The Physiological Response of Picophytoplankton to Temperature and Its Model Representation

Abstract

Picophytoplankton account for most of the marine (sub-)tropical phytoplankton biomass and primary productivity. The contribution to biomass among plankton functional types (PFTs) could shift with climate warming, in part as a result of different physiological responses to temperature. To model these responses, Eppley's empirical relationships have been well established. However, they have not yet been statistically validated for individual PFTs. Here, we examine the physiological response of nine strains of picophytoplankton to temperature; three strains of picoprokaryotes and six strains of picoeukaryotes. We conduct laboratory experiments at 13 temperatures between -0.5 and 33 • C and measure the maximum growth rates and the chlorophyll a to carbon ratios. We then statistically validate two hypotheses formulated by Eppley in 1972: The response of maximum growth rates to temperature (1) of individual strains can be represented by an optimum function, and (2) of the whole phytoplankton group can be represented by an exponential function Eppley (1972). We also quantify the temperature-related parameters. We find that the temperature span at which growth is positive is more constrained for picoprokaryotes (13.7-27 • C), than for picoeukaryotes (2.8-32.4 • C). However, the modeled temperature tolerance range ( T) follows an unimodal function of cell size for the strains examined here. Thus, the temperature tolerance range may act in conjunction with the maximum growth rate to explain the picophytoplankton community size structure in correlation with ocean temperature. The maximum growth rates obtained by a 99th quantile regression for the group of picophytoplankton or picoprokaryotes are generally lower than the rates estimated by Eppley. However, we find temperature-dependencies (Q 10 ) of 2.3 and of 4.9 for the two groups, respectively. Both of these values are higher than the Q 10 of 1.88 estimated by Eppley and could have substantial influence on the biomass distribution in models, in particular if picoprokaryotes were considered an independent PFT. We also quantify the increase of the chlorophyll a to carbon ratios with increasing temperature due to acclimation. These parameters provide essential and validated physiological information to explore the response of marine ecosystems to a warming climate using ocean biogeochemistry models.

The physiological response of seven strains of picophytoplankton to light, and its representation in a dynamic photosynthesis model

“…These parameters included the maximum carbon specific rate of photosynthesis ( ), the affinity to light (α chl ), the light inhibition term (β chl ) and the respiration rate (resp) ( see Supporting Information Fig. A1; Table A1 and Stawiarski ). There were no trends which would indicate a systematic bias in how the photosynthesis model represents acclimation to light intensity.…”

“…We calculated a slightly lower μ max for the low light adapted Prochlorococcus sp. strain which may be explained by strain related differences, as the temperature was chosen to be at its optimum (Stawiarski et al ). The higher maximum growth rates of picoeukaryotes are consistent with previous findings (Glover et al ; Six et al ).…”

“…Each oxygraph chamber was filled with a 3 mL sample of the acclimated culture and the oxygen concentration was measured continuously at a constant temperature of 218C. There was no significant change of Chl a to carbon ratios between the acclimation temperature and the temperature used in the oxygraph chamber for all species (linear regression, ANOVA (p > 0.05), Stawiarski et al 2016). Hence, this difference of 18C should not affect the photosynthesis measurements.…”

Constraints on global oceanic emissions of N<sub>2</sub>O from observations and models

“…The available uptake rate data do not include the supporting data to allow conversion to the K 1/2 for growth. We are only aware of measurements of the K 1/2 for growth by Stawiarski (2014). Based on the latter values of 0.09 ± 0.15 µmol L −1 for picoeukaryotes, the K 1/2 of phytoplankton for NH + 4 was set to 0.1 to 5 µmol L −1 , increasing linearly with nominal size (Buitenhuis et al, 2013b).…”

Estimating Primary Production of Picophytoplankton Using the Carbon-Based Ocean Productivity Model: A Preliminary Study

“…The data from limited number strains does not represent the full range of growth characteristic of Prochlorococcus and Synechococcus , although these datasets were widely used in other modeling studies (Boyd et al, 2015; Hynes et al, 2015; Stawiarski et al, 2016; Grossowicz et al, 2017). We recognize that the high phenotypic diversity of Prochlorococcus and Synechococcus combined with the limited number of cultured strains for which there are growth rates data represents an inherent limitation of model parameterizations.…”