4. Algae and plankton
Contents
4.1 Microbes
Microbes are organisms that one cannot see without a microscope. All bacteria and archaea are microbes.
Most protists are also microbes. Organisms such as paramecia and amoebas are examples of protists. They thrive in wet living environments. Some fungi, such as molds and yeasts, are also microbes.
Viruses, such as the influenza virus, are also often called microbes, even though they are not technically living organisms.
The traditional categorization of living organisms. Prokaryotic organisms (bacteria and archaea) do not have nuclei in their cells. Protists, plants, animals, and fungi are all eukaryotic: their cells have nuclei.
Small microbes dry up easily. Because of this, they require a wet living environment in order to survive. Water also makes it possible for them to move from one place to another. This is why most microbes live in the water.
Phytoplankton and cyanobacteria are significant producers in water ecosystems. They use sunlight to produce chemical energy. This energy is then transferred up the food chain to consumers. This happens for example when a water flea eats a small alga.
Many microbes function as decomposers in the soil. Decomposers free up nutrients that are bound to dead organisms or their fecal matter and release these nutrients back to the environment. When this happens, the nutrients are then used by other organisms.
Some microbes are pathogens. This means that they cause diseases. For example, drinking dirty water can result in diarrhea caused by a microbe.
Microbes were the first organisms on planet Earth. Over time, unicellular bacteria developed into more complex multicellular organisms that had specialized cells for different functions. Multicellularity also made it possible for organisms to develop structures that protected them from drought. This made it possible for life to move from water to land. Photosynthetic bacteria and unicellular algae also produced oxygen that formed a planetary atmosphere that could sustain more complex forms of life.
4.2 Plankton
Plankton are small water-dwelling organisms. Most of them cannot be seen without a microscope. Plankton sometimes form large, concentrated distributions that can color the water green or red.
Plankton flow in the water. When they do this, they do not actively swim or float on the surface of the water. Instead, they remain close to the surface by using structures such as gas vacuoles or small drops of oil.
Unicellular plankton organisms reproduce asexually by division. The asexually produced daughter cells are genetically identical with their mother cell. Asexual reproduction can happen extremely fast when the conditions are right.
Algae belong to the group of phytoplankton, and other microscopically small organisms belong to the group of animal plankton.
Animal plankton can reproduce either sexually or asexually.4.3 Phytoplankton
Organisms belonging to the group of phytoplankton are autotrophic. This means that they produce energy through photosynthesis, a process where water and carbon dioxide are transformed into oxygen and sugar with the help of sunlight.
The amount of phytoplankton in the oceans and freshwater ecosystems of planet Earth is enormous. They form the basis for the food chains and food webs of water ecosystems. When phytoplankton photosynthesize, they produce oxygen that is released into the surrounding water. Therefore, phytoplankton contribute significantly to the oxygen concentration of water ecosystems.
Phytoplankton are found everywhere in the water, but photosynthesis can only happen only near the surface of the water, where there is enough sunlight. This is why phytoplankton are mostly found near the surface of the water.
Some phytoplankton produce poisonous substances. Most of the time these substances are not dangerous. However, when large concentrations of plankton occur, they can be harmful even to humans. Some mass concentrations of blue-green algae or cyanobacteria contain poisonous substances that are harmful to humans and other animals. If blue-green algae can be seen on the surface of the water, one should not swim in it, as contact can result in nausea and a rash.
Phytoplankton are studied attentively, as changes in species can tell a lot about changes in water quality. When the amount of nutrients in the water increases, the number of diatoms and cyanobacteria in the water likewise grows. This phenomenon is known as eutrophication.
Mass concentrations of cyanobacteria are easily distinguishable.
4.4 Animal plankton
The group of animal plankton includes small heterotrophic organisms that inhabit water ecosystems. Heterotrophic organisms receive energy by consuming other living organisms.
Some animal plankton, such as ciliates, are unicellular protists. The group of animal plankton also includes small multicellular crustaceans such as copepods and water fleas.
The smallest animal plankton are only 50 micrometres in diameter, whereas the largest are a few millimetres in diameter. When you gather animal plankton in a test tube, you can distinguish small movements even without a microscope!
Most animal plankton move by flowing along water currents. Some animal plankton can also swim by using their antennas or cilia.
Animal plankton use smaller phytoplankton or other small organisms as their source of energy. They themselves are also the food of other organisms. Especially small or young fishes like to eat animal plankton.
The group of animal plankton is often considered to contain only small, microscopic animals. However, our waters are inhabited by a number of larger species that can be considered plankton. The most famous of these is the jellyfish found in the Baltic Sea. Large jellyfish can swim, but most of the time they flow along water currents.
The jellyfish is a cnidarian. It can swim by contracting its jelly-like body cavity.
4.5 Terminology
Flowing
For the most part, plankton move by flowing along water currents.
Autotrophic
An autotrophic organism does not require food. Instead, it can produce its own energy by using sunlight to photosynthesize. All producers are autotrophic.
Eutrophication
The drastic increase in the amount of producers (such as algae and cyanobacteria) that is caused by the increase in the amount of nutrients (especially nitrogen and phosphorous) in a water ecosystem.
Asexual reproduction
Reproduction without fertilization, for example by division. Offspring that are produced asexually are genetically identical to their parent. Asexual reproduction is the simplest and fastest method of reproduction for living organisms.
Sexual reproduction
Reproduction with the use of gametes or sex cells. Sexual reproduction increases the genetic diversity of the individual.
Heterotrophic
An organism that receives its energy by consuming other living or dead organisms. Consumers and decomposers are both heterotrophic.