Award: OCE-1635748

Coccolithophore Mixotrophy- Project Outcomes Report INTELLECTUAL MERIT OF THIS WORK: Mixotrophy is the process for organisms to use a combination of different sources of energy and carbon, instead of having a single trophic mode. It occurs on the continuum from complete heterotrophy on one extreme to autotrophy at the other extreme. In the case of marine phytoplankton, it describes the ability of cells to take up carbon via photosynthesis, phagocytosis (cellular engulfing of particles) or uptake of dissolved organic carbon (DOC, a.k.a. osmotrophy). The purpose of this project was to understand how coccolithophores (calcifying, unicellular algae) survive in complete darkness. This question was posed given the survival of the plant class following the major asteroid impact some 225 million years ago at the Cretacious/Tertiary boundary which was alledged to have darkened Earth for several months yet they have no resting stage like other phytoplankton classes. Moreover, there are observations of some coccolithophore species surviving in almost complete darkness in the ocean water column, far too deep for photosynthesis. There was some previous tantalizing unpublished evidence from 60 years ago, too, that this algal group could take up DOC in the dark and use it to grow on but those results were only part of a Ph.D. dissertation and were never published in the open literature. We therefore performed laboratory experiments in which we confirmed that coccolithophores could take up and assimiilate a wide variety of organic material. Then we focused in on a few of these compounds, first performing 30d growth experiments in darkness plus we used specific radio-labelled compounds to examine how their uptake rates varied as a function of concentration and, finally, how this nutritional mode compared to autotrophy (photosynthesis). Finally, we took these techniques to sea on the R/V Endeavor to perform the first such experiments on natural coccolithophore populations. As luck would have it, the time of the cruise happened to be during the largest coccolithophore bloom observed on the New England contintinental shelf in 30 years. We performed similar experiments using the radio-labelled organic compounds and then we concentrated the cells using a filtration device called a "cell trap", which then allowed us to use an instrument called a "flow cytometer" to sort out the coccolithophores, away from all other phytoplankton. This allowed us to have sufficient signal to see whether natural assemblages of coccolithophores could take up dissolved organic matter. The surprise in the results was that we discovered that no only did the coccolithophores take up DOC, but that within 24 hours, they fixed half of it into particulate organic carbon (POC) and half of it was fixed into coccolithophore calcite (a.k.a. particulate inorganic carbon or PIC). BROADER IMPACTS OF THIS WORK: This work is important because PIC is one of the four forms of carbon compounds on Earth, along with POC, DOC and dissolved inorganic carbon (DIC; forms such as CO2). Moreover, there are entire paradigms in oceanography such as the biological carbon pump (BCP) paradigm which now warrent re-examination given our results. The BCP explains how organic carbon is fixed into phytoplankton whiich is ballasted to sink to the sea floor by coccolith PIC (thought to originate from CO2, DIC) and is subsequently sequestered away from the atmosphere. Instead, here, we have discovered that the PIC coccoliths are synthesized from DOC, too, which is the largest carbon pool in the sea. Another paradigm is the Ocean Alkalinity Pump, in which organisms like coccolithophores fix calcium ions and bicarbonate (DIC) from the surface ocean and fix it into CaCO3, where it sinks, carrying that alkalinity to depth. whereupon the coccoliths then dissolve, releasing the alkalinity back into the water at depth (hence, "pumping" alkalinity to depth). Our results from this work demonstrate that PIC coccoliths are not necessarily transporting only surface alkalinity (as DIC) but could be effectively transporting DOC, as well. The bottom line is that our work illustrates a previously unknown (and rapid) pathway for transformation of one major carbon pool into another for deposition at depth. As part of this grant, we trained three undergraduate students, one doctoral student from Old Dominion University, and a post-doc in the Balch Lab at Bigelow Laboratory. We gave seminars to the public about our research. We also published an E-book with scanning electron microscope images of coccolithophores for public dissemination. The book not only describes the natural history of coccolithophores but also delves into their nutritional strategies, the subject of this proposal. Last Modified: 11/22/2021 Submitted by: William M Balch