Author: Jovana Kostić-Vuković
Keywords: decomposers, food chain, food web, energy flow, matter cycling
Short intro: In this article, you will learn about the importance of microorganisms in aquatic ecosystems and their role in water self-purification and aquatic food web. These organisms are the most ecologically aware organisms and naturally born recyclers.
The article provides also some great ideas for outdoor activities/experiments you can do to explore the topic. We will explore the relation of microorganisms to oxygen by activity „Make a home for microbes“, and you will also be challenged to make a food chain.
1. Ecological Roles of freshwater microbes
When we’re swimming in a river or lake, we really don’t want to think about all those little microorganisms that surround us, do we? Still, although not visible they play an extremely important role in the health and life of aquatic ecosystems. Those little creatures do some pretty big jobs for the entire world. They process elements and nutrients into new forms, decompose dead organisms much bigger than they are and turn them into nutrients. One big difference between humans and bacteria is that we eat food that is much smaller than us. Just imagine – bacteria can eat a whole elephant!
A healthy river will have the right mix of good microorganisms to keep all environmental factors in balance. When one or more of these factors get out of balance, the consequences can disrupt other organisms living in or using the river. Just as a doctor measures specific factors in your body to evaluate your health, observing the progression of changes in a river’s microorganisms can give us an information if the water is getting healthier or sicker. The stuff and waste that humans put into the river can have an enormous impact on the balance of aquatic microorganisms.
Like all ecosystems, freshwater ecosystems require energy inputs to sustain the organisms within. In lakes and streams, plants and certain microbes perform photosynthesis and harvest the energy of the sun. Microbial photosynthesizers include algae and cyanobacteria which are considered as primary producers. Other organisms called consumers, feed on these organisms and form a link in the food chain. Decomposers are third, an especially important group of organisms in the food chain. They consume dead bodies of plants, animals, and other microbes. and convert detritus (dead and decaying matter) and organic matter into simple nutrients, such as nitrate, phosphate, and sulfate. Decomposers are essential for major biogeochemical cycles by which nutrients are exchanged between living and nonliving parts of the ecosystem. Without microbial decomposers, minerals and nutrients critical to plant and animal growth would not be available to support other levels of the freshwater food chain (www.waterencyclopedia.com).
Biogeochemical cycle (www.cnx.org)
1. Aerobic and Anaerobic decomposition
In water, aerobic decomposers need oxygen to do their work and survive. The lapping waves and babbling brook increase the level of dissolved oxygen that is crucial to so many creatures in aquatic ecosystems, none more so than the bacteria. If there is not enough oxygen in the water, many parts of the system suffer: the aerobic decomposers cannot digest dead matter, insects cannot develop and mature, and the fish cannot play their part. Eventually, the stream or pond will be changed, starting at the microbial level (www.waterencyclopedia.com).
Human activities can jeopardize parts of this system in a variety of ways, but especially through the runoff of fertilizers or sewage into a waterbody. Both contain nutrients that plants, algae, and cyanobacteria can use to grow. Excessive amounts of nutrients can lead to their very rapid growth and depletion of dissolved oxygen supply, which will lead to severe changes in the aquatic ecosystem. If the condition becomes severe enough, only a few species known as anaerobes which are tolerant to low-oxygen conditions will survive. This process, called cultural eutrophication, can have profound and lasting consequences on the waterbody.
Cultural eutrophication (www.creeklife.com)
When the oxygen cycle is disrupted, reduction of organic matter will be performed by anaerobic bacteria. Thus, the self-purification capacity of natural waters represents a delicate balance between the amount of oxygen absorbed from the atmosphere, the amount of oxygen produced by the vegetation, and the amount of oxygen used to oxidize present organic matter. This balance will be easily disturbed if high amounts of organic waste are introduced in water, whose decomposition will lead to depletion of oxygen faster than it may be restored. Constant input of untreated waste in waterways in amounts that surpass the self-cleaning capacity of the river will quickly lead to the establishment of anaerobic conditions with consequent deterioration of plant, animal, fish life, due to the lack of oxygen and silting up of water streams with the partially decomposed organic matter. In such conditions, natural waters become an unfavorable environment for life, with unpleasant odor and deprived of the living world.
Depletion of oxygen due to the excess input of organic waste (www.blog.nus.edu.sg)
Activity: Make Winogradsky column – a home for aerobic and anaerobic microbes (Recipe: https://www.amnh.org/ology/features/how-to-make-a-winogradsky-column/)
3. Food Chains and Food Webs
The energy and matter produced by autotrophic organisms are distributed to other organisms in an ecosystem through pathways known as food chains and food webs.
A food chain represents the simple linkage between producers and consumers based on the feeding relationship. For example, an insect, small fish eating an insect, large fish eating small fish and humans eating large fish are connected in a food chain. The trophic level is an organism’s position in the food chain as determined by the number of energy-transfer steps required to reach that level. A fish that has consumed an insect, which itself has just consumed algae, is at a higher trophic level than the insect (www.orangesenqurak.com).
An example of food chain (www.nps.gov)
On the other hand, food web represents a network of linked food chains, since usually organisms consume, and are consumed by more than one type of organism.
Food web (www.picrevise.net)
Food webs connect autotrophs, which form the very base of the web, to the herbivores (primary consumers) and then to various carnivores (secondary consumers). In rivers, as in the majority of other aquatic and terrestrial systems, the energy at the base of a food web comes from the sun and it is fixed by autotrophic organisms through photosynthesis. Energy from the sun is transferred through the ecosystem by passing through various trophic levels. The energy pyramid has a large base and becomes smaller at the top. At the base or the first level are the producers. Here, the energy and the number of organisms (biomass) are very high. At the second level are the various consumers. For each level or feeding stage, the energy and number of organisms decrease so the pyramid becomes smaller at the top. Roughly, only 10% of the energy is transferred from one trophic level to the next, while 90% is lost at each level because it is used for metabolic activities of organisms or it is given off as heat.
