Interdependence Among Organisms Within Their Environment

Lessons

Senior 5 · Biology

Interdependence Among Organisms Within Their Environment

4 lessons

Lesson 4 of 4

Saprophytism, mutualism and commensalism

25 min

Where does a fallen tree go?

When a large tree falls in Nyungwe forest, the trunk is not there for ever. Within a few years it turns soft and crumbly, bracket fungi and mushrooms push out of it, and at last it becomes dark soil. Nothing ate it in the way a lion eats a bushbuck. Fungi and bacteria digested it from the outside and absorbed the products. These saprotrophs depend on other organisms as completely as a parasite does, but on dead ones. This lesson also looks at partners that both gain (mutualism) and at partners where only one gains (commensalism).

Definition

Saprophytism (saprotrophic nutrition): feeding on dead or decaying organic matter by secreting digestive enzymes onto it and absorbing the soluble products. The organism is a saprotroph (older term: saprophyte). Examples: the bread mould Rhizopus, mushrooms, and many soil bacteria.

Extracellular digestion: digestion that happens outside the cells of the organism, by enzymes it has secreted.

Decomposer: a saprotroph that breaks down dead organisms and wastes, releasing simple inorganic substances such as carbon dioxide, ammonium and phosphate.

How a saprotroph feeds

A fungus such as Rhizopus grows as a mass of thin threads called hyphae, together forming a mycelium. The hyphae spread through the food and feed in four steps (Figure 1):

The hyphal tips secrete enzymes by exocytosis: amylase, protease, lipase and, in wood-rotting fungi, cellulase and enzymes that break down lignin.
The enzymes hydrolyse large insoluble molecules outside the hypha.
The small soluble products, such as glucose and amino acids, are absorbed by diffusion and active transport over the huge surface area of the mycelium.
They are used in respiration and growth, or stored as glycogen and lipid droplets.

Why saprotrophs matter

Recycling: they release carbon as CO₂ for photosynthesis, and turn the nitrogen of proteins into ammonium (ammonification), the start of the nitrogen cycle you studied in Senior 3.
Soil fertility: part-digested matter forms humus, which holds water and minerals.
Uses: composting crop waste and manure; the yeast used to brew urwagwa and to raise bread; the bacteria that produce biogas from cow dung.
Harm: moulds spoil stored food, and some, such as Aspergillus flavus on badly dried maize and groundnuts, produce poisonous aflatoxins. Drying grain well before storage prevents this.

Worked example 1: a litter-bag experiment

Students put 20.0 g of dry leaves into each of two mesh bags and buried them in a garden. The fine mesh lets in only microorganisms; the coarse mesh also lets in earthworms and insects. Illustrative results after 8 weeks:

Bag	Start / g	After 8 weeks / g
Fine mesh	20.0	15.0
Coarse mesh	20.0	9.0

(a) Calculate the percentage of mass lost from each bag. (b) Calculate the mean rate of loss in the coarse bag in g per week. (c) Explain the difference.

(a) Fine: $\frac{20.0 - 15.0}{20.0} \times 100 = 25\,\%$ . Coarse: $\frac{20.0 - 9.0}{20.0} \times 100 = 55\,\%$ .
(b) $\frac{11.0\,\mathrm{g}}{8\,\text{weeks}} = 1.4\,\mathrm{g\ per\ week}$ (to 2 significant figures).
(c) In both bags, saprotrophic fungi and bacteria digest the leaves. In the coarse bag, earthworms and insects (detritivores) also eat the leaves and break them into small pieces, which gives the saprotrophs a much larger surface area to work on.

Definition

Mutualism: a close association between two species in which both benefit (+/+). If neither partner can survive without the other, the mutualism is obligate.

Commensalism: an association in which one species, the commensal, benefits and the other, the host, is neither helped nor harmed (+/0).

Mutualism: both partners gain

Partners	What the first gains	What the second gains
Rhizobium bacteria and legumes (beans, peas, groundnuts)	Bacteria: sugars from the plant, and a home in root nodules where the protein leghaemoglobin keeps oxygen low enough for their enzyme nitrogenase to work	Plant: ammonium made by the bacteria from nitrogen gas, so beans grow well in soil poor in nitrate
Mycorrhizal fungi and plant roots	Fungus: sugars from the plant	Plant: the hyphae act as extra, very fine roots, absorbing phosphate and water from a far larger volume of soil
Fungus and alga in a lichen	Fungus: sugars made by the alga's photosynthesis	Alga: shelter from drying out and strong light, and water and minerals held by the hyphae
Cow and the microorganisms of its rumen	Microbes: a warm, wet, oxygen-free home with a steady supply of grass	Cow: microbes digest cellulose, which the cow has no enzyme for, into fatty acids that it absorbs
Fungus-growing termites (Macrotermes) and the fungus Termitomyces	Termites: food, as the fungus breaks down tough plant material in combs inside the mound	Fungus: a supply of chewed plant matter and a protected, humid home. Its mushrooms appear on mounds after rain

A closer look at a lichen

A lichen looks like one organism, but it is a fungus with algal cells (or cyanobacteria) living among its hyphae. Lichens can grow on bare rock and tree bark, where neither partner could live alone. Because they absorb water and dissolved substances over their whole surface, many lichens are killed by sulphur dioxide in polluted air, so they are used as indicators of air quality.

