Climate change impacts on microscopic organisms

Rising sea temperatures, changes to ocean currents and ocean acidification are the main factors that will affect microscopic organisms. This group is fundamentally important to the Great Barrier Reef.

Because they are short-lived (and go through many generations in a short time), microscopic organisms will have a better chance of adapting to environmental changes than many other species groups.

What are microscopic organisms?

Microscopic organisms on the Great Barrier Reef include plankton (tiny, drifting animals and algae) and microbes (such as bacteria, fungi and viruses). They make up the majority of living organisms in the ocean. Some of them (the phytoplankton) take carbon dioxide from the atmosphere and convert it to food using the sun’s energy. This makes them vital processors of the world’s greenhouse gas emissions.

Larger plankton is food for bait fish, which in turn feed seabirds, predatory fish, dolphins and whales.

Microscopic organisms live together with many other reef creatures in close partnerships known as symbioses (for example, the zooxanthellae that give corals their colour).

Others are the cause of diseases, including some diseases that may be more common as the global climate changes.

Rising sea temperatures

Rising sea temperatures will affect the growth, reproduction and survival of microscopic organisms. Different species will react in different ways, and this will alter the interactions between organisms as well.

One well-known consequence of rising sea temperature is coral bleaching. Bleaching occurs when corals expel the microscopic algae (known as zooxanthellae) that live within the coral tissue. Some of these algae can tolerate higher temperatures than others and corals have been found to swap heat-sensitive algae for more resistant strains to try to prevent bleaching.

Higher sea temperatures are also expected to favour a range of microscopic organisms that cause disease. Some types of coral diseases already appear to be occurring more often as the climate changes.

Changes to ocean currents

Rising sea temperatures are also likely to reduce the supply of essential nutrients for the tiny plants that support the open ocean food web. These microscopic plants (known as phytoplankton) rely on nutrients that are brought up from deeper in the ocean.

As surface water warms, it becomes less dense and does not mix as readily with the cooler water below. This makes it harder for nutrients to reach the surface, reducing the production of phytoplankton.

This in turn affects the food supply for the entire open ocean food chain, including fish, seabirds, whales and dolphins. Other changes to ocean currents will also affect the transport of microscopic eggs and larvae within and between coral reefs and other Great Barrier Reef habitats.

Ocean acidification

Many microscopic organisms are also vulnerable to ocean acidification. Some plankton make hard shells from calcium carbonate (limestone). These species are likely to be directly affected by ocean acidification, which will impact the rate of shell formation (calcification).

Because some species will be affected more than others, ocean acidification will not only change the activity of individual phytoplankton types, but will also tend to favour some types over others. Resulting shifts in the make-up of plankton communities will affect the animals that depend on them for food.

It will also influence the cycling of nutrients (such as carbon) through open ocean ecosystems. Bacteria and other types of plankton might also be affected by ocean acidification. Scientists are just beginning to understand these impacts, but it is likely that changes could affect the structure and functioning of the entire marine ecosystem.