Energy pyramid (www.en.wikipedia.org)
Energy derived from terrestrial plants enters the water in the form of plant parts, such as leaves or twigs, or in the form of dissolved organic matter. This material is used as a source of energy by microorganisms such as fungi and bacteria, and by invertebrates. Plants in the river are also important in food webs. Microscopic algae are often eaten while alive, while larger aquatic plants mainly enter food chains after they have died.
Cascade interactions occur in food webs when one group of organisms indirectly affects another group, by feeding on animals that eat the other group. For example, when predators consume herbivores, the plants that the herbivores would otherwise have consumed will multiply. Also, in the case of the oil spill, the number of bacteria and fungi that break down detritus and provide nutrients for autotrophs will change leading to further changes at all levels of the food web.
Looking at ecosystems in terms of food chains and webs, however, can help us understand how species introduction or removal impacts the environment (www.ramp-alberta.org).
4. Energy and matter in ecosystems
Ecosystem ecologists are often most interested in tracing the flow of energy and cycling of matter through ecosystems. A movement of energy and matter can be followed if we consider food webs – networks of organisms that feed on one another, and biogeochemical cycles – pathways taken by chemical elements during movement through the biosphere.
Both energy and matter are conserved, they cannot be created or destroyed, but they take different routes through ecosystems:
• The matter is recycled, in a way that the same atoms are reused over and over again.
• Energy flows through the ecosystem, usually entering as light and exiting as heat.
4.1 Matter is recycled
The matter is recycled through Earth’s ecosystems, and it may move from one ecosystem to another as it does when nutrients are washed away into a river. The same atoms are used over and over again, assembled into different chemical forms and incorporated into the bodies of different organisms (www.khanacademy.org).
As an example, let’s see how chemical nutrients move through an aquatic ecosystem. Primary producers (plants, algae, cyanobacteria) at first trophic level take in carbon dioxide from the atmosphere and other nutrients, such as nitrogen and phosphorous, from the water to build complex molecules that make up their cells. When an animal eats the primary producer, it uses its molecules to obtain energy and as building material for its own cells, often rearranging atoms and molecules into new forms. These animals which are feeding on plants are called herbivores or primary consumers, and they represent a second trophic level. Animals that feed on herbivores are called carnivores or secondary consumers, and they represent a third trophic level. The top carnivores are called tertiary consumers, and they represent the fourth trophic level.
When plants and animals carry out cellular respiration they break down complex molecules as fuel, releasing the carbon dioxide into the atmosphere. Similarly, when they excrete waste or die, their chemical compounds are used for energy and building material by bacteria and fungi. These decomposers release simple molecules back into the water and atmosphere, where they can be taken up by primary producers.
Movement of chemical nutrients through an aquatic ecosystem
Thanks to this recycling, the atoms that make up your body right now have long, unique histories. They’ve most likely been part of plants, animals, other people, and even dinosaurs (www.khanacademy.org).
4.2 Energy flow is unidirectional
Energy, unlike matter, cannot be recycled in ecosystems. Energy flow through an ecosystem is a one-way street, usually, from light to heat. Energy usually enters ecosystems as sunlight and is captured in form of a chemical compound by photosynthesizers like plants and algae. The energy is then passed through the ecosystem, changing forms as organisms metabolize, produce waste, eat one another, and eventually, die and decompose. Each time energy changes forms, some of it is converted to heat. Heat still counts as energy, and thus no energy has been destroyed, but it generally can’t be used further as an energy source by living organisms. Ultimately, the energy that entered the ecosystem as sunlight is dissipated as heat and radiated back into space. This one-way flow of energy through ecosystems means that every ecosystem needs a constant supply of energy, usually from the sun, in order to function.
Many ecologists think that the biodiversity of an ecosystem plays a key role in stability. For example, if there were just one plant species with a particular role in an ecosystem, a disturbance that harms that one species—say, a drought for a drought-sensitive species—might have a severe impact on the ecosystem as a whole. In contrast, if there were several plant species with similar functional roles, there would be a better chance of one of them being drought-tolerant and helping the ecosystem as a whole survive the drought period.
Ecosystem resistance and resilience are important when we consider the effects of disturbances caused by human activity. If a disturbance is severe enough, it may change an ecosystem beyond the point of recovery—push the ecosystem into a zone where it is no longer resilient. A disturbance of this sort could lead to permanent alteration or loss of the ecosystem (www.khanacademy.org).
Activity: Make food chain – try to make food chain from what you had for breakfast!
As primary producers and decomposers, microorganisms are irreplaceable members of the aquatic food web. Human activities and pollution pressure on rivers may endanger the delicate balance between oxygen production and consumption in aquatic ecosystems. In the absence of oxygen, aerobic microorganisms that perform complete decomposition of organic matter will not be able to do their job and provide the aquatic ecosystem with primary nutrients. In this way, ecological processes that perform natural water purification will be disturbed. To find out more about good and bad microorganisms in our body, as well in the rivers, read the next article „Good and bad guys among bacteria“.
Aerobes – an organism that need oxygen to live and function.
Anaerobes – an organism that lives and grows in the absence of molecular oxygen.
Autotroph – an organism that are capable of producing their own food
Heterotroph – an organism that cannot produce their own food and derives its intake of nutrition from other sources of organic carbon.
Herbivore – an animal that gets its energy from eating only plants.
Carnivore – an animal that feeds primarily or exclusively on animal matter
Trophic level – a position it occupies in a food chain
Biodiversity – the existence of a wide variety of plant and animal species living in their natural environment.