Photophysiological responses of marine diatoms to elevated CO2 and decreased pH: a review

Abstract

Abstract. Diatoms dominate nearly half of current oceanic productivity, so their responses to ocean acidification are of general concern regarding future oceanic carbon sequestration. Community, mesocosm and laboratory studies show a range of diatom growth and photophysiological responses to increasing pCO 2 . Nearly 20 studies on effects of elevated pCO 2 on diatoms have shown stimulations, no effects or inhibitions of growth rates. These differential responses could result from differences in experimental setups, cell densities, levels of light and temperature, but also from taxon-specific physiology. Generally, ocean acidification treatments of lowered pH with elevated CO 2 stimulate diatom growth under low to moderate levels of light, but lead to growth inhibition when combined with excess light. Additionally, diatom cell sizes and their covarying metabolic rates can influence responses to increasing pCO 2 and decreasing pH, although cell size effects are confounded with taxonomic specificities in cell structures and metabolism. Here we summarise known diatom growth and photophysiological responses to increasing pCO 2 and decreasing pH, and discuss some reasons for the diverse responses observed across studies.

Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO<sub>2</sub> tolerance in phytoplankton productivity

“…Acclimation to increased CO 2 has been reported in a number of studies, resulting in shifts in carbon and energy utilisation (Sobrino et al, 2008;Hopkinson et al, 2010;Hennon et al, 2014;Trimborn et al, 2014;Zheng et al, 2015). Numerous photophysiological investigations on individual phytoplankton species also report species-specific tolerances to increased CO 2 (Gao et al, 2012a;Gao and Campbell, 2014;Trimborn et al, 2013Trimborn et al, , 2014, and a general trend toward smaller-celled communities with increased CO 2 has been reported in ocean acidification studies globally . Changes in community structure were observed with increasing CO 2 , with taxon-specific thresholds of CO 2 tolerance (Hancock et al, 2017).…”

“…This cellular energy is then utilised in a second, light-independent pathway, which uses the carbon-fixing enzyme RuBisCO to convert CO 2 into sugars through the Calvin cycle. However, under certain circumstances the relative pool of energy may also be consumed in alternative pathways, such as respiration and photoprotection (Behrenfeld et al, 2004;Gao and Campbell, 2014). Increases in en- ergy requirements for these alternate pathways have been demonstrated, where measurements of maximum photosynthetic rates (P max ) and photosynthetic efficiency (α) display changes that result in no change to saturating irradiance levels (E k ) (Behrenfeld et al, 2004(Behrenfeld et al, , 2008Halsey et al, 2010).…”

A red tide alga grown under ocean acidification upregulates its tolerance to lower pH by increasing its photophysiological functions

“…Other reports have indicated neutral responses with insignificant influences on growth (Arnold et al, 2013), calcification (Langer et al, 2006(Langer et al, , 2009 or photosynthesis (Wu et al, 2008;Trimborn et al, 2013). While elevated CO 2 in air, and hence in water, might stimulate algal photosynthesis, the CO 2 -induced pH drop and change in carbonate chemistry of seawater could bring about different physiological impacts on phytoplankton (Raven, 2011;Gao and Campbell, 2014). It is known, for instance, that OA stimulates nonphotochemical quenching when diatoms or surface phytoplankton assemblages are grown under bright sunlight (Gao et al, 2012b).…”

“…Effects of the CO 2 enrichment on phytoplankton have been widely studied (see the reviews, and literature therein, by Beardall et al, 2009;Tanaka et al, 2013;Brussaard et al, 2013;Gao and Campbell, 2014). Algal responses to elevated CO 2 concentrations have indicated a stimulation of growth or photosynthesis (Gao et al, 1991;Hein and Sand-Jensen, 1997;Zou et al, 2011;Trimborn et al, 2013), reduced calcification (Riebesell et al, 2000;Gao and Zheng, 2010) or growth rates (Gao et al, 2012b;Trimborn et al, 2013) and stimulation of respiration (Wu et al, 2010;Yang and Gao, 2012).…”

Functional Redundancy Facilitates Resilience of Subarctic Phytoplankton Assemblages toward Ocean Acidification and High Irradiance

“…While the consequences of OA have received a lot of attention over the past decades, interactive effects of multiple drivers have been the focus of more recent studies (Riebesell and Gattuso, 2015). Interactions between light and pCO 2 levels, for example, have been demonstrated in phytoplankton isolates and natural assemblages from temperate and subtropical regions (Gao and Campbell, 2014). Beneficial effects of OA have been shown to be more pronounced under low light levels (Kranz et al, 2010;Rokitta and Rost, 2012), while OA appears to inhibit phytoplankton growth under high or variable light (Gao et al, 2012;McCarthy et al, 2012;Jin et al, 2013).…”