Mortality rates in key phytoplankton functional types: the nature of cell death and its biogeochemical consequence

In the sunlit upper layers of the sea, countless billions of floating microbes convert the energy of the sun into living tissue through photosynthesis. These tiny one-celled creatures are called 'phytoplankton', and their photosynthesis draws carbon dioxide (CO2) down from the atmosphere and into the ocean. They use the CO2 and nutrients from the water to build the cell components that they need to grow and multiply and whilst doing this they also give off oxygen. To catch the sunlight the phytoplankton use molecules called pigments, such as chlorophyll. Scientists have developed satellite methods so that they can look at chlorophyll from space and see where the phytoplankton are. Scientists who study the oceans are interested in more than just how much chlorophyll there is. They want to know how much CO2 is used, or fixed, into new tissue by photosynthesis: this is known as primary production. Primary production is important because the more there is the more zooplankton and fish can thrive by eating the phytoplankton. Scientists measure the rate of phytoplankton primary production, and then compare rates at different places and times to understand the way different marine ecosystems work. Primary production is therefore a very fundamental measurement for the marine sciences because it describes how much energy is generated at the base of the food web. Developments in the use of sophisticated Earth-observing satellites, offshore buoys and weather stations for measuring ocean properties of the ocean (such as chlorophyll, temperature, and salinity) are bringing great advances, but we still cannot estimate biological processes like competition and mortality in the ocean and these are important in determining how much phytoplankton will grow. It use to be assumed that phytoplankton could divide indefinitely i.e. that they were 'functionally immortal' and that population losses came only from being eaten by zooplankton, infected by viruses or sinking out of the sunlit waters. But now we know that phytoplankton are mortal, and that they will grow old and die, or die because they cannot grow. We do not know how often this happens, because it is difficult to recognise death in unicellular organisms and a 'one size fits all' rule may not apply because phytoplankton are highly diverse - some are less related to each other than humans are to trees, and there is also great variation in form, function and life-history. Nevertheless, these essential microbes control the processes, such as oxygen production, which sustain all other life on Earth. Indeed, the phytoplankton made the Earth's oxygen atmosphere a billion years ago. In the last 15 years or so scientists have revealed how important the natural death of phytoplankton could be for the energy flow of marine ecosystems: in some cases, more than half of the surface-dwelling phytoplankton may be dead. Dead cells cannot grow and divide, but may still contain chlorophyll, so it seems that detecting chlorophyll is not as good an indicator of primary production as we once thought and suggests that our ideas of how energy flows in the food web may be simplistic. The research that we propose here aims to better understand how populations of phytoplankton grow, divide and die in the vast expanses of our blue planet.