Ocean Acidification in Coastal Upwelling Regions Shown to Impact Phytoplankton Nutrient Acquisition

Scientists at the Scripps Institution of Oceanography at the University of California San Diego and the J. Craig Venter Institute (JCVI) have for the first time shown that increased acidification of ocean water in an upwelling region reduces the availability of iron for phytoplankton, thereby threatening to reduce overall phytoplankton productivity.

Given that phytoplankton sit at the base of the oceanic food web, ocean acidification is a concern to all life in these upwelling regions. Upwelling regions are among the most productive due to the concentration of nutrients brought from deep water, driven by coastal winds. Results of this study are published in the journal Nature Communications.

“This study provides critical insight into how key organisms in this ecosystem may respond to future conditions,” said lead author Robert Lampe, a graduate student at Scripps Oceanography and JCVI. “Our current projections for how organisms and biological processes will respond to climate change are still quite uncertain and this brings us a step closer towards understanding change in the ecosystem.”

Aboard R/V Atlantis, a research vessel owned by the U.S. Navy and operated by Woods Hole Oceanographic Institution, JCVI and Scripps Oceanography scientists spent 32 days in the California Current, a cold-water Pacific Ocean Current that runs southward along the western coast of North America. The team began their experiments—to better understand how acidification affects marine microbial life—near Big Sur, California and moved progressively farther from shore, performing four experiments in total.

Each of the experiments consisted of carefully collecting water with a trace metal clean rosette, a special sampling device built with minimal exposed metal on the overall structure and none inside the collection bottles to prevent water from being contaminated with iron. Once on the ship, the bottles of ocean water were taken into positive pressure clean areas for sampling and experimental setup. In incubators in the ship’s lab, carbon dioxide was bubbled into the water, closely matching the phenomenon in nature of carbon dioxide dissolving in the water, making it more acidic and reducing the bioavailability of iron. The primary driver for the increase in carbon dioxide and resulting ocean acidification is burning fossil fuels.

In the JCVI lab, RNA and proteins were extracted from the experimental samples. Using a “multi-omics” approach, the researchers were able to get a molecular-level view of both the types of organisms present as well as what mechanisms were in play during the acidic water stress test.

The phytoplankton appeared to take advantage of alternative biological mechanisms to absorb iron that are less hindered by acidification and reduced their overall iron utilization. Growth, nutrient absorption, and community compositions remained largely unaffected, suggesting that these strategies may confer short-term resistance but longer-term exposure to acidification may result in a reduction of photosynthetic activity and contractions in the phytoplankton community.

“Potential impacts of ocean acidification on diatoms—which are responsible for around 20% of global primary production—have not been obvious,” said senior author Andrew Allen, a professor at JCVI and Scripps. “This study shows that if we look at plankton ecosystems with an increased sensitivity, with our experimental design and methodology, we can see significant impacts of ocean acidification on diatoms. Ocean acidification clearly exacerbates iron stress in diatoms. Interestingly, diatoms apparently have a battery of diverse iron uptake mechanisms that we were not previously aware of.”

In addition to Allen, the project was conducted under the direction of co-principal investigators Katherine Barbeau and Andreas Andersson at Scripps, in conjunction with UC San Diego’s California Current Ecosystem Long Term Ecological Research (CCE LTER) program. Collaborators included Erin Bertrand at Dalhousie University and Miroslav Oborník at the Czech Academy of Sciences.

Funding for the project was provided by the National Science Foundation (NSF; grants OCE-1756884, OCE-1637632, OCE-1756860, and OCE-2224726), National Oceanic and Atmospheric Administration (NOAA; grant NA19NOS4780181), and Simons Foundation Collaboration on Principles of Microbial Ecosystems (PriME) grant 970820.

The complete study, “Short-term acidification promotes diverse iron acquisition and conservation mechanisms in upwelling-associated phytoplankton,” may be found in the journal Nature Communications.

•  Adapted from JCVI