Sensitivity of phytoplankton to future changes in ocean carbonate chemistry: current knowledge, contradictions and research directions

Abstract

Despite their microscopic size, marine phytoplankton are responsible for about half of the global primary production and represent the basis of the marine food web. This diverse group of organisms drives important biogeochemical cycles, exporting massive amounts of carbon to deep waters and sediments, and strongly influencing ocean-atmosphere gas exchanges. Anthropogenic climate change will result in significant alterations in the marine environment over the next 100 yr and beyond. The increase in atmospheric CO 2 has already caused significantly higher aquatic CO 2 concentrations and lower pH values ('ocean acidification') than in pre-industrial times. Rising temperatures will also impact surface ocean stratification, which in turn will affect the surface-water light regime and nutrient input from deeper layers. Phytoplankton will be affected by these environmental changes in many ways. In this article we assess the possible responses of different phytoplankton groups with regard to the expected physico-chemical changes. In addition to summarizing laboratory and field studies, we outline the current understanding of the underlying mechanisms that cause processes such as photosynthesis, calcification, and nitrogen fixation to be sensitive to ocean acidification. We describe different approaches to manipulate carbonate chemistry (e.g. acid/base or CO 2 addition), discuss their potential to simulate future ocean acidification, and allude to common problems in experiments caused, for instance, by high biomass or the use of buffers. In addition to guidelines for CO 2 perturbation experiments, we argue that it is essential to look at multiple environmental factors in combination with CO 2 , to aim for process-understanding rather than correlation, and to assess a wider diversity of phytoplankton species both in laboratory and field studies.KEY WORDS: Ocean acidification · CO 2 manipulations · Photosynthesis · Carbon acquisition · Calcification · Nitrogen fixation Resale or republication not permitted without written consent of the publisher Contribution to the Theme Section 'Effects of ocean acidification on marine ecosystems'OPEN PEN

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

“…Perhaps the greatest challenge to determine the effect of climate change on DOM release by phytoplankton is accounting for interactions between different abiotic and biotic components of marine ecosystems (Rost et al, 2008). Much of the work that has been carried out so far has looked at either warming or acidification in isolation, whereas in reality the ocean's pH is decreasing as it warms.…”

“…These results suggest that research considering the effects of climate change on phytoplankton permeability and DOM release should consider the microenvironment immediately surrounding the cell in addition to the bulk medium. Collectively, these results suggest that researchers need to pay more attention to the design of ocean acidification experiments with phytoplankton cultures, in terms of how to control conditions within the experiments and accurately monitor DIC (Rost et al, 2008;Hurd et al, 2009;Hoppe et al, 2012).…”

“…Acidification is generally achieved by titrating the culture to the desired pH (using HCl and NaOH) or by bubbling the system with air containing an elevated mixing ratio of CO 2 (Rost et al, 2008;Hurd et al, 2009). Manipulating the pH and DIC system in seawater is complex, as CO 2 both dissolves in and reacts with seawater (Dring, 1982).…”

“…Creating experimental conditions that realistically simulate ocean acidification remains a challenge (Rost et al, 2008;Hurd et al, 2009). Acidification is generally achieved by titrating the culture to the desired pH (using HCl and NaOH) or by bubbling the system with air containing an elevated mixing ratio of CO 2 (Rost et al, 2008;Hurd et al, 2009).…”

Patterns of magnesium content in Arctic bryozoan skeletons along a depth gradient

“…Ocean acidification is a decrease in pH in the oceans caused by the uptake of anthropogenic CO 2 from the atmosphere (e.g., Orr et al 2005) and is recognized to have negative effects on many groups of planktonic and benthic marine invertebrates with calcareous skeletons (for reviews see Hoegh-Guldberg et al 2007;Fabry 2008;Pörtner 2008;Fabry et al 2008;Rost et al 2008;Doney et al 2009;Pelejero et al 2010). It may have a biotic impact at various levels, from cells to larvae, mature individuals, populations and entire ecosystems/regions.…”