Day length as a key factor moderating the response of coccolithophore growth to elevated pCO₂

Abstract

The fate of coccolithophores in the future oceans remains uncertain, in part due to key factors having not been standardized across experiments. A potentially moderating role for differences in day length (photoperiod) remains largely unexplored. We therefore cultured four different geographical isolates of the species Emiliania huxleyi, as well as two additional species, Gephyrocapsa oceanica (tropical) and Coccolithus braarudii (temperate), to test for interactive effects of pCO 2 with the light : dark (L : D) cycle. We confirmed a general regulatory effect of photoperiod on the pCO 2 response, whereby growth and particulate inorganic carbon and particulate organic carbon (PIC : POC) ratios were reduced with elevated pCO 2 under 14 : 10 h L : D, but these reductions were dampened under continuous (24 h) light. The dynamics underpinning this pattern generally differed for the temperate vs. tropical isolates. Reductions in PIC : POC with elevated pCO 2 for tropical taxa were largely through reduced calcification and enhanced photosynthesis under 14 : 10 h L : D, with differences dampened under continuous light. In contrast, reduced PIC : POC for temperate strains reflected increases of photosynthesis that outpaced increases in calcification rates under 14 : 10 h L : D, with both responses again dampened under continuous light. A multivariate analysis of 35 past studies of E. huxleyi further demonstrated that differences in photoperiod account for as much as 40% (strain B11/92) to 55% (strain NZEH) of the variance in reported pCO 2 -induced reductions to growth but not PIC : POC. Our study thus highlights a critical role for day length in moderating the effect of ocean acidification on coccolithophore growth and consequently how this response may play out across latitudes and seasons in future oceans.

Physiological responses of <i>Skeletonema</i> <i>costatum</i> to the interactions of seawater acidification and combination of photoperiod and temperature

“…neutral effects of higher pCO2 were observed. However, different patterns were reported for other species, such as E. huxleyi and the macroalgae Ulva linza (Bretherton et al, 2019;Yue et al, 2019). For these two species, reduced growth rate at elevated pCO2 were found when the daylength was longest.…”

“…For these two species, reduced growth rate at elevated pCO2 were found when the daylength was longest. High temperature might accelerate nutrient uptake and metabolic rates, which may alleviate the negative effects of longer daylength under higher pCO2 environment (Bretherton et al, 2019;Yue et al, 2019). Maximum photosynthetic rate increased significantly under higher pCO2 in spring condition.…”

“…Under the combined influence of photoperiod and OA, physiological performance of phytoplankton might be different from that under single factor. For example, continuous light moderates the negative effect of OA on coccolithophore growth, although species isolated from different regions show diverse responses (Bretherton et al, 2019). The changes of photoperiod are often accompanied by increase or decrease temperature, and impacts of OA on diatoms can also be changed by temperature.…”

Effects of Temperature and Light on Methane Production of Widespread Marine Phytoplankton

“…By contrast, growth rates of E. huxleyi have been reported to be independent of daylength (Nielsen, 1997) or to be inhibited by continuous light (Van Rijssel & Gieskes, 2002). The response to daylength has been suggested to be strain specific (Bretherton et al, 2019) and is dependent on other environmental parameters, for example, on seawater CO 2 concentration (Bretherton et al, 2019; Zhang, Bach, et al, 2015) and light quality (Glover et al, 1987). This could be one reason why the response of growth in relation to daylength differs between studies.…”

Diel variations in cell division and biomass production of Emiliania huxleyi—Consequences for the calculation of physiological cell parameters

“…One suggested approach to obtain daily means in cellular pool sizes and production rates is to fully desynchronize phytoplankton cultures by applying continuous light, in which case any sample taken intrinsically represents the daily mean (Jochem and Meyerdierks 1999;Shi et al 2009;Müller et al 2017). This approach is straight-forward, but it can only be applied when the observed phytoplankton truly desynchronizes in response to continuous light, and when its growth is unaffected by this treatment (Brand and Guillard 1981;Chisholm and Brand 1981;Bretherton et al 2019).…”