Inorganic carbon availability and the growth of large marine diatoms

Abstract

To address the question of whether free CO2 can be a growth-limiting nutrient in oceanic waters 3 large marine diatoms, Stephanopyxis palmeriana (Greville) Grunow, Ditylum brightwellij (T. West) Grunow in Van Heurck 1883, and Coscinodiscus sp., were grown in pH-drift experiments under batch culture conditions. The cultures were maintained under quiescent conditions without added buffer in the growth medium, allowing the pH to rise and the free CO2 concentration to fall as growth proceeded. Growth rates were constant for sustained periods and only decreased as the pH rose to as high as 8.5-8.6 and the free CO2 concentration fell to -4 1.1mol l-' Such a low free CO2 concentration is far below the expected half saturation coefficient for ribulose biphosphate carboxylase-oxygenase (RUBISCO) and suggests that these species were capable of utilizing HC03-in some fashion so as not to allow free CO2 to become growth-limiting. Additional experiments were conducted with these and other diatoms to demonstrate that turbulent mixing at high pH levels had no effect on inorganic carbon uptake or growth rate and that HCO, uptake probably was occurring. Turbulent mixing should have enhanced inorganic carbon uptake by lowering diffusion gradients had these species been obligate users of free COz. These species, being the most susceptible to diffusion-controlled uptake of free COz because of their long diffusion paths, represent the worst case scenario for free CO2 limitation. Finally, by developing a chemical-biological model it was possible to show that, in order for the pH of marine waters to rise even several tenths above ambient levels, biomass concentrations must increase to levels that are never found, except poss~bly on occasion in estuarine or coastal waters. Thus ~t seems unlikely that free CO2 limits phytoplankton growth in the oceans.

Individual and interactive effects of warming and CO<sub>2</sub> on <i>Pseudo-nitzschia subcurvata</i> and <i>Phaeocystis antarctica</i>, two dominant phytoplankton from the Ross Sea, Antarctica

“…However, at CO 2 concentrations beyond current atmospheric levels of ∼ 400 ppm, growth rates of P. subcurvata or P. antarctica were CO 2 -saturated. Although a general model prediction suggests that an atmospheric CO 2 increase from current levels to 700 ppm could increase the growth of marine phytoplankton by 40 % (Schippers et al, 2004), our results instead correspond to previous studies which showed negligible effects of elevated CO 2 on various groups of phytoplankton (Goldman, 1999;Fu et al, 2007;Hutchins and Fu 2017). In particular, Trimborn et al (2013) found that increasing CO 2 had no effect on growth rates of Southern Ocean isolates of P. subcurvata and P. antarctica.…”

Effects of Increased CO2and Temperature on the Growth of Four Diatom Species (Chaetoceros debilis, Chaetoceros didymus, Skeletonema costatum and Thalassiosira nordenskioeldii) in Laboratory Experiments

“…) (like results of Goldman, 1999). Cultures of the diatom Chaetoceros muelleri showed lower growth and/or no response in elevated CO2 concentrations (Gao et al, 2012).…”

Influence of zinc and iron enrichments on phytoplankton growth in the northeastern subarctic Pacific

“…This resulted mainly from a very high value in the small fraction, although the reason for the difference between the ϩFe and ϩZnϩFe treatments in this small fraction is unknown. The highest chlorophyll-specific POC production at the end of the incubations was for the ϩZnϩFe treatment in the large fraction, consistent perhaps with carbon limitation of large diatoms at relatively low pCO 2 at the end of the incubation; Zn could allow synthesis of CA and could therefore mitigate the potential CO 2 limitation to which larger cells with longer diffusion paths might be susceptible (e.g., Morel et al 1994;Goldman 1999).…”

“…Indeed, there Ϫ 3 appears to be an increased demand for Zn, Co, or both, and Cd under low-pCO 2 conditions (Morel et al 1994;Cullen et al 1999). The rate of diffusion of free CO 2 into the cell can be a limitation for some species, even in air-equilibrated seawater (Riebesell et al 1993), a point of particular potential relevance to larger cells (with longer diffusion paths) that might alternatively be directly using HCO (Goldman 1999).…”