2 July 2013

Oceanic food-chain undergoing dramatic changes

by Will Parker

Increasing levels of carbon dioxide appear to be changing the biodiversity of the oceanic ecosystem, most notably the keystone bacterial organisms that form the foundation of the ocean's food-chain. Reporting their findings in Nature Geoscience, the researchers say they are uncertain as to what the long term effects of these climate-driven selection pressures will be.

While the outcomes may be unclear, the study warns that changes to populations of nitrogen-fixing cyanobacteria (bacteria that obtain energy through photosynthesis, otherwise known as blue-green algae) will have implications for every living thing in the ocean.

Nitrogen-fixing is when organisms, such as cyanobacteria, convert inert nitrogen gas into a reactive form that most other creatures need to survive. Without nitrogen fixers, most life in the ocean could not survive.

Researcher David Hutchins, from the University of Southern California, explained that his team's study focused on two major groups of nitrogen-fixing cyanobacteria: Trichodesmium and Crocosphaera.

Trichodesmium forms large floating colonies big enough to see with the naked eye and makes vast "blooms" in the open ocean. Crocosphaera is also very abundant but is a single-celled, microscopic organism.

Previous research showed that these two types of cyanobacteria should be some of the biggest "winners" of climate change, thriving in high CO2 levels and warmer oceans. However, those previous studies only examined one or two strains of the organisms.

Now, using a culture library of many different strains and species of the organisms, Hutchin's team was able to show that some strains grow better at low CO2 levels not seen since the start of the Industrial Revolution, while others thrive in high CO conditions.

"Our findings show that CO2 has the potential to control the biodiversity of these keystone organisms in ocean biology, and our fossil fuel emissions are probably responsible for changing the types of nitrogen fixers that are growing in the ocean," said Hutchins.

The findings may have significant ramifications for changes in ocean food-chains and productivity, potentially affecting resources we harvest from the ocean. "It's not that climate change will wipe out all nitrogen fixers; we've shown that there's redundancy in nature's system. Rather, increasing atmospheric carbon dioxide changes specifically which nitrogen fixers are likely to thrive," Hutchins explained. "And we're not entirely certain how that will change the ocean of tomorrow."

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Source: University of Southern California