What are Algae? Fondriest.com
Algae are aquatic, plant-like organisms. They encompass a variety of simple structures, from single-celled phytoplankton floating in the water, to large seaweeds (macroalgae) attached to the ocean floor. Algae can be found residing in oceans, lakes, rivers, ponds and even in snow, anywhere on Earth.
So what makes algae only plant-like, instead of plants? While algae are often called primitive plants, other terms, like protists, can be used. Protist may be a more accurate term, particularly for the single-celled phytoplankton. However, larger, more complex algae, including kelp and chara, are often mistaken for submerged plants.
The difference between these seaweeds and submerged plants is in their structure. Macroalgae are simpler, and attach themselves to the seabed with a holdfast instead of true roots. Aquatic plants, whether floating, submerged, or emergent (starting in the water and growing out) have specialized parts such as roots, stems and leaves. Most plants also have vascular structures (xylem and phloem), which carry nutrients throughout the plant. While algae contain chlorophyll (like plants), they do not have these specialized structures.
As algae can be single-celled, filamentous (string-like) or plant-like, they are often difficult to classify. Most organizations group algae by their primary color (green, red, or brown), though this creates more problems than it solves. The various species of algae are vastly different from each other, not only in pigmentation, but in cellular structure, complexity, and chosen environment. As such, algal taxonomy is still under debate, with some organizations classifying algae under different kingdoms, including Plantae, Protozoa and Chromista. While the overarching kingdom classiffication is not always agreed upon, the species, genus, family, class and phylum of each alga generally are.
To further complicate this nomenclature, single-celled algae often fall under the broad category of phytoplankton.
Phytoplankton are microorganisms that drift about in water. They are single-celled, but at times they can grow in colonies large enough to be seen by the human eye. Phytoplankton are photosynthetic, meaning they have the ability to use sunlight to convert carbon dioxide and water into energy. While they are plant- like in this ability, phytoplankton are not plants. The term “single-celled plants” is a misnomer, and should not be used. Instead, phytoplankton can be divided into two classes, algae and cyanobacteria. These two classes have the common ability of photosynthesis, but have different physical structures. Regardless of their taxonomy, all phytoplankton contain at least one form of chlorophyll (chlorophyll A) and thus can conduct photosynthesis for energy.
Phytoplankton, both algae and cyanobacteria, can be found in fresh or saltwater. As they need light to photosynthesize, phytoplankton in any environment will oat near the top of the water, where sunlight reaches. Most freshwater phytoplankton are made up of green algae and cyanobacteria, also known as blue-green algae. Marine phytoplankton are mainly comprised of microalgae known as dinoflagellates and diatoms, though other algae and cyanobacteria can be present. Dinoflagellates have some autonomous movement due to their “tail” (flagella), but diatoms are at the mercy of the ocean currents.
Despite their ability to conduct photosynthesis for energy, blue-green algae are a type of bacteria. This means that they are single-celled, prokaryotic (simple) organisms. Prokaryotic means that the cyanobacteria do not have a nucleus or other membrane-bound organelles within their cell wall.
Cyanobacteria are the only bacteria that contain chlorophyll A, a chemical required for oxygenic photosynthesis (the same process used by plants and algae). This process uses carbon dioxide, water and sunlight to produce oxygen and glucose (sugars) for energy. Chlorophyll A is used to capture the energy from sunlight to help this process. Other bacteria can be considered photosynthesizing organisms, but they follow a different process known as bacterial photosynthesis, or anoxygenic photosynthesis. This process uses bacteriochlorophyll instead of chlorophyll A. These bacteria cells use carbon dioxide and hydrogen sulfide (instead of water) to manufacture sugars. Bacteria cannot use oxygen in photosynthesis, and therefore produce energy anaerobically (without oxygen). Cyanobacteria and other phytoplankton photosynthesize as plants do, and produce the same sugar and oxygen for use in cellular respiration.
In 2011, Lake Erie experienced the worst blue-green algae bloom in decades (Photo Credit: MERIS/NASA; processed by NOAA/NOS/NCCOS )
In addition to chlorophyll A, blue-green algae also contain the pigments phycoerythrin and phycocyanin, which give the bacteria their bluish tint (hence the name, blue-green algae). Despite not having a nucleus, these microorganisms do contain an internal sac called a gas vacuole that helps them to oat near the surface of the water.
What is chlorophyll?
