The physiological response of marine diatoms to ocean acidification: differential roles of seawater pCO₂ and pH

Abstract

Although increasing the pCO2 for diatoms will presumably down‐regulate the CO2‐concentrating mechanism (CCM) to save energy for growth, different species have been reported to respond differently to ocean acidification (OA). To better understand their growth responses to OA, we acclimated the diatoms Thalassiosira pseudonana, Phaeodactylum tricornutum, and Chaetoceros muelleri to ambient (pCO2 400 μatm, pH 8.1), carbonated (pCO2 800 μatm, pH 8.1), acidified (pCO2 400 μatm, pH 7.8), and OA (pCO2 800 μatm, pH 7.8) conditions and investigated how seawater pCO2 and pH affect their CCMs, photosynthesis, and respiration both individually and jointly. In all three diatoms, carbonation down‐regulated the CCMs, while acidification increased both the photosynthetic carbon fixation rate and the fraction of CO2 as the inorganic carbon source. The positive OA effect on photosynthetic carbon fixation was more pronounced in C. muelleri, which had a relatively lower photosynthetic affinity for CO2, than in either T. pseudonana or P. tricornutum. In response to OA, T. pseudonana increased respiration for active disposal of H+ to maintain its intracellular pH, whereas P. tricornutum and C. muelleri retained their respiration rate but lowered the intracellular pH to maintain the cross‐membrane electrochemical gradient for H+ efflux. As the net result of changes in photosynthesis and respiration, growth enhancement to OA of the three diatoms followed the order of C. muelleri > P. tricornutum > T. pseudonana. This study demonstrates that elucidating the separate and joint impacts of increased pCO2 and decreased pH aids the mechanistic understanding of OA effects on diatoms in the future, acidified oceans.

Diatoms Dominate and Alter Marine Food-Webs When CO2 Rises

“…Decreases in respiratory metabolism may synergise with the increased rate of C uptake due to increased photosynthesis, promote increased growth rates, and explain the greatly boosted abundance in our site with increasing pCO 2 levels. Although species-specific responses are likely for diatoms, some generalisations have been suggested, for example, diatoms with lower CCM efficiencies are more likely to show a pronounced response [57], and larger centric diatoms are more likely to profit relative to smaller species [11,12]. This body of work aligns with our novel observations of the effects of ocean acidification on B. biddulphiana.…”

Elevated pCO2 Level Affects the Extracellular Polymer Metabolism of Phaeodactylum tricornutum

“…Elevated environmental CO 2 concentrations may facilitate carbon fixation by phytoplankton, as suggested by several studies of algal cells (Wu et al, 2010;Shi et al, 2019), including the present study. Thus, it is generally expected that increased seawater CO 2 concentrations will enhance marine primary productivity.…”

“…Thus, the possible biological consequences of elevated pCO 2 levels has recently received extensive research attention. In particular, several studies have reported the effects of elevated pCO 2 levels on diatoms at the physiological and ecological scales (Wu et al, 2010;Gao and Campbell, 2014;Shi et al, 2019). However, few studies have evaluated the molecular mechanisms underlying diatom responses to elevated pCO 2 levels.…”

Effects of Ocean Acidification on Marine Photosynthetic Organisms Under the Concurrent Influences of Warming, UV Radiation, and Deoxygenation

“…Under OA conditions, the coastal diatom Thalassiosira weissflogii shows a faster particulate carbon production rate, whereas a pelagic species, Thalassiosira oceanica, grows slower, implying that under diel pH fluctuations the large coastal diatom appears to be more tolerant of the pH decline (Li et al, 2016). While there are species-specific responses to OA, diatoms with lower CCM efficiencies showed more pronounced responses to OA in terms of growth rate (Shi et al, 2019).…”