U.S. Department of Energy

Pacific Northwest National Laboratory

PNNL Helps Reveal How Ocean Algae Under Stress Sustain Growth

Algal relationships and Micromonas growth under changes in phosphate availability.

As the Earth's oceans warm, the eukaryotic phytoplankton that so consequentially populate marine surfaces are predicted to face the kind of nutritional limitations that occur just seasonally today, including declining levels of phosphate.

Marine algae perform about half of the planet's global carbon fixation. That's the process by which carbon dioxide (CO2) in the atmosphere is converted to organic carbon compounds, including carbohydrates used for food.

This anticipated increase in the "desertification" of open oceanic regions will likely impact marine food webs, and could reduce the photosynthetic uptake of CO2.

A paper appearing today (June 25) in Nature Microbiology reports how algae continue to grow under these types of conditions, in part by dissipating excess light and by promoting phosphate homeostasis.

One particular protein linked to harvesting light, LHCSR, is an ancient mechanism for blunting the effects of light stress. The authors call it an important key to sustaining algal growth during extended periods of phosphate limitation, even when light is constant.

PNNL researchers had a significant role in the study by performing proteomic analyses at the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy user facility located on the PNNL campus.

The PNNL co-authors are Charles Ansong, Samuel Purvine, Richard D. Smith, and Stephen Callister, who also helped further analyze the proteomics data.

Callister said the study "represented an outstanding opportunity to utilize PNNL's proteomics capabilities."

Unlocking Little-Known Processes

Phytoplankton acclimate to variations in oceanic surface conditions through processes that are not well understood. This especially true for eukaryotic algae in open-water regions, where desertification is expected to hit hardest.

The researchers-led by Alexandra Worden and her team at the Monterey Bay Aquarium Research Institute in California-devised custom-built photo-bioreactors to monitor photosynthetic efficiency and other parameters under various nutrient-level regimes. 

To investigate likely algal responses, the paper's authors employed Micromonas commoda as a model system. This tiny, fast-swimming, flagellated genus is found in a range of ocean environments, from tropical to polar, including the Atlantic Ocean's phosphate-limited Sargasso Sea.

"We now understand some of the cellular biology behind how algae deal with modern-day seasonal changes," said Worden, as well as new insights into mechanisms that will have a role in how ocean algae adapt to future conditions.

The new study, she added, "allows us to go out into the field and probe the real-time experience of algae with much greater sensitivity than possible before."

Proteomic Responses

The researchers chose Micromonas for its attractively small genome. It also contains just 10,306 protein-encoding genes and is closely related to similar oceanic organisms common in coastal, frontal, and open-ocean environments.

"The cells are so tiny that we can't say much by looking through a microscope," said Worden of Micromonas. Instead, the genes and proteins they express, she added, "are our hook into visualizing growth and stress in the ocean."

The authors studied physiological and proteomic responses to chronic nutrient limitation, and watched for changes in protein expression.

Together with Worden's team, Callister said, "we were able to accomplish the difficult task of looking at the protein phenotype of this very tiny eukaryote." 

The study's findings, he added, belong in a much bigger context: increased phosphate limitation as the world's oceans warm, and "what could happen to the growth and photosynthetic capabilities of this widespread ocean organism."

Knowing more about the mechanism of ancient LHCSR protein is a start, the authors say. It will open inquiries into other interactive effects that arise as multiple ecological stressors cause oceans to change.

As ecological perturbations unfold in the world's future oceans, the authors add, some of these interactive effects could reduce how efficiently algae maintain and optimize marine photosynthesis.

Date: 
June 2018
| Pacific Northwest National Laboratory