Satellites use chlorophyll’s green color to detect biological activity in the oceans. The lighter-green swirls are a massive December 2010 plankton bloom following ocean currents off Patagonia, at the southern tip of South America. Credits: NASA’s Earth Observatory

Guest essay by Eric Worrall

Warmer weather apparently makes it more difficult for pond scum to grow and consume CO2.

Key Biological Mechanism is Disrupted by Ocean Acidification

Inability of phytoplankton to acquire iron imperils marine ecosystems

Mar 14, 2018

A team led by scientists from Scripps Institution of Oceanography at the University of California San Diego and the J. Craig Venter Institute (JCVI) has demonstrated that the excess carbon dioxide added to the atmosphere through the combustion of fossil fuels interferes with the health of phytoplankton which form the base of marine food webs.

Phytoplankton are microscopic plants whose growth in ocean surface waters supports ocean food webs and global marine fisheries. They are also key agents in the long-term removal of carbon dioxide (CO2)

As reported in the March 14 edition of Nature, the team shows that a mechanism widely used by phytoplankton to acquire iron has a requirement for carbonate ions. Rising concentrations of atmospheric CO2 are acidifying the ocean and decreasing carbonate, and the team shows how this loss of carbonate affects the ability of phytoplankton to obtain enough of the nutrient iron for growth. Ocean acidification is poised to decrease the concentration of sea surface carbonate ions 50 percent by the end of this century.

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“Ultimately our study reveals the possibility of a ‘feedback mechanism’ operating in parts of the ocean where iron already constrains the growth of phytoplankton,” said Jeff McQuaid, lead author of the study who made the discoveries as a PhD student at Scripps Oceanography. “In these regions, high concentrations of atmospheric CO2 could decrease phytoplankton growth, restricting the ability of the ocean to absorb CO2 and thus leading to ever higher concentrations of CO2 accumulating in the atmosphere.”

“Studies investigating the effects of high CO2 on phytoplankton growth have shown mixed results to date. In some cases, certain phytoplankton seem to benefit from high CO2,”added Andrew E. Allen, a biologist with a joint appointment at Scripps and JCVI who is senior author and initiator of the study. “Most of these studies, however, have been conducted under high-iron conditions. Our study uncovers a widespread cellular mechanism that suggests high CO2 might be particularly problematic for phytoplankton growth in low-iron regions of the ocean.”

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Read more: https://scripps.ucsd.edu/news/key-biological-mechanism-disrupted-ocean-acidification

The abstract of the study;

Carbonate-sensitive phytotransferrin controls high-affinity iron uptake in diatoms

Jeffrey B. McQuaid, Adam B. Kustka, Miroslav Oborník, Aleš Horák, John P. McCrow, Bogumil J. Karas, Hong Zheng, Theodor Kindeberg, Andreas J. Andersson, Katherine A. Barbeau & Andrew E. Allen

In vast areas of the ocean, the scarcity of iron controls the growth and productivity of phytoplankton. Although most dissolved iron in the marine environment is complexed with organic molecules, picomolar amounts of labile inorganic iron species (labile iron) are maintained within the euphotic zone and serve as an important source of iron for eukaryotic phytoplankton and particularly for diatoms. Genome-enabled studies of labile iron utilization by diatoms have previously revealed novel iron-responsive transcripts, including the ferric iron-concentrating protein ISIP2A8, but the mechanism behind the acquisition of picomolar labile iron remains unknown. Here we show that ISIP2A is a phytotransferrin that independently and convergently evolved carbonate ion-coordinated ferric iron binding. Deletion of ISIP2A disrupts high-affinity iron uptake in the diatom Phaeodactylum tricornutum, and uptake is restored by complementation with human transferrin. ISIP2A is internalized by endocytosis, and manipulation of the seawater carbonic acid system reveals a second-order dependence on the concentrations of labile iron and carbonate ions. In P. tricornutum, the synergistic interaction of labile iron and carbonate ions occurs at environmentally relevant concentrations, revealing that carbonate availability co-limits iron uptake. Phytotransferrin sequences have a broad taxonomic distribution and are abundant in marine environmental genomic datasets, suggesting that acidification-driven declines in the concentration of seawater carbonate ions will have a negative effect on this globally important eukaryotic iron acquisition mechanism.

Read more (paywalled): https://www.nature.com/articles/nature25982

I’m skeptical of this claim that slightly warmer temperatures would inhibit plant growth to such an extent it would significantly impact the ability of ocean plankton to absorb CO2.

My large salt water pool hits a maximum temperature of around 80F (27C) in Summer, warmer than most seawater. I’ve never noticed warmer temperatures inhibiting the growth of microscopic plants – peak Summer where I live is a continuous battle to stop the pool turning green.