Abstract
Predicting spatiotemporal distributions of phytoplankton biomass and community composition heavily relies on experimental studies that document how environmental conditions influence population growth rates. In unicellular phytoplankton, the net population growth rate is the difference between the cell division rate and the death rate. Along with predation and disease, phytoplankton mortality arises from abiotic stress. Although the effect of temperature on the net population growth rate is well understood, studies examining thermally induced death rates in phytoplankton are scarce. We investigated how cell division and death rates of the diatom Phaeodactylum tricornutum varied within its thermal tolerance limits (thermal niche), and at temperatures just above its upper thermal tolerance limit. We show that death rates were largely independent of temperature when P. tricornutum was grown within its thermal niche, but increased significantly at temperatures that approached or exceeded its upper thermal tolerance limit. Furthermore, the sensitivity of mortality increased with the duration of exposure to heat stress and was affected by the pre-acclimation temperature. Heat waves can be expected to significantly affect phytoplankton mortality episodically. The increasing frequency of heat waves accompanying global warming can be expected to drive changes in phytoplankton community structure due to interspecific variability of thermal niches with potential implications for food web dynamics and biogeochemical cycles.
Highlights
Phytoplankton growth generates half of the atmosphere's oxygen through photosynthesis, providing the energy that fuels pelagic food chains and playing key roles in biogeochemical cycles of oxygen, carbon and nutrient elements
We address the following key questions: (i) ‘Does the death rate (d) depend on temperature within the thermal tolerance limits of the model organism P. tricornutum, and if so by how much?’ and (ii) ‘What role does thermal acclimation have in mortality, if any?’
Death rates were largely independent of temperature when P. tricornutum was exposed to temperatures within the thermal niche, and only become significant when cultures were exposed to temperatures that approached or exceeded the Tmax
Summary
Phytoplankton growth generates half of the atmosphere's oxygen through photosynthesis, providing the energy that fuels pelagic food chains and playing key roles in biogeochemical cycles of oxygen, carbon and nutrient elements. R = 1 ⋅ e( 2 ⋅ T) − d0 − d1 ⋅ e(d2 ⋅ T), where μ1 is the cell division rate at 0°C, and μ2 is the exponential change in this rate with increasing temperature, d0 is a temperature- independent death rate, while d1 and d2 jointly describe the exponential increase in death rate with temperature All of these approaches yield similar TPCs for net growth, zero net growth rates are achieved with differing assumptions for the temperature dependence of mortality and depict strikingly different physiological responses to heat stress. Both the Serra-Maia et al (2016) and Thomas et al (2017) models assume high death rates within the thermal niche, datasets that quantify rates of phytoplankton mortality are scarce and would be valuable for those ecosystem models that incorporate mortality.
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