Climate change could cause the levels of greenhouse gases emitted by freshwater lakes to double, University of Cambridge scientists have predicted.

Researchers found that there are thousands of different organic molecules in every drop of fresh water.

Sunlight penetrating a lake (21782492)

These are a source of food for microbes in the lake sediments, which break them down, releasing carbon dioxide and methane as by-products.

An increase in the level of organic molecules leads to a rise in microbial activity, and therefore more greenhouse gases.

The same microbes can make these gases from many different organic molecules.

This means the diversity of organic molecules is more significant for greenhouse gas concentrations than the diversity of microbes.

A greater variety of organic molecules could also mean more that can be broken down by sunlight penetrating water.

Dr Andrew Tanentzap in Cambridge’s Department of Plant Sciences, who led the reasearch published in PNAS, said: “Climate change will increase forest cover and change species composition, resulting in a greater variety of leaves and plant litter falling into waterways. We found that the resulting increase in the diversity of organic molecules in the water leads to higher greenhouse gas concentrations

“Understanding these connections means we could look at ways to reduce carbon emissions in the future, for example by changing land management practices.”

Small shallow lakes dominate the world’s freshwater area. Sediments within them are known to produce at least one-quarter of all carbon dioxide and more than two-thirds of all methane released by lakes.

The researchers suggest climate change may increase the levels of greenhouse gases emitted by freshwater northern lakes to increase by between 1.5 and 2.7 times.

To conduct the study, the scientists filled containers with varying ratios of rocks and organic material - deciduous and coniferous litter from nearby forests. These were submerged in the shallow waters of two Canadian lakes, then analysed two months later using ultrahigh resolution mass spectrometry and next generation DNA sequencing.

This showed the diversity of organic molecules was connected to the diversity of microbes in the water, and an increase in diversity of both as the amount of organic matter increased.

“What we’ve traditionally called ‘carbon’ in freshwater turns out to be a super-diverse mixture of different carbon-based organic molecules,” said Dr Tanentzap.

“We’ve been measuring ‘carbon’ in freshwater as a proxy for everything from water quality to the productivity of freshwater ecosystems. Now we’ve realised that it’s the diversity of this invisible world of organic molecules that’s important.”

The team will expand the study by taking samples from 150 lakes across Europe to understand the broader ecological consequences of organic molecule diversity in natural freshwater systems.

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