Effects of temperature and salinity on respiratory losses and the ratio of photosynthesis to respiration in representative Antarctic phytoplankton species

Abstract

The Southern Ocean (SO) is a net sink for atmospheric CO2 whereby the photosynthetic activity of phytoplankton and sequestration of organic carbon (biological pump) plays an important role. Global climate change will tremendously influence the dynamics of environmental conditions for the phytoplankton community, and the phytoplankton will have to acclimate to a combination of changes of e.g. water temperature, salinity, pH, and nutrient supply. The efficiency of the biological pump is not only determined by the photosynthetic activity but also by the extent of respiratory carbon losses of phytoplankton cells. Thus, the present study investigated the effect of different temperature and salinity combinations on the ratio of gross photosynthesis to respiration (rGP/R) in two representative phytoplankton species of the SO. In the comparison of phytoplankton grown at 1 and 4°C the rGP/R decreased from 11.5 to 7.7 in Chaetoceros sp., from 9.1 to 3.2 in Phaeocystis antarctica strain 109, and from 12.4 to 7.0 in P. antarctica strain 764, respectively. The decrease of rGP/R was primarily dependent on temperature whereas salinity was only of minor importance. Moreover, the different rGP/R at 1 and 4°C were caused by changes of temperature-dependent respiration rates but were independent of changes of photosynthetic rates. For further interpretation, net primary production (NPP) was calculated for different seasonal conditions in the SO with specific combinations of irradiance, temperature, and salinity. Whereas, maximum photosynthetic rates significantly correlated with calculated NPP under experimental 'Spring', 'Summer', and 'Autumn' conditions, there was no correlation between rGP/R and the respective values of NPP. The study revealed species-specific differences in the acclimation to temperature and salinity changes that could be linked to their different original habitats.

Fig 1. Physiological key parameters (GP_max, R, rGP/R, NPQ_max) of Chaetoceros sp. and Phaeocystis antarctica.

Mean values (± standard deviation, n = 4–11) of physiological parameters measured in Chaetoceros sp. and P. antarctica (strains 109 and 764) grown under different combinations of temperature (-1, 1, 4°C) and salinity of the growth medium (20, 35, 50, 70 PSU; white, light grey, dark grey, black bars, respectively): a) Maximum gross oxygen-based photosynthesis (GP_max, [μmol O₂ (mg Chla)^-1 h^-1]), b) Respiration rate (R, [μmol O₂ (mg Chla)^-1 h^-1]), c) Ratio of maximum gross photosynthesis rate to respiration rate (rGP/R), d) Maximum value of non-photochemical quenching (NPQ_max), ‘n’ depicts the number of biological replicates. For P. antarctica no data were obtained at the condition -1°C/70 PSU (marked with ‘n.d.’).

Fig 2. Relationship between a) ratio of gross photosynthesis to respiration (rGP/R) and respiration and b) rGP/R and maximum gross photosynthetic rates (GP_max) in Chaetoceros sp. and Phaeocystis antarctica (strains 109 and 764).

Cultures of Chaetoceros sp. (filled triangles), P. antarctica strain 109 (filled circles), and strain 764 (open circles) were grown under different combinations of temperature (-1, 1, 4°C) and salinity of the growth medium (20, 35, 50 PSU). The correlation was calculated using Spearman rank correlation (correlation coefficient r_s).

Fig 3. Physiological key parameters (P_F/P_O, a*_phy) of Chaetoceros sp. and Phaeocystis antarctica.

Mean values (± standard deviation) of physiological parameters measured in Chaetoceros sp. and Phaeocystis antarctica (strains 109 and 764) grown under different combinations of temperature (-1, 1, 4°C) and salinity of the growth medium (20, 35, 50, PSU; white, light grey, dark grey, respectively): a) Ratio maximum fluorescence-/maximum oxygen-based photosynthetic rate (P_F/P_O, n = 4–11), b) Chlorophyll-specific absorption coefficient (a*_phy, [cm² (mg Chla)^-1], n = 3). ‘n’ depicts the number of biological replicates. The same column colours with respect to medium salinity were applied for all subfigures.

Fig 4. Ratio gross photosynthesis to respiration and calculated net primary production (NPP) for different seasonal conditions.

The ratio P/R and NPP were derived from mean values (± standard deviation) measured in Chaetoceros sp. (C. sp., filled triangles), Phaeocystis antarctica (P.a., strains 764, open circles, and 109, filled circles) grown under experimental conditions that represent specific seasonal in situ-conditions: a) Spring, b) Summer, c) Autumn, d) Winter (see text for details). NPP (μmol O₂ [mg Chla]^-1 d^-1) was calculated from fitted gross oxygen production rates (GP) minus measured respiratory losses considering the light conditions under seasonal conditions. The asterisks represent significant differences between the species (* p < 0.05, ** p < 0.01, *** p < 0.001).

Fig 5. Relationship between calculated net primary production (NPP) and GP_max and between NPP and rGP/R.

NPP (μmol O₂ [mg Chla]^-1 d^-1) was calculated from fitted gross oxygen production (GP) minus measured respiratory losses in Chaetoceros sp. (C. sp.), Phaeocystis antarctica (P.a., strains 764 and 109). Estimation of NPP is based on mean values of fitted Photosynthesis-Irradiance curves for different experimental conditions that represent specific seasonal in situ-conditions (see text for details). The calculated NPP values were plotted against the respective mean values of maximum gross photosynthesis (GP_max) and against rGP/R, respectively. The correlation was calculated using Spearman rank correlation (correlation coefficient r_s). In a), the correlation between NPP and GP_max data was calculated separately for ‘Spring’, ‘Summer’, and ‘Autumn’ (filled squares) and for the ‘Winter’ condition (open squares).