The continuous rise in carbon dioxide levels in the means that it is crucial to better understand the Earthâ€™s carbon cycle. One key aspect is the oceanâ€™s so-called â€œbiological carbon pumpâ€ i.e. the biologically driven sequestration of atmospheric carbon to the deep...
The continuous rise in carbon dioxide levels in the means that it is crucial to better understand the Earthâ€™s carbon cycle. One key aspect is the oceanâ€™s so-called â€œbiological carbon pumpâ€ i.e. the biologically driven sequestration of atmospheric carbon to the deep ocean and its sediments. Major vehicles for this transfer of organic matter from the surface to the deep are so-called â€œmarine snow aggregatesâ€ and, in particular, aggregates composed of diatoms. Diatomsâ€™ heavy silicified cell walls and long cell chain formation mean that they can form fast-sinking aggregates, and given their abundance as one of the most common types of phytoplankton found in the ocean, can they dominate the particle flux to the ocean floor. Diatom aggregates are like small microbial hotspots as they sink to the seafloor and are frequently glued together by transparent exopolymeric particles (TEP), which is a gel-like transparent sugary substance released by diatoms themselves and bacteria. As the aggregates sink there is an exchange of solutes and they become a food source for microbes and larger organisms such as zooplankton. Despite their importance the small-scale processes occurring around these aggregates as they sink are difficult to study. The key objective of this project was to implement advanced technology to gain a better understanding of the processes, e.g., TEP production, that influence the relative importance of flow and diffusion around and within diatom aggregates and hence the exchange of gases, nutrients and solutes between the aggregate and the surrounding water. We examined the flow around both permeable as well as impermeable aggregates and, further, explored the production of TEP as well as aggregate formation of the cosmopolitan diatom species \'Skeletonema marinoi\' in detail. Specifically, we determined how TEP production is affected by dynamic nutrient conditions at a clonal level and if subjected to grazing pressure.
During the project we have:
1. Conducted a large scale collaborative study to investigate the adaptation of the cosmopolitan diatom species \'Skeletonema marinoi\' to dynamic nutrient conditions. More specifically, we have examined how this affects the production of transparent exopolymeric particles (TEP) and the formation of aggregates, whilst our collaborators focused on examining carbon and nitrate assimilation processes. The analysis of TEP samples from both experiments is in progress and intended for publication. The researcher is co-author on the study of carbon and nitrate assimilation published in Environmental Biology (https://doi.org/10.1111/1462-2920.14434) which revealed a high diversity of nutrient demand not only at a clone-specific level but also at the single-cell level allowing the population to sustain itself and adapt to dynamic nutrient conditions.
2. Investigated the effect of signaling molecules (copepodamides) produced by copepods on the production of TEP and aggregate formation of Skeletonema marinoi. Two experiments were run as well as two preliminary experiments with data analysis in progress of all TEP and aggregate measurements. First indications of the data analysis reveal effects of the copepodamides on aggregate size, and sinking velocity as well as associated TEP content. A manuscript outline has been prepared and an abstract submitted for results to be presented in May 2019 at the â€œMarine Particles and Phycospheresâ€ conference in Ascona, Switzerland.
3. Examined flow field data measured around permeable and impermeable aggregates in detail. Results indicate that highly irregularly shaped permeable aggregates formed from the diatom Chaetoceros affinis behaved similarly to more compact spherical aggregates formed by Skeletonema marinoi as well as impermeable model aggregates. The presence of TEP dominated the interstitial spaces of Chaetoceros aggregates compared to the cell-to-cell stickiness found in Skeletonema aggregates. Subsequently, the transport of gases, nutrients, and solutes occurs by diffusion even within large, apparently porous diatom aggregates during sinking. Results from this were presented at the Gordon Research Conference â€œThe Biologically-Driven Ocean Carbon Pumpsâ€ in June 2016 as well as at the EuroMarine Foresight Symposium in October 2016: â€œThe Biological Carbon Pump in a Changing Worldâ€ which was co-organized by the researcher. The final manuscript is currently being assembled.
4. Set up digital holographic microscopy imaging capabilities at UGOT and used this to explore the structure of aggregates and the presence of TEP within. Access to an oLine D3HM (Ovizio Imaging Systems, Belgium) were provided via Prof. Filip Meysman of the partner organization, whilst a QMod module (Ovizio Imaging Systems, Belgium) for UGOT microscope facilities was obtained via Swedish funding. All experiments benefitted from the access to these facilities. In addition, we investigated embedded natural aggregates through collaborative efforts with Dr. Morten Iversenâ€™s lab in Germany (Marum) which will require further study.
5. Described particle sources and transport in stratified Nordic coastal seas in the Anthropocene in a review initiated as a departmental effort at UGOT. We highlighted the particular importance of particles in stratified coastal waters and estuaries, their roles in natuÂ¬ral processes, their formation and transport (including aggregation processes), and their interactions with anthropogenic activities. E. Zetsche coordinated the contribution which has been published in Elementa Science of the Anthropocene (https://doi.org/10.1525/elementa.149).
To date, results from this project were presented at one international conference and one workshop, which was also co-organized by the researcher. A third presentation is scheduled for May 2019. The initially planned attendance of a second international conference, where a session was also co-organized was cancelled
This project has significantly advanced our understanding of small-scale processes, particularly with respect to TEP production, in marine snow aggregates, specifically diatom aggregates. The research under this grant has been interdisciplinary combining different aspects of biology, physics and chemistry but also evolutionary genetics and chemical ecology. It has also built on existing collaborations and developed new ones, both within UGOT but also internationally.
It is hoped that the fundamental knowledge gained within this project will further improve our understanding of diatom aggregatesâ€™ contribution to the biological carbon pump and consequently to the global carbon cycle. Given the importance of the oceanâ€™s ability to sequester carbon in the currently changing global climate it is critical that we better understand all transfer processes both on small as well as large scales.