Microorganisms: top of the rots

Allison Cartwright is Events Officer for our Early Career Scientists (ECS) group. Here, she ponders how microbes get busy during autumn as a vital component for decay. 

As autumn approaches and the trees start to lose their leaves, microbes and fungi are hard at work, clearing up the debris and releasing the material for utilisation in spring.

As children we love to collect autumn leaves for their bright colours, or jump in leaf piles, but without microbes, dead leaves would pile up, forming a thick layer of anoxic sludge, like the smelly mud sometimes found in lakes. To demonstrate how vital microbes are in decay, I’ve selected 3 unusual examples: cockroaches, crane flies and caddisflies.


Fans of the forest

Many view cockroaches as a pest; vermin that signifies poor hygiene. These insects can result in the closure of restaurants. But, are they that bad?

There are around 4,600 species of cockroaches with only four species considered pests. However, for many of the other species, including Cryptocercus spp., their natural home is the forest floor where they live on decaying plant matter.


Plant life matters

To be able to feed on plants, their guts contain bacteria which break down cellulose to provide the cockroach with nourishment. To get these bacteria, the larvae are fed by their parents on a bacteria-rich secretion.

This is vital for the young critters. If they don’t obtain the microbes from their parents to digest plant matter, they can’t develop into an adult cockroach and have cockroaches of their own. Microorganisms in the cockroach gut define whether the creature will live to recycle decaying plants, or die.


Daddy issues

The need for decomposers may seem more critical on land as we can see the detritus, but it is even more important in aquatic environments. The crane fly (also known as daddy-long legs) are a familiar sight in the summer if you live by water.

Although the adult’s sole purpose is reproduction (some also feed on nectar), their larvae have a very different life.

The larvae like wet habitats such as damp hollows in trees, or streams where they feed on decaying vegetation. As the fully aquatic larvae develop, they feed on submerged leaves using special bacteria in their guts.


Break it down

To feed on the leaves, they also need to consume additional microorganisms (including fungi) which have already colonised the leaf’s surface. Combined, these microorganisms break down the organic waste.

However, this is not based on a symbiotic relationship with the same microorganisms, as this changes based on where the developing crane fly lives and its diet. This means that bacteria decomposition is not a ‘one-fits all’ principle, but specific to different vegetation types, and even river conditions.

Crane fly

Crane fly

It must be larvae

Initially, I had hoped to provide an example of insect decomposers at land, freshwater and seas, but the sea is nearly free of insects (their role is replaced by crustaceans).

A few insects float on the ocean surface where they lay their eggs, but for most, proximity to land is key for their existence.

So, instead I will mention my favourite aquatic insect larvae – the caddisfly (common in freshwater). The case bearing larvae from this group don’t need to worry about finding a home, they simply make their own by spinning together vegetation or sediment.



Mobile home

This hand-made home is carried everywhere they go, offering shelter whenever a predator lurks. They manually tear up pieces of vegetation to make their homes, and they also chose to eat vegetation (including leaves) where, like the crane fly larvae, they selectively choose to feed on leaves colonised be fungi.

They even choose to feed on specific fungi over others, and they can detect them before the fungal growth is visible. We may use our eyes to detect fungi, but these critters can find fungi before it is visible.


Microbes matter

If microbes are vital for these creatures, does it not demonstrate how much the world relies on microorganisms? The loss of microbes could cause the breakdown of many processes vital for life.

If for example, we removed microbes (and fungi) responsible for decay, we would be left in a world where nutrients, vital for new life would not be recycled, a world where plant and animal matter would pile up, unable to decay- and that would be a rotten world to live in.

Further reading:

Arsuffi, T.L., Suberkropp, K. (1985) Selective feeding by stream caddisfly (Trichoptera) detritivores on leaves with fungal-colonized patches. Oikos 45: 50-58.

Kaufman, M.G., Walker, E.D., Odelson, D.A., Klug, M.J. (2000) Microbial community Ecology and Insect Nutrition. American Entomologist 46: 173-184.

The Dragonfly Wonan (2012) Five reasons there are few marine insects [Online]. Available from: https://thedragonflywoman.com/2012/05/25/marine-insects/.

Categories: Feature Articles

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