Exploring Our Fluid Earth

Introduction to Aquatic Plants and Algae

Autotrophs are organisms that can make their own food. A plant is a familiar example of an autotroph (Fig. 2.2). Plants produce their own food through the process of photosynthesis. Photosynthesis converts light energy into chemical energy and food. Photosynthetic autotrophs are also called photo-autotrophs. This term distinguishes them from chemo-autotrophs, which are capable of converting carbon dioxide into chemical energy and food without light energy.

Fig. 2.2. (A) Rhizophora mangle, a red mangrove in the Dominican Republic

Image courtesy of Anton Bielousov, Wikimedia Commons

Fig. 2.2. (B) Posidonia oceanica, a Mediterranean seagrass

Image courtesy of Dr. Alberto Romeo, Wikimedia Commons

Fig. 2.2. (C) Flowering Elodea canadensis, a submerged freshwater plant

Image courtesy of Christian Fischer, Wikimedia Commons

Fig. 2.2. (D) Spartina alterniflora, a coastal salt marsh grass

Image courtesy of U.S. Department of Agriculture (USDA)

Photosynthesis is the process of converting light energy from the sun into chemical energy. Photosynthesis begins with the capture of light energy in the form of photons. Photons are tiny packets of energy. Like all other forms of energy, photons have the capacity to do work or move things. For more information about photosynthesis, please see the Biogeochemical Cycles unit.

Photons are absorbed by pigments inside autotroph cells. Pigments are chemical compounds that appear colorful and absorb light and, in doing so, appear as a specific color. Pigments occur in lots of places—flowers, corals, and even human skin have pigments. The most common pigment involved in photosynthesis is the green pigment Chlorophyll.

When a pigment absorbs photon energy, electrons from the pigment molecules are excited. The excited electrons enable a series of chemical reactions that build carbon-based sugar molecules, like glucose, from carbon dioxide (CO₂) and water (H₂O). The chemical reaction for photosynthesis is shown below:

This photosynthetic process transforms light energy into chemical energy. Chemical energy is energy stored in chemical bonds. A common form of chemical energy inside a living organism is a sugar molecule called glucose (C₆H₁₂O₆). Glucose can be used to build cell structures, like cell membranes and cell walls. Glucose can also be combined with other nutrients to form amino acids and proteins.

Living things must release the chemical energy stored in glucose before they can use it for growth, movement, reproduction, and other life processes. Cellular respiration is the process of breaking apart a glucose molecule to release stored chemical energy. Cellular respiration is the reverse of photosynthesis.

In addition to producing glucose, photosynthesis also releases oxygen. Oxygen is very important to life on Earth because it is necessary for cellular respiration. Photosynthetic autotrophs make life on Earth possible for other organisms. Interestingly, microscopic autotrophs produce more oxygen than aquatic and land plants combined. They are also responsible for making the earth habitable. The first autotrophs were bacteria called blue-green algae or cyanobacteria. Cyanobacteria first evolved on Earth over three billion years ago, and they radically changed the planet’s atmosphere by increasing the amount of oxygen.

Aquatic Plants

Plants are familiar examples of photosynthetic autotrophs. Most plant species are found in terrestrial habitats. Humans depend heavily on land plants such as wheat, corn, and tomatoes for food. Many plants also thrive in the water. Aquatic plants are plants that live in shallow coastal zones, wetlands, rivers, and lakes. Aquatic plants provide important food and habitat for other organisms.

Coastal aquatic plants such as mangroves (Fig. 2.2 A) and marsh grasses (Fig. 2.2 D) can tolerate wet conditions that would typically drown terrestrial plants. Many of these coastal species have also adapted to survive in salty seawater or brackish water. Some aquatic plant species have even adapted to live completely submerged in seawater (Fig. 2.2 B) or freshwater lakes (Fig. 2.2 C) away from their required sunlight and carbon dioxide gas.

Almost all plant species have evolved vascular, or vein-like, tissue that transports water and nutrients throughout the plant. Most plants also have distinct roots, shoots, and leaves. Aquatic plants have retained these characteristics. However, there are some plants without vascular tissues—the moss-like species that roughly resemble some seaweeds or algae.

Algae

The term algae (singular: alga) refers to a diverse group of photosynthetic organisms that thrive in aquatic environments (Fig. 2.3). The term algae includes over 350,000 species with representatives in multiple different phyla. Algae range from single-celled microbes floating in the water (Figs. 2.3 A and 2.3 B) to towering giant seaweeds (Fig. 2.3 D). All algae and plants are photosynthetic autotrophs.

