Evidence of Common Ancestry and Diversity (2024)

Origins of Algae Lineages

The evolutionary history of algae is remarkable. Over 350,000 species of algae have evolved in several branches of the prokaryotic and eukaryotic trees of life. The evolutionary relationships of algae are closely tied to the evolution of certain organelles in the domain Eukaryota.

Mitochondria and chloroplasts are both organelles that came from a prokaryotic ancestor. In both cases, a eukaryotic host cell engulfed a prokaryotic cell. Over time, the prokaryotic cell became dependent on the host eukaryotic cell.

Division Cyanophyta: Blue-green Algae

The division Cyanophyta is made up of the blue-green algae (Fig. 2.25). Blue-green algae are actually prokaryotic bacteria, also known as cyanobacteria. Cyanobacteria are capable of photosynthesis (with very few exceptions). As a group, they have representatives across aquatic and terrestrial habitats—from the tropics to the poles.

Cyanobacteria algae were the dominant life forms on earth for more than 1.5 billion years. They were the first organisms to photosynthesize and to produce chlorophyll and other pigments. Cyanobacteria represent the evolutionary origin of chloroplasts in all eukaryotic algae and vascular plants.

Visible evidence of cyanobacteria is often recorded in the fossil record as mound-shaped structures called stromatolites. Stromatolites are layers of sediment and calcium carbonate deposited by cyanobacteria over long periods of time (Fig. 2.26).

In general, cyanobacteria are small and relatively inconspicuous. They are found as simple, unicellular forms or as filamentous forms. The majority of cyanobacteria use chlorophyll a and pigments like phycocyanin. These pigments make them blue-green and give the group its common name: blue-green algae.

Some species of cyanobacteria have other colors of pigments, like the pink pigment phycoerythrin and the orange pigments known as carotenoids. Cyanobacteria themselves can be blue, purple, brown, black, or green in color. In fact, the Red Sea gets its name from occasional blooms of a reddish cyanobacteria Oscillatoria, and African flamingos get their pink color from eating the cyanobacteria Spirulina.

Cyanobacteria are also the only algae group able to transform atmospheric nitrogen into more usable forms, like ammonia, which can then be converted to amino acids and proteins. The process of nitrogen conversion is called nitrogen fixation. Nitrogen fixation is important because, although atmospheric nitrogen (N₂) is very abundant, most organisms cannot use it. Thus, nitrogen can be a limiting nutrient for growth. Nostoc is a genus of filamentous cyanobacteria that has specialized cells, called heterocysts, that can fix nitrogen (Fig. 2.27).

Division Chlorophyta: Green Algae and Vascular Plants

Chlorophyta is a division of algae known as the green algae. Most species occur in freshwater and coastal marine habitats. Scientists estimate that there are 4,000 to 7,000 species of green algae. Figure 2.28 shows some representatives of green algae.

Not all green algae are green. Chlamydomonas nivalis is a species known for thriving in snow and ice. Its common name, watermelon snow, reflects the alga’s appearance as a red or pink tint on the surface of summer snow (Fig. 2.29).

All green algae (Chlorophyta) and plants share a common evolutionary ancestor. They both contain the photosynthetic pigments chlorophyll a and chlorophyll b. The two lineages diverged between 630 million and 510 million years ago. Terrestrial plants lacking vascular tissue appear in the fossil record around 476 million years ago. These early land plants were likely similar to modern liverworts, hornworts, and mosses (Fig. 2.30).

Vascular plants evolved around 430 million years ago. Their vein-like, vascular tissues distribute water and nutrients throughout the organism. These structures allow vascular plants to grow to larger sizes than non-vascular plants. The vast majority of the 300,000 to 315,000 living plant species are vascular plants. Vascular plant species include ferns, conifer trees, and flowering plants.

Most of the food crops humans eat are flowering plants, also called angiosperms. Almost all aquatic plants are also angiosperms. They appear to have evolved aquatic adaptations independently across several of different flowering plant families. Modern aquatic plants present a wide diversity of forms ranging from mangrove trees to seagrasses to waterlilies.

Division Rhodophyta: Red Algae

The division Rhodophyta are the red algae. There are between 5,000 and 6,000 species of red algae, most of which are marine. However, there are approximately 150 species of freshwater red algae. Red algae range from unicellular to multicellular, with a large diversity of macroalgae forms in the intertidal and subtidal zones (Figs. 2.31 B and 2.31 C). Some red algae are calcified, meaning they have calcium carbonate in their tissue (Fig. 2.31 D). Some of these calcified forms lay flat and form an encrusting surface as they grow. These crusts can actually help stabilize coral reef habitats and provide a surface for new coral to grow on.

Most red algae are red or pink in color due to the photosynthetic pigment phycoerythrin. This pigment is red and covers up the green color of chlorophyll a. Phycoerythrin is very efficient at absorbing blue and green wavelengths of light. This can be useful in subtidal habitats were much of the red light has been absorbed or reflected by water. In fact, a species of red crustose coralline algae is the deepest known living photosynthetic organism. It was found at over 260 meters deep in the San Salvador Islands, Bahamas.

