Ecosystem is defined as communities of microorganisms and their physical and chemical environment that function as a self-regulating unit
All living beings must obtain nutrition from their environment. This relationship can be represented in a simplistic form called the food chain that gives an idea of the general trophic level – producers, consumers and decomposers.
A food chain always starts with producers that are capable of producing their own nutrition by fixing CO2 using solar energy. They convert the light energy from sun to chemical energy. Producers vary from habitat to habitat
In open oceans, primary production is through photosynthesis by planktons and to a lesser extent by chemoautotrophs.
In hydrothermal vents, chemoautotrophic bacteria are predominant that require H2S to obtain energy
Consumers are the ones that feed on other animals or plants. They are classified as:
Primary consumers – Herbivores
Secondary consumers – Carnivores
Tertiary consumers – Scavengers. Those that feed on the secondary carnivores
Quaternary consumers – Decomposers. Fungi and protists
In estuaries, mostly higher animals are the direct consumers of vegetation
Decomposers are primarily microbes that decompose dead plants, animals and other microorganisms. They are functionally very important. Without them the ecosystem would stagnate at all nutritional levels and die
Saprophytic microbes are found at all nutritional levels and break complex organic materials into simple inorganic matter thereby recycling nutrients in the biosphere
However, in nature, the feeding relationships are interconnected in a very complex manner that can be represented as multichannel food chains together, known as food web. It helps to identify the feeding patterns of herbivores, carnivores and omnivores
Food web
Nutrient cycling in Aquatic environments
Lakes, estuaries and other coastal regions have a relatively high rates of primary production. Nitrogen and phosphorus, however, are the limiting nutrients. Nearby agricultural and urban activity frequently produces runoff that provides substantial nutrient inputs to these environments.
On the contrary, nutrient levels in the open ocean are very low, which is unaffected by rivers, streams and terrestrial run-offs. In this region, nitrogen, phosphorus, iron and even silica, which diatoms require to build their frustules, are limiting. Thus, the major source of organic matter are phytoplanktons, autotrophic microbes floating in the photic zone of the ocean.
Planktonic cyanobacteria like Prochloron and Synechococcus, also known as picoplanktons, represent about 20-80% of the total planktonic biomass. Eukaryotic autotrophs such as diatoms, also contribute significantly to carbon fixation.
Planktons obtain N and P from the surrounding marine water. The nutrient composition of the water affects the final C:N:P ratio (Redfield ratio) which is assumed to be 106:16:1
Microbial loop is very crucial in nutrient cycling in aquatic habitats. In any food chain, primary producers are numerous. They provide all the organic carbon consumed by herbivores. Herbivores are consumed by carnivores which occupy different trophic levels. Microbes predominantly constitute decomposers, mineralizing most of the waste products produced in an ecosystem
In this loop, prokaryotes and eukaryotes consume dissolved organic matter (DOM). DOM include the liquid wastes of zooplanktons and materials that leak from phytoplanktons, sometimes called
Viruses are also sources of DOM. They lyze their host cells on infection that contributes significantly to recycling of nutrients back into the microbial loop
Protists consume smaller bacteria termed as particulate organic matter (POM). Protists are in turn consumed by zooplanktons returning both DOM and POM into the food web for use at a number of trophic levels
