Scientists have uncovered what they say is a new threat from ocean acidification.

Scientists investigating the effect of ocean acidification on a key group of microscopic marine organisms, phytoplankton, say ocean acidification is negatively impacting the extent to which diatoms in Southern Ocean waters incorporate silica into their cell walls.

The oceans act as a carbon sink and have already absorbed more than 40 per cent of anthropogenic carbon emissions.

The majority of this CO2 has been taken up by the Southern Ocean making these waters hotspots of ocean acidification.

Dr Katherina Petrou from the University of Technology Sydney (UTS) says that although changes in ocean pH have been shown to impact marine calcifying organisms, the consequences for non-calcifying marine phytoplankton are less clear.

“Previous studies reported a range of responses to [ocean acidification in phytoplankton] yet rarely considered how environmental pH shifts might affect silicification rates in diatoms,” she says.

“Diatoms are unique phytoplankton in that they need silicic acid to produce silica cell walls.

“Under the microscope they look like beautiful glass jewellery boxes, but importantly, this dense, glass-like armour promotes sinking, which makes diatoms an important conduit for transport of carbon to the deep ocean where it can be stored for millennia.”

Diatoms are responsible for around 40 per cent of ocean productivity which means they play a major role in supporting marine food webs, sustaining life for millions of creatures, including humans.

In recent experiments carried out the Australian Antarctic base, Davis station, researchers used large 650L tanks, a temperature controlled 15-metre container and natural coastal water to investigate the effects of predicted future changes in ocean acidity on the community structure of the Antarctic phytoplankton.

“We were alarmed to find that diatoms were so negatively affected, with some species likely to have diminished silica production before the end of this century,” says Dr Petrou.

In the context of global climate change, these findings are important because they reveal that ocean acidification can not only alter phytoplankton community composition, but also reduces diatom ballast (sinking ability).

Loss of silica production and thus ballast could mean that fewer diatoms end up on the ocean floor, resulting in less atmospheric CO2 being removed from our atmosphere and transported for storage in the deep ocean.

The study is accessible here.