The red algae are very important economically. Many useful commercial products come from red algae. For example, products such as agar, agarose, and carrageenan are used in foods and pharmaceuticals—from toothpaste to ice cream to medicine. These compounds are difficult to synthesize industrially, so they are commonly harvested from red algae (Fig. 2.32).

Class Phaeophyceae: Brown Algae

The class Phaeophyceae consists of almost 2,000 species of brown macroalgae. These algae are commonly found near intertidal and subtidal coastlines. The largest forms of brown algae are sometimes referred to as kelp (Fig. 2.33 B). Some of these species can grow up to two feet per day and reach heights of 150 feet. Giant kelp grows in large forests along the coastline, and supports a large variety of marine animals.

Brown algae have unique structures called pneumatocysts, which are gas-filled bladders that provide buoyancy and allow them to grow upright. The small, round structures in the brown alga, Sargassum natans, are pneumatocysts that allow this species to float on the ocean surface (Fig. 2.33 A).

Scientists believe brown algae evolved through endosymbiosis of a red alga. Because of this, brown algae have historically been included in a diverse group of eukaryotic organisms called stramenopiles. Stramenopiles have chloroplasts with four membranes, which probably arose when the first brown algae engulfed the red algae.

Most stramenopiles also have two unequal length flagella at some point in their life cycle. Stramenopiles are primarily photosynthetic, however not all stramenopiles are brown algae. For example, the organisms that cause potato rot and seed rot are both stramenopiles.

Diatoms

Diatoms are one of the most common types of phytoplankton, and are found in both marine and freshwater environments. Diatoms are also found in damp environments such as soil. Although most diatoms exist as single cells, they can form colonial chains of organisms.

The diatom group is large with more than 200 different genera, 100,000 species, and a wide variety of shapes and sizes (Fig. 2.34). Diatoms first appear in the fossil record of marine areas during the cretaceous period, about 100 million years ago. The chloroplasts of diatoms are very similar to those of red algae.

Their silica cell wall makes diatoms unique in most aquatic environments. The diatom silica wall is rigid and highly resistant to decay. These algae are incredibly important to the global ocean; diatoms make up a majority of phytoplankton, and they are responsible for as much as half of the ocean’s photosynthesis.

An interesting feature of diatoms is their ability to convert ammonia to urea using the urea cycle. The conversion to urea is important for marine organisms because urea is much less toxic to tissues than ammonia. Before scientists discovered that diatoms could convert ammonia to urea, the urea cycle had only been found in animals.

Diatoms are incredibly useful to scientists because they act as water quality indicators in aquatic environments. Diatoms have been used to study how local environmental conditions have changed as a result of pollution and also to examine long-term changes due to climate change. Diatoms found in rivers are particularly useful for studying the effects of pollution because they are attached to the riverbed, and their population distribution can be compared to that of unpolluted sites.

Dinoflagellates

Dinoflagellates are single-celled, eukaryotic organisms found in freshwater and marine environments. Many marine dinoflagellates are photosynthesizers that create food using light, but some are also predatory, meaning that they feed on prey. There are more than 1,500 described species of dinoflagellates (Fig. 2.35). They can be found as free-living planktonic organisms or as symbionts in reef habitats.

Certain dinoflagellates undergo bioluminescence, emitting light energy from chemical reactions within the cell. These bioluminescent dinoflagellates emit a blue-green light when they are disturbed by the crashing of waves or the motor of a boat (Fig. 2.35 B). They are thought to use bioluminescence as a defense, by startling predators and calling attention to even larger predators.

Symbiodinium is a genus of dinoflagellates that form a symbiotic relationship with some corals and anemones (animals in the phylum Cnidaria). The Symbiodinium are collected from the water column and engulfed by the coral tissue. The Symbiodinium dinoflagellates then reproduce within the coral tissue.

Symbiodinium absorb sunlight and complete photosynthesis to produce oxygen and glucose (Fig. 2.35 C). Symbiodinium generate as much as 85 percent of the chemical energy used by coral. Symbiodinium are critically important in maintaining a healthy reef ecosystem.

Corals owe most of their bright coloring to the Symbiodinium microalgae living within their tissues. When a coral becomes stressed, such as with a sudden increase in seawater temperature, the coral expels the Symbiodinium into the water column—resulting in coral bleaching (Fig. 2.36). If water temperatures cool down and the coral is no longer stressed, it can recruit new Symbiodinium from the water column back into its tissues. However, if the heat persists, the corals starve and die.

A red tide is a bloom of certain dinoflagellate species that result in a reddish-brown color on the ocean surface (Fig. 2.37). These dinoflagellates can produce toxins that result in the death of marine mammals, fish, and birds. For example, the dinoflagellate Karenia brevis produces a brevetoxin that causes eye and respiratory irritation to people near the shoreline.

Although many algae blooms occur naturally, red tides and other harmful algae blooms are caused by increased nutrient levels as the result of human activities. Coastal water pollution and runoff leads to an increase in phosphates and nitrates, providing the nutrients needed for algae blooms to occur. Toxins from harmful algae blooms can accumulate in filter-feeding animals such as oysters and clams and present a health hazard to human consumers.