Dissolved Phosphorus Processing by Trichodesmium Consortia: Quantitative Partitioning, Role of Microbial Coordination, and Impact of Nitrogen Fixation

Trichodesmium colonies will be collected at sea during a cruise in the western (sub)tropical North Atlantic. Colonies will be incubated in the presence of ³³P-labeled dissolved phosphorus molecules, which will be traced into phospholipid molecules that are specific for Trichodesmium and their epibionts, thereby revealing the partitioning of the P from these substrates. Metagenomic and metatranscriptomic sequencing will be employed to identify the metabolic capabilities of the consortia, constrain which organisms are expressing these metabolisms, and understand these organisms' responses to the different experimental treatments. These data will be used to elucidate how phosphorus metabolism is coordinated in Trichodesmium consortia, and to discern the role of quorum sensing in phosphorus acquisition and partitioning. In the end, all of these pieces of information will be tied together to assess whether coordination and quorum sensing ultimately affect phosphorus stress and N₂ fixation by Trichodesmium.

Colonies of the cyanbacterium Trichodesmium are responsible for a large fraction of N₂ fixation in nutrient-poor, open-ocean ecosystems. Because this nitrogen ultimately fuels primary production in both Trichodesmium and in the broader planktonic community, Trichodesmium thus plays a critical role in their environment. However, in some parts of the ocean, the scarcity of dissolved phosphorus limits rates of Trichodesmium N₂ fixation. Trichodesmium colonies employ an arsenal of strategies to mitigate the effects of phosphorus limitation. However, Trichodesmium contain not only Trichodesmium cells but also complex consortia of epibiotic bacteria. The roles these epibionts play in phosphorus acquisition are completely unknown, even though the epibionts might contain as much as half of phosphorus in the colony. The PIs have recently discovered evidence for coordinated phosphorus acquisition, based on quorum sensing, amongst members of these consortia. The PIs posit that this strongly suggests that epibionts play a key role in colonial phosphorus metabolism. To guide their research, they pose four questions about the relative roles Trichodesmium and epibionts in dissolved phosphorus acquisition.

Question 1 - How are phosphorus atoms from phosphate and DOP partitioned between Trichodesmium and their epibionts?

Question 2 - Is the acquisition and partitioning of phosphate and DOP