Chlorophyll is a color pigment found in plants, algae and phytoplankton. This molecule is used in photosynthesis, as a photoreceptor. Photoreceptors absorb light energy, and chlorophyll specifically absorbs energy from sunlight. Chlorophyll makes plants and algae appear green because it reflects the green wavelengths found in sunlight, while absorbing all other colors.
However, chlorophyll is not actually a single molecule. There are 6 different chlorophylls that have been identified. The different forms (A, B, C, D, E and F) each reflect slightly different ranges of green wavelengths. Chlorophyll A is the primary molecule responsible for photosynthesis. That means that chlorophyll A is found in every single photosynthesizing organism, from land plants to algae and cyanobacteria. The additional chlorophyll forms are accessory pigments, and are associated with different groups of plants and algae and play a role in their taxonomic confusion. These other chlorophylls still absorb sunlight, and thus assist in photosynthesis. As accessory pigments, they transfer any energy that they absorb to the primary chlorophyll A instead of directly participating in the process.
Chlorophyll B is mainly found in land plants, aquatic plants and green algae. In most of these organisms, the ratio of chlorophyll A to chlorophyll B is 3:1. Due to the presence of this molecule, some organizations will group the green algae into the Plant Kingdom. Chlorophyll C is found in red algae, brown algae, and ↑ dinoagellates. This has lead to their classification under the Kingdom Chromista. Chlorophyll D is a minor pigment found in some red algae, while the rare Chlorophyll E has been found in yellow-green algae. Chlorophyll F was recently discovered in some cyanobacteria near Australia. Each of these accessory pigments will strongly absorb different wavelengths, so their presence makes photosynthesis more efficient.
Chlorophyll is not the only photosynthetic pigment found in algae and phytoplankton. There are also carotenoids, and phycobilins (biliproteins). These accessory pigments are responsible for other organism colors, such as yellow, red, blue and brown. Like chlorophylls B, C, D, E and F, these molecules improve light energy absorption, but they are not a primary part of photosynthesis. Carotenoids can be found in nearly every phytoplankton species, and reflect yellow, orange and/or red light. There are two phycobilins found in phytoplankton: phycoerythrin and phycocyanin. Phycocyanin reflects blue light and is responsible for cyanobacteria’s common name – blue-green algae. Phycoerythrin reflects red light, and can be found in red algae and cyanobacteria.
Some algae will appear green despite the presence of these accessory pigments. Just as in plants, the chlorophyll in algae has a stronger relative absorption than the other molecules. Like a dominant trait, the more intense, reflected green wavelengths can mask the other, less-reflected colors. In green algae, chlorophyll is also found at a higher concentration relative to the accessory pigments. When the accessory pigments are more concentrated (such as in red algae, brown algae and cyanobacteria), the other colors can be seen.
The different forms of chlorophyll absorb slightly different wavelengths for more efficient photosynthesis.
What is Photosynthesis?
Each pigment absorbs and reflects different wavelengths, but they all act as accessory pigments to chlorophyll A in photosynthesis.
Photosynthesis is the process by which organisms use sunlight to produce sugars for energy. Plants, algae and cyanobacteria all conduct oxygenic photosynthesis. That means they require carbon dioxide, water, and sunlight (solar energy is collected by chlorophyll A). Plants and phytoplankton use these three ingredients to produce glucose (sugar) and oxygen. This sugar is used in the metabolic processes of the organism, and the oxygen, produced as a byproduct, is essential to nearly all other life, underwater and on land.
Photosynthesis uses water, carbon dioxide and sunlight to produce energy and oxygen.
Underwater Photosynthesis… Phytoplankton drifting about below the surface of the water still carry out photosynthesis. This process can occur as long as enough light is available for the chlorophyll and other pigments to absorb. In the ocean, light can reach as far as 200m below the surface. This region where sunlight can reach is known as the euphotic zone. Phytoplankton and other algae can be found throughout this zone.
What Affects Photosynthesis? As light is required for photosynthesis to occur, the amount of light available will affect this process. Photosynthetic production peaks during the day and declines after dark. However, not all light can be used for photosynthesis. Only the visible light range (blue to red) is considered photosynthetically active radiation. Ultraviolet light has too much energy for photosynthesis, and infrared light does not have enough. If phytoplankton are exposed to too much UV light, the excessive solar energy can break molecular bonds and destroy the organisms’ DNA.
Within the visible light spectrum, chlorophyll strongly absorbs red and blue light while reflecting green light. This is why phytoplankton, particularly cyanobacteria, can thrive at the bottom of the euphotic (sunlit) zone, where only blue light can reach. As blue light is both high in energy and strongly absorbed by chlorophyll, it can be used effectively in photosynthesis.
