proteins and lipids and store calcium.
• The Golgi apparatus or complex sorts, processes, and packages proteins from the ER.

The algae shown in Figure 2 is E. viridis, specifically a unicellular mixotroph.

Euglena is an autotroph regularly but will behave like a heterotroph when deprived of sunlight and prey on smaller organisms.

Figure 2: Algae characteristics labeled. Daniela Lin, Study Smarter Originals.

3 types of algae

Although algae are a varied group of protists, the three most common types are:

Brown algae

• Brown algae are part of the Phaeophyceae class.
• They can range from a few centimeters to more than 100 meters and are multicellular.
• They are predominantly marine, as less than 1% are in freshwater environments.
• They have chlorophyll and other types of pigments or carotenoids.
• The brown color of the algae comes from the xanthophyll pigment fucoxanthin. This is because fucoxanthin is more dominant than chlorophyll a and c, beta-carotene, etc.
• They can reproduce both sexually and asexually.

They have plant-like structures such as:

• A holdfast is a root-like structure that anchors the algae to the floor. Unlike real roots, they do not absorb nutrients.
• The blade or lamina is a leaf-like structure that allows photosynthesis.
• Stipes are stem-like structures that provide support for the blades. Unlike stems, stipes do not transport nutrients.
• Some algae have pneumatocysts, a floating bladder that helps lift the blades to the surface for photosynthesis to be able to occur.

Red algae

• Red algae are part of the classRhodophyceae.
• They are predominantly marine, as only around 3% are in freshwater environments.
• Red algae have chlorophyll pigment A, which gives them their characteristic red color.
• They also reproduce sexually and asexually.
• They also have plant-like structures such as stipes, blades, and a holdfast.

Green algae

• Green algae are part of the class Chlorophyceae.
• They can be found in both marine and freshwater environments.
• They come in unicellular, colonial, or multicellular forms.
• They share the same pigments as land plants or chlorophyll a and b.
• They also possess plant-like structures such as stipes, blades, and a holdfast.
• Green algae or charophytes are considered to be land plant ancestors. This is because they have the same pigments and are thought to have diverged around 450-500 million years ago.

Algae examples

There are many examples of algae, but some of the most famous ones are:

Volvox

Volvoxes are green algae or hollow balls of around 500 to 60,000 cells called a colony. Each of the cells has flagella or hair-like appendages and chloroplast with pyrenoids. These flagella allow the colony to swim around, and the chloroplast contains chlorophyll pigments that enable the volvox to capture sunlight and photosynthesize. Pyrenoids are sub-compartments in many algae that help with carbon dioxide fixation in photosynthesis.

Figure 3: Volvox illustrated. Daniela Lin, Study Smarter Originals.

Euglena

Euglena are unicellular organisms that have both plant and animal traits. They live in freshwater and saltwater environments. They have several chloroplasts with three membranes and pyrenoids. Euglena also falls under the green algae category. Euglena is a mixotroph that does not possess cell walls, usually has 1-2 flagella, is pear-shaped, and has one contractile vacuole.

They are motile and can act as either autotrophs or Heterotrophs, depending on the presence of sunlight. They use flagella for movement and have a contractile vacuole to regulate fluid pressure or osmoregulation.

Diatoms

Diatoms are unicellular photosynthetic organisms that are motile. They can reproduce both asexually and sexually. They are usually identifiable based on their silica cell walls called frustules. Their fossils are typically used in filters, paints, and varnishes. They are microscopic sea organisms that serve as the basis of the food chain.

Fungus and algae relationship

When Fungi and algae interact in nature, they often form mutualistic relationships.

We usually call the green algae or cyanobacteria associated with lichen photobionts, as they contain chlorophyll pigments and are responsible for using photosynthesis to transform light energy into nutrients.

This symbiotic relationship allowed lichens to increase in extreme environments such as environments with low nutrients. Lichens are pioneer species in primary succession as they are usually the first organisms to colonize rocks, mountains, etc. Lichens chemically weather rocks and gather organic matter allowing grasses and mosses to grow later. Overall, lichen is self-sustaining, allowing it to grow almost anywhere.