When you think of eukaryotes, what comes to mind? Plants? Animals? In that case, you might be surprised that protists are also eukaryotes! Most eukaryotes are protists! A famous protist that you might know of is seaweed. Seaweed can be found in cosmetics, ice cream, and crude oil! Other things you can attribute to algae are algal blooms. Algal blooms occur when there's an accumulation of algae and is toxic to animals and humans. Researchers study algae as they are essential to our environment and have many valuable applications.
Algae definition
Protists are eukaryotic organisms that are not plants, animals, or fungi. Like other eukaryotic organisms, protists have membrane-bound organelles and a nucleus. In comparison, prokaryotes are all unicellular organisms without membrane-bound organelles or a nucleus.
While prokaryotes and eukaryotes have cell membranes that regulate what goes in and out of the cell, only eukaryotes have internal membranes containing their organelles. These inner membranes allow eukaryotes to have specialized functions when compared to prokaryotes.
For example, the nucleus is where eukaryotes such as protists store their genetic information or DNA (deoxyribonucleic acid). This compartmentalized structure is essential to eukaryotes as it allows for greater control over gene expression resulting in more regulation and a reduction of mutation rates compared to prokaryotes.
Algae are plant-like protists or "aquatic plants" as illustrated in Figure 1. Unlike plants, they are primarily photosynthetic aquatic organisms that lack stems, true roots, and leaves.
Algae are essential to our ecosystem because they produce around 50-80% of our earth's oxygen and reduce carbon emissions by storing it. Algae also act as a vital food source for almost all aquatic organisms and serve as a source of nutrition in many cultures. Other uses for algae include crude oil, medicines, and solidifying agents like agar.
Figure 1: Plant-like protists illustrated. Daniela Lin, Vaia Originals.
Scientists think that the oldest eukaryotes are protists. This makes sense because protists generally have the most straightforward organization or body plans out of all the eukaryotes.
So how did the protists evolve? Well, researchers believe that protists evolved from prokaryotes.
The endosymbiotic theory explains how protists evolved from a symbiotic relationship between larger and smaller prokaryotic cells. Larger ones engulfed smaller prokaryotic cells. The smaller prokaryotes were afforded a safe home and nutrients, while the larger ones received energy. Over time, the smaller prokaryotes evolved into organelles of the larger ones.
Algae characteristics
Algae are mainly photosynthetic eukaryotic organisms that can be both single-celled and multicellular.
For example, Euglena viridis are single-celled, while Volvox and seaweed are 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.
Autotrophs are primary producers that produce their food, usually using light or other chemicals.
Heterotrophs are consumers that cannot produce their food and instead must consume other organisms.
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 Type | Chloroplast Type/Evolution | Example |
| Euglena | - Three membranes
- Green chlorophyll from green algae
| E. viridis |
| Red algae | - Two membranes
- Brown or red chlorophyll like cyanobacteria
| Lithothamnium calcareum or Marine Red Algae |
| Green algae | - Two membranes
- Green chlorophyll like cyanobacteria
| Volvox aureus |
| Dinoflagellates | - Three membranes
- Red or brown color chlorophyll from red algae
| Dinophyceae |
Algae can reproduce sexually or asexually.
Asexual reproduction involves spore formation.
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.
A mixotroph is an organism that can act as both a heterotroph and an autotroph.
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, Vaia 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.
An example of a species under the genus Volvox is V. aureus.
Figure 3: Volvox illustrated. Daniela Lin, Vaia 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.
An example of Euglena is E. viridis.
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.
Mutualistic relationships are relationships where both participants or organisms benefit. Symbiotic relationships are long-term established associations between two organisms, usually for their benefit.
A lichen is an example of a fungi and algae relationship. Within lichen, fungi help algae by aiding in water uptake and protecting it. Simultaneously, algae provide nutrients for fungi through photosynthesis, as these fungi lack chlorophyll pigments.
Around 90% of lichens are made up of associations between green algae/cyanobacteria and usually ascomycetes fungi. Ascomycetes fungi are also called sac-fungi.
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.
Algae - Key takeaways
- Algae are plant-like protists or "aquatic plants". Unlike plants, they are primarily photosynthetic aquatic organisms that lack stems, true roots, and leaves.
- Protists are eukaryotic organisms that are not plants, animals, or fungi. Like other eukaryotic organisms, protists have membrane-bound organelles and a nucleus.
- 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 essential to our ecosystem because they produce around 50-80% of our earth's oxygen and reduce carbon emissions by storing it. Algae also act as a vital food source for almost all aquatic organisms and serve as a source of nutrition in many cultures.
- A lichen is an example of a fungi and algae relationship. Within lichen, fungi help algae by aiding in water uptake and protecting it.
References
- Nina Parker Mark Schneegurt et al., 5.4 Algae, 2016.
- John D. Wehr, Freshwater Algae of North America (Second Edition), 2015.
- Robert G. Sheath and John D. Wehr, Freshwater Algae of North America, 2003.
- L. Naselli-Flores and R. Barone, Encyclopedia of Inland Waters, 2009.
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