Commensalism: one gains, the other is unaffected

Cattle egrets walk beside grazing cattle and buffalo and snap up the insects disturbed from the grass. The birds gain food; the grazer seems unaffected.
Epiphytes such as orchids, ferns and mosses grow on the branches of forest trees in Nyungwe. They gain a place in the light without growing a long stem; they take no food from the tree.
Remora fish attach to sharks with a sucker on the head and feed on scraps from the shark's meals.
Entamoeba coli lives in the human large intestine, feeding on bacteria and food remains without harming us. (Its relative Entamoeba histolytica is a parasite that causes amoebic dysentery.)

Worked example 2: is it really commensalism?

Illustrative field data: a cattle egret feeding beside a cow caught 2.4 insects per minute; feeding alone, it caught 0.8 insects per minute. (a) By what factor does feeding beside a cow increase the egret's catch? (b) What evidence would you need to call this relationship commensalism rather than mutualism?

(a) $\frac{2.4}{0.8} = 3.0$ , so three times as many insects per minute.
(b) The data prove only the + for the egret. To show 0 for the cow, measure something that matters to the cow, such as fly bites, tick numbers or weight gain, with and without egrets nearby. No difference supports commensalism; fewer ticks with egrets would suggest mutualism.

Evaluation: the same test has changed a famous example. Oxpeckers on buffalo and cattle were long called mutualists because they eat ticks. Field studies found they also peck at wounds and drink blood, keeping the wounds open, so the relationship may be partly parasitic. A "0" or "+" needs measuring, not assuming.

National Geographic Short Film Showcase: "What's in a Lichen? How Scientists Got It Wrong for 150 Years". Watch for how a lichen is built from partners, and how DNA evidence in 2016 revealed a yeast living in many lichens as well as the fungus and the alga.

Warning

Common mistakes

"Saprotrophs are parasites." A parasite feeds on a living host; a saprotroph feeds on dead matter.
"Fungi digest food inside their cells." Saprotrophic fungi digest outside the hypha and absorb the products.
"Symbiosis means mutualism." In the broad sense used here, symbiosis covers mutualism, commensalism and parasitism. State which one you mean.

Tip

Name both sides: for any mutualism answer, write one line for what each partner gives and one for what each receives. Examiners want both directions.

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Practice: saprophytism, mutualism and commensalism

Define saprotrophic nutrition and give two examples of saprotrophs.
Describe the four steps by which a fungal hypha obtains glucose from starch in bread.
Explain why saprotrophs are essential to the nitrogen cycle.
For the Rhizobium–bean mutualism, state what each partner gives and receives.
Explain why lichens are absent from the bark of trees in areas with high sulphur dioxide pollution.
Distinguish between commensalism and mutualism, using one example of each.
In a litter-bag test, 25.0 g of leaves fell to 17.5 g in 6 weeks. Calculate the percentage loss and the mean rate in g per week.
Evaluate the claim that "oxpeckers and buffalo are a perfect example of mutualism".

Show answersHide answers

Feeding on dead organic matter by secreting enzymes onto it and absorbing the soluble products. Examples: Rhizopus (bread mould), mushrooms, decomposer bacteria.
Hyphal tips secrete amylase by exocytosis; amylase hydrolyses starch outside the hypha to maltose and glucose; the glucose is absorbed by diffusion and active transport; it is used for respiration and growth or stored.
They break down proteins and other nitrogen compounds in dead organisms and wastes to ammonium (ammonification), which nitrifying bacteria and plants can then use; without them the nitrogen would stay locked in dead matter.
Bacteria give ammonium made from nitrogen gas and receive sugars and a low-oxygen home in nodules; the plant gives sugars and the nodule environment and receives fixed nitrogen.
Lichens absorb water and dissolved substances over their whole surface and cannot exclude SO₂ dissolved in rain; it damages the alga's chlorophyll, so the lichen dies.
Commensalism +/0: cattle egrets eat insects disturbed by cattle, which are unaffected. Mutualism +/+: Rhizobium gets sugars and the bean gets fixed nitrogen.
Loss $= \frac{25.0 - 17.5}{25.0} \times 100 = 30\,\%$ ; rate $= \frac{7.5}{6} = 1.25 \approx 1.3\,\mathrm{g\ per\ week}$ .
Partly true: oxpeckers eat ticks, which benefits the buffalo. But they also feed at wounds and drink blood, keeping wounds open, which harms the host. The balance may change with tick numbers, so the relationship can be mutualistic or partly parasitic: not "perfect".

Key points

Saprotrophs digest dead matter outside their cells and absorb the products; they recycle carbon, nitrogen and phosphorus.
Mutualism +/+: Rhizobium and legumes, mycorrhizae, lichens, rumen microbes, termites and Termitomyces.
Commensalism +/0: cattle egrets, epiphytes, remoras, Entamoeba coli.
Labels such as "0" and "+" must be tested with evidence; oxpeckers show how an accepted example can change.

1Where does a fallen tree go?

2How a saprotroph feeds

3Why saprotrophs matter

4Mutualism: both partners gain