Ocean acidification can alter the growth, abundance, and elemental composition of diatoms, with possible consequences for marine food webs and for the ocean’s capacity to transport carbon to the depths, according to a study by Flinders University published in the journal Marine Ecology.
Diatoms are unicellular microalgae found in oceans, lakes, rivers, and even moist soils. Despite their small size, they play an important role in aquatic ecosystems: they account for between 40% and 50% of the oceans’ primary production and contribute to the export of organic carbon to the deep ocean, through the fixation of carbon dioxide (CO₂) in surface waters.
The investigation examined how different stress factors, including the reduction of pH caused by rising CO₂ concentrations, affect the uptake of trace elements by diatoms. These elements, among which are iron, zinc and cadmium, play important roles in the metabolism of these organisms.
“We showed that changes in the ocean’s pH can affect the growth, abundance, and elemental composition of these diatoms,” explains Sophie Leterme, a professor at Flinders University and the lead author of the study.
Acidification alters the availability of elements
When atmospheric CO₂ dissolves in seawater, it triggers changes in its chemistry and a lowering of pH. According to the researchers, the average ocean pH has already fallen by about 0.1 units since the end of the Industrial Revolution, and could suffer a further reduction of 0.3 to 0.6 units by the end of this century.
These changes can influence how phytoplankton absorb essential trace elements. As diatoms are at the base of many marine food webs, changes in their growth and functioning may ripple through other organisms.
The researchers also warn of possible effects on the carbon cycle. Diatoms contribute to the transfer of carbon from surface waters to the deep ocean, so alterations in their physiology could affect this process.
“We need to investigate how these changes interact among the various trace elements and how they may lead to broader ecological impacts, such as disruption of marine food webs, reduced export of carbon and silicon, and increased microbial activity and nutrients,” says Leterme.
To carry out the study, the team used seawater samples collected in St Vincent Gulf, South Australia, and cultures from the CSIRO algae collection. Two species of marine diatoms were analyzed, Thalassiosira pseudonana and Nitzschia navis-varingica, using a technique of neutron activation analysis, with support from the Australian Nuclear Science and Technology Organisation (ANSTO).
Diatoms are also used as bioindicators of water quality. Monitoring changes in these organisms could, therefore, help to understand the effects of acidification and the warming of the oceans on marine ecosystems.
The authors suggest that a better understanding of these interactions could also contribute to the development of biotechnological applications, including new biofilms designed to reduce pollution associated with maritime transport in port areas.
