Phytoplankton Cell Death Under the Global Change Scenario

Phytoplankton are aquatic unicellular eukaryotic (i.e. protists) or prokaryotic photosynthetic organisms thriving in the world oceans. They are at the base of aquatic food webs and play a key role in determining the effects of the environmental change on the ocean surface. Phytoplankton are responsible for around 50% of the net amount of carbon assimilated annually by photoautotrophs. The removal of atmospheric CO₂ does not only depend on CO₂ fixation, but long-term carbon sequestration also requires the export of the organic matter fixed by phytoplankton in the surface layer (euphotic zone) into the deep ocean via a suite of processes collectively referred to as the biological carbon pump (BCP). The BCP likely transports 5-12 Pg C year^-1 into the deep ocean where it is buried and where it remains between thousands and millions of years. Approximately 50% of the CO₂ emitted by human activity accumulates in the atmosphere, causing an increase in the average temperature of the Earth's surface and oceans. Around 40% of the CO₂ emitted into the atmosphere is absorbed by the surface layers of the oceans, causing ocean acidification (OA) due to the increase in CO₂ concentration in the water. We are therefore currently facing one of the greatest challenges of the twenty-first century. The scenario based on the SSP5-8.5 climate simulation (IPCC, AR6 2021) closely matches the total historical accumulated CO₂ emissions (within 1%); moreover, this SSP5-8.5 scenario best represents mid-century greenhouse gas concentrations with current and declared emission policies, while forecasting very plausible atmospheric CO₂ levels of 1,200 μatm by 2,100. Considering that even small changes in the BCP can have substantial effects on atmospheric CO₂ levels, thus, the response of the BCP to future climate change is uncertain. Phytoplankton dynamics are driven by imbalances between growth and loss processes that are controlled by a combination of physical, chemical, and biological factors (drivers). Usually, grazing by zooplankton and viral infections stand at the base of phytoplankton population collapses. However, cell death (CD) also accounts for phytoplankton losses. Accumulating evidence suggests that CD, accidental (ACD) or regulated (RCD), occurs in phytoplankton under diverse environmental stresses, both in natural phytoplankton communities and in laboratory controlled experiments. GC affects phytoplankton species composition and size structure and favours species traits best acclimated and/or adapted to changing conditions associated with global change. Shifts in phytoplankton can have far-reaching consequences for the entire food web. The way phytoplankton dies critically determines marine ecosystem structure, functioning, and services. We aim in this chapter at capturing, re-grouping, and thoroughly analysing all the GC drivers studied up to date that cause cell death and/or severe stress in phytoplankton, trying to gain further insight on what could be the subsequent cell death outcome regarding the BCP.