Guest essay by Eric Worrall

According to scientists who bubbled CO2 through a tank full of wild algae, we need to be concerned about the health of the world’s seas 80 years from now.

Acid oceans are shrinking plankton, fuelling faster climate change

August 27, 2019 5.59am AEST
Katherina Petrou Senior Lecturer in Phytoplankton Ecophysiology, University of Technology Sydney
Daniel Nielsen Casual Academic, University of Technology Sydney

Increasingly acidic oceans are putting algae at risk, threatening the foundation of the entire marine food web. 

Our research into the effects of CO₂-induced changes to microscopic ocean algae – called phytoplankton – was published today in Nature Climate Change. It has uncovered a previously unrecognised threat from ocean acidification.

In our study we discovered increased seawater acidity reduced Antarctic phytoplanktons’ ability to build strong cell walls, making them smaller and less effective at storing carbon. At current rates of seawater acidification, we could see this effect before the end of the century.

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Read more: https://theconversation.com/acid-oceans-are-shrinking-plankton-fuelling-faster-climate-change-121443

The abstract of the study;

Acidification diminishes diatom silica production in the Southern Ocean
Katherina Petrou, Kirralee G. Baker, Daniel A. Nielsen, Alyce M. Hancock, Kai G. Schulz & Andrew T. Davidson 

Diatoms, large bloom-forming marine microorganisms, build frustules out of silicate, which ballasts the cells and aids their export to the deep ocean. This unique physiology forges an important link between the marine silicon and carbon cycles. However, the effect of ocean acidification on the silicification of diatoms is unclear. Here we show that diatom silicification strongly diminishes with increased acidity in a natural Antarctic community. Analyses of single cells from within the community reveal that the effect of reduced pH on silicification differs among taxa, with several species having significantly reduced silica incorporation at CO₂ levels equivalent to those projected for 2100. These findings suggest that, before the end of this century, ocean acidification may influence the carbon and silicon cycle by both altering the composition of the diatom assemblages and reducing cell ballasting, which will probably alter vertical flux of these elements to the deep ocean.

Read more: https://www.nature.com/articles/s41558-019-0557-y

Essentially they exposed algae collected from seawater to different levels of CO2, and observed those exposed to higher levels of CO2 appeared less healthy and less dense. The scientists are concerned this means the captured CO2 would not be dragged as quickly to the ocean floor when the algae died.

Though they admit that in the real world, there are a lot of uncertainties;

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The future of oceanic carbon sequestration remains ambiguous because of the uncertainties associated with potential changes to the biological carbon pump²⁵. Current predictions of climate-driven changes to ocean productivity are incomplete because many of the effects of these environmental changes on phytoplankton groups, on the interactions among lower trophic levels and on the feedbacks to climate change are poorly understood. The effect of ocean acidification on species selection and silicification and the consequences for carbon export may be mediated by exposure to coincident environmental stresses imposed by changing climate^53,54,55,56 or higher trophic interactions. Yet, this study establishes that silicification is sensitive to ocean acidification, with potentially crucial consequences for both trophodynamics and elemental cycling in Antarctic coastal waters and beyond.

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Read more: Same link as above

In my opinion the headline claim that CO2 is putting algae at risk seems far fetched. The Algae in question exist in vast numbers, and multiply rapidly, which implies the ability to quickly adapt to changed conditions.

Diatoms appear in the fossil record for at least the last 185 million years, so the ancestors of current species have thrived during periods of far higher CO2 levels than today.