Healthy microbes make for a healthy coral reef. And if that microbiological community is disrupted by overfishing, pollution or climate change, it can contribute to the decline of reefs.
A three-year study
published this month in Nature Communications, conducted on a reef in the Florida Keys, United States, has shed light on how microbes living on corals are instrumental to keeping coral reefs healthy, and how overfishing, pollution and climate change can destabilise the coral’s natural defence and disrupt ecological communities.
According to the lead author of the study Dr Rebecca Thurber, from Oregon State University, healthy corals normally recover easily from small injuries, such as fish bites.
“In our experiment, 100% of the corals bitten in normal waters recovered. But in the presence of elevated nutrients, 66% died after they were bitten by fish, showing that nutrient pollution increases the vulnerability of corals to normal every day events,” she said.
Although this study focused on Caribbean ecosystems, it can inform threats to the Great Barrier Reef, said Dr Jon Brodie from James Cook University, who was not involved in the study.
The addition of overfishing and nutrient pollution interact with the elevated temperatures creating more disease-causing bacteria, and this may make reefs less resilient to disruptive events such as cyclones.
According to Dr Zoe Richards, from Western Australian Museum, who was not involved in the study, the study shows “how easily an innocuous interaction like a fish feeding on a coral can turn deadly in overfished and polluted habitats, especially in summer”.
The results suggest it’s especially important to manage overfishing around important reefs, says Richards. This will help sustain the population of fish that feed on microbes that might otherwise increase in numbers and disrupt the normal microbial ecology.
“This will help suppress algal overgrowth and blooms of harmful bacteria, which are major drivers of coral mortality,” said Richards.
Another strategy to protect reefs is to protect the environment around them.
“Rehabilitating catchment areas, preventing clearing and erosion, along with protecting natural waterways and limiting herbicide and pesticide run-off are integral components of reducing nutrient pollution,” said Richards.
Even though climate change is warming the Great Barrier Reef, reducing the impact of other stressors could help maintain a healthy microbial balance.
“If we reduce ocean pollution and ensure that there are abundant fishes to remove the algae on reefs, corals can likely tolerate some increases in water temperatures,” said Thurber
Microbial services are not limited to recycling. Many microbes, including cyanobacteria, function like tiny plants by using sunlight to produce oxygen and sugars. Due to their extraordinary number in the world’s oceans, the amount of oxygen these organisms produce is equal to that of all plants on land.
Until recently, finding out just how many different types of microbes there are was relatively difficult. How do you identify and study millions of different organisms that are not visible to the naked eye?
Different areas of harbours, such as Sydney Harbour, have distinct bacterial communities. These patterns may be driven by circulation. The outer harbour, which is flushed with seawater on every tidal cycle, is dominated by photosynthetic cyanobacteria. The upper harbour, with less flushing and more runoff, is dominated by soil-related bacteria and those adapted to nutrient-rich environments.
In our waterways, pollutants such as metals bind to fine particles and settle as sediment. This exposes sediment-dwelling organisms to a multitude of toxic products. What effect do these toxic substances have on sediment microbes?
It would be great if we could use particular microbes to diagnose human impacts. For instance, certain microbes can indicate water quality.
A technique called metagenomics is revealing the true depth of microbial diversity by pooling DNA sequences from all the species in a sample. It then works backwards to construct a genetic overview of the entire community.
However, while metagenomics can give us important information about the identity of microbes in a community, it can’t tell us what they are doing or how their functions change in response to environmental stressors and human activities.
Metatranscriptomics takes the sequencing approach one step further and characterises gene expression in a microbial community, which can be linked to crucial ecosystem services such as nutrient cycling.
Similar to their use for diagnosis of ailments in humans, molecular tools are being used to diagnose human impacts on earth by observing changes in microbes across polluted and unpolluted environments. They can even detect very small amounts of toxic substances. Because of their diversity, they can potentially be used to detect a wide range of human impacts.
This allows us to identify environmental impacts early, potentially limiting greater loss in larger organisms.
With the new tools to “see” microbes and their importance, we are now perfectly poised to advance our understanding of how microbes are responding to environmental change. They are sentinels of our increasingly human-affected waterways.