Algae are difficult to define because the term describes such a wide diversity of organisms. Many species of algae, like larger seaweeds and giant kelp, appear similar to plants (Figs. 2.3 C and D). However, these algae are not true plants. Algae lack the vein-like vascular system found in most plants.

Algae are considered the most important photosynthetic organisms on Earth. Without algae, life in the ocean would be very different than it is. Small, microscopic algae serve as the base for most marine food webs (Figs. 2.3 A and 2.3 B). Larger, macroscopic algae provide food and three-dimensional habitats for other organisms (Figs. 2.3 C and 2.3 D). This role of algae in the ocean is similar to the way trees provide both food and habitat in a forest (Fig. 2.3 D).

Fig. 2.3. (A) A single diatom cell seen through a light microscope. Numbered ticks are 10 micrometers (µm) apart.

Image courtesy of Bob Blaylock, Wikimedia Commons

Fig. 2.3. (B) A bloom of microscopic phytoplankton near Gotland, Sweden as seen from space

Image courtesy of National Aeronautics and Space Administration (NASA)

Fig. 2.3. (C) Green macroalga sea lettuce, Ulva sp., in a coastal, rocky intertidal zone

Image courtesy of Anthony Giorgio, Wikimedia Commons

Fig. 2.3. (D) The brown macroalga giant kelp, Macrocystis pyrifera, in a Pacific kelp forest

Image courtesy of Claire Fackler, National Oceanic and Atmospheric Administration (NOAA)

Fig. 2.4. The tree of life showing placement of the algae groups within the domains Bacteria and Eukarya.

Image by Byron Inouye

Understanding the evolutionary relationships among organisms is important to biologists who want to know how communities of species have evolved. Molecular genetics has been helpful in providing information about many of these relationships in algae. However, there is still much that is not understood. Algae as a group have members in several distantly-related branches of the tree of life (Fig. 2.4). For example, cyanobacteria, or “blue-green algae”, are single-celled organisms in the domain Bacteria. Red algae, Rhodophyta, are multicellular organisms in the domain Eukarya. And, within the domain Eukarya, algae are spread among many of the major divisions. For more information about phylogenetic trees, see the Classification of Life topic.

Identifying and classifying algae species is difficult. Most textbooks and field guides have divided algae into broad groups based on the type of photosynthetic pigments they contain. Large plant-like seaweed algae, or macroalgae, are generally classified into three groups: Chlorophyta (green algae), Rhodophyta (red algae), and Phaeophyceae (brown algae). Microscopic algae include diatoms and dinoflagellates. Cyanobacteria (also called blue-green algae) are technically bacteria. However, cyanobacteria are included with algae because they photosynthesize and form large colonies.

Prokaryotes and Eukaryotes

Fig. 2.5. Diagram of evolutionary tree of life showing three domains: Bacteria, Archaea, and Eukaryota. Bacteria and Archaea species (shown in blue and red, respectively) are considered prokaryotic. Species in the domain Eukaryota (sometimes “Eukarya;” show in brown) are considered eukaryotic.

Image courtesy of National Aeronautics and Space Administration (NASA), modified by David Lin

All living organisms can be placed into two broad categories: prokaryotes and eukaryotes (Fig. 2.5). Prokaryotes include all organisms from the domains Bacteria and Archaea (Fig. 2.5). Prokaryotes were the first organisms to exist on Earth. They evolved approximately 3.5 billion years ago.

The first eukaryotic organisms, by comparison, evolved much later—a mere 1.6 to 2.2 billion years ago. Eukaryotes include all organisms from the domain Eukaryota and include such things as plants, animals, and fungi. Scientists have estimated that there are 8.7 million species of eukaryotes alive on Earth today, but only 1.2 million species have been formally described so far.

Fig. 2.6. Comparative cell anatomy of eukaryotes and prokaryotes

Image courtesy of Science Primer, National Center for Biotechnology Information, modified by David Lin

The main differences between prokaryotes and eukaryotes are shown in Table 2.1. The cells (the term cell is defined in the Properties of Life topic) of eukaryotes are generally more complex than the cells of prokaryotes (Fig. 2.6). Most algae are eukaryotes, although the term algae is also used to describe photosynthetic prokaryotes such as cyanobacteria.

The main difference between prokaryotes and eukaryotes is their cell structure (Fig. 2.6). Prokaryotes are single-celled organisms that range in size from one to 10 micrometers (μm). They are generally less complex than eukaryotic cells.