Turbidity, or the presence of suspended particles in the water, affects the amount of light that reaches into the water. The more sediment and other particles in the water, the less light will be able to penetrate. With less Blue and red light are used more efficiently in photosynthesis. on the lakebed, as more light is available. . light available, photosynthetic production will decrease. In turbid water, photosynthesis is more likely to occur at the water’s surface than…
Water temperature will also affect photosynthesis
rates. As a chemical reaction, photosynthesis is
initiated and sped up by heat. As
photosynthesis production increases, so will
phytoplankton reproduction rates. This factors
into the large, seasonal swings of phytoplankton
populations. However, the extent to which
temperature affects photosynthesis in algae and
cyanobacteria is dependent on the species. For all
phytoplankton, photosynthetic production will
increase with the temperature, though each
organism has a slightly different optimum
temperature range. When this optimum
temperature is exceeded, photosynthetic activity
will in turn be reduced. Too much heat will
denature (break down) the enzymes used during
the process, slowing down photosynthesis instead of speeding it up.
To survive, every living thing needs organic carbon. Organic carbon can be found in many different things including sugars (glucose = C6H12O6), plants and animals. Phytoplankton produce their required sugar through photosynthesis. As they are able to produce their own energy with the help of light, they are considered autotrophic (self-feeding). Phytoplankton and other autotrophs are called primary producers, and make up the bottom of the food web. These organisms are called “primary” because all other organisms rely on them (directly or indirectly) as a food source.
Phytoplankton are generally consumed by zooplankton and small marine organisms like krill. These creatures are then consumed by larger marine organisms, such as fish. This chain continues up to apex predators, including sharks, polar bears and humans.
During the photosynthetic process, phytoplankton produce oxygen as a byproduct. Due to their vast and widespread populations, algae and cyanobacteria are responsible for approximately half of all the oxygen found in the ocean and in our atmosphere. Thus oceanic lifeforms not only feed o the phytoplankton, but also require the dissolved oxygen they produce to live.
Before plants, algae and phytoplankton used
water for photosynthesis, bacteria used H2S
and other organic compounds to x CO2.
Early cyanobacteria were the first organism to
use water to x carbon. The use of H2O
introduced free oxygen (O2) into the
environment as a byproduct. The start of
oxygenic photosynthesis was a turning point
for Earth’s history. This process slowly changed
the inert Precambrian atmosphere into the
oxygen-rich environment known today. Though microscopic, early cyanobacteria have made a permanent impact on the Earth’s environment.
In addition to providing food and oxygen for nearly all life on Earth, phytoplankton help to regulate inorganic carbon (carbon dioxide) in the atmosphere. During photosynthesis, carbon dioxide and water molecules are used to make sugar for energy. The process of incorporating inorganic carbon into organic carbon (glucose and other biologically useful compounds) is called carbon fixation, and is part of the biological carbon pump.
As carbon fixation and oxygen production are part of the same process, the extent of phytoplankton’s participation is on the same scale. Phytoplankton consume a similar amount of carbon dioxide as all land plants combined. While phytoplankton can pull carbon dioxide from the atmosphere or the ocean, it will have a similar effect. CO2 that is taken from the water is replaced by CO2 from the atmosphere, thanks to Henry’s law (the dissolved gas content of water is proportional to the percentage of gas in the air above it. This consumption helps keep carbon dioxide levels in check, reducing its presence as a greenhouse gas.
Algae and cyanobacteria help to regulate the climate by fixing carbon dioxide from the atmosphere. This carbon is then consumed or decomposed by other organisms, making its way through the cycle until it is released as dissolved carbon dioxide in water or deposited in sediment.
When carbon dioxide is consumed, the carbon molecules become incorporated into the phytoplankton’s structure, allowing the organism to function and grow. If the phytoplankton is not eaten by another organism (passing on the carbon up the food chain), then it will sink into the ocean when it dies. As with other detritus (non-living organic material), the phytoplankton will be decomposed by bacteria, and the carbon is either released back into the ocean as dissolved carbon dioxide or eventually deposited into the seafloor sediment. Thanks to phytoplankton, this biological carbon pump removes approximately 10 trillion kilograms (10 gigatonnes) of carbon from the atmosphere every year, transferring it to the ocean depths.
In climate terms, this process helps to maintain global surface temperatures. Without this cycle, atmospheric CO2 would rise approximately 200 ppm (current levels are around 400 ppm). Even small changes in phytoplankton populations could have an effect on the atmosphere and world climate.