Distinguished Professor of Microbiology Stephen Giovannoni was part of an international research team that discovered two phytoplankton groups -- unlike any known species -- in climate-sensitive areas around the world. While they appear relatively rare compared to other phytoplankton, the scientists say their prevalence in warm waters suggests they could be important in future ocean ecosystems.
The findings, published this week in the journal Current Biology, traced the phytoplankton genes to their potential ancient origins and matched them with sequences in around 200 contemporary samples. The new phytoplankton groups were increasingly abundant in warmer, low-nutrient surface waters at sites including the Sargasso Sea, Bay of Bengal and North Pacific Gyre.
"These new phytoplankton appear to thrive in the world's most desert-like waters where most other eukaryotic species decrease," said Alexandra Z. Worden.
Worden is a senior fellow at the Canadian Institute for Advanced Research, who led the team from the Monterey Bay Aquarium Research Institute along with collaborators from Oregon State University, Woods Hole Oceanographic Institution and the National Institute of Oceanography in India.
Both phytoplankton groups were found in larger numbers in warmer, low-nutrient surface waters compared to numbers in cooler, higher-nutrient regions. This pattern surprised researchers because larger, eukaryotic phytoplankton often decline to vanishingly low numbers under these conditions, which typically favor small cyanobacteria.
Ocean surface warming creates a layer of low-nutrient water separate from cooler, nutrient-rich water below. This process occurs annually in large regions of the open ocean where punctuated winter mixing allows for a short "bloom" of phytoplankton life that is followed by a summer season of warm, low-nutrient surface waters. Most of the bloom species disappear during summer because they are not effective competitors for nutrients at low concentrations. Ocean surface warming is leading to an expansion of these low-nutrient environments in a process known as ocean desertification.
"As microbes in our oceans are forced to adapt to climate change, these are the types of organisms we really need to understand," Worden said.
Scientists have had difficulty measuring the impact of ocean warming on resident microbial groups due to a lack of consistent information on microbes, like the phytoplankton that carryout marine photosynthesis.
Worden's team established the Baselines Initiative to overcome this hurdle with more than 6,000 full-length RNA sequences and time-series sampling, where the same location is sampled repeatedly through the seasons and over the years. The ultimate goal of these oceanographic time-series is to capture current day information against which future changes in the ocean can be assessed.
Giovannoni's lab at OSU contributed the data from the Sargasso Sea and jointly developed the software that made this study possible.