A novel hydrogen isotope proxy for trophic behaviour in phytoplankton

Carbon fixation by oceanic primary production provides a substantial sink for carbon, responsible for the sequestration of about 7.2 Pg C yr via the biological pump. Traditional paradigms assume a strict distinction between autotrophic phytoplankton and heterotrophic zooplankton, and consequently, that all primary production can be attributed to strictly autotrophic phytoplankton. However, recent research has shown that mixoplankton (protists and bacteria capable of both phototrophy and phagotrophy) comprise a substantial part of the global plankton community. These mixotrophs significantly alter the structure of marine food webs and increase the rate of biomass transfer up the food chain, which recent modelling suggests may ultimately enhance the efficiency of the biological pump by up to 35%. This discovery has major implications for fields ranging from climate modelling to the management of fisheries.

However, to fully investigate the impact of mixotrophy upon global biogeochemical cycles and marine food webs robust techniques to estimate mixotrophic grazing will be required. Currently methods which measure grazing by phagotrophs are unable to distinguish between mixotroph and heterotroph contributions, while those that measure primary production are unable to distinguish between mixotrophs and pure phototrophs.

The most successful techniques to date have employed fluorescently labelled bacteria or algae. However, these techniques often require the prey to be killed, and some protist species avoid inert particles when grazing. This behaviour is more frequent in species which prey on organisms similar to them in size, which presents problems for examining the impacts of mixotrophy. as some major algal grazers, like dinoflagellates, actively discriminate against inert prey.

The use of live fluorescent prey also presents difficulties. These methods require the observed system to be altered through the introduction of fluorescently labelled prey organisms, resulting in the artificial increase in prey abundance. Additionally, grazers are not selective towards or against the labelled prey. An alternative method is to use acidotrophic probes to stain food vacuoles, but this requires flow cytometry, which can fail to detect larger grazers such as ciliates and dinoflagellates. Ultimately, all currently used methods require prior knowledge of the system in order to determine the trophic behaviour of plankton within it.

Currently, no methods capable of reconstructing both the spatial and temporal variations in heterotrophic growth of microplankton exist. This project will develop a novel tool to allow this reconstruction across a range of plankton groups.

We will use the hydrogen isotope signature of specific plankton lipids to measure the metabolic changes occurring in plankton in response to changing conditions. The dinoflagellate P. Micans will be used as a model for the metabolic fractionation of hydrogen due to its previous cultivation under autotrophic, mixotrophic and heterotrophic conditions, and their ability to produce the dinoflagellate-specific lipid, dinosterol. The use of this biomarker will allow us to track the metabolic behaviour of dinoflagellates across the current oceans and throughout geological time, providing both spatial and temporal resolution of the metabolic behaviour of these protists. This approach will be applied to other groups such as prymnesiophytes and diatoms, using alkenones and brassicasterol as the respective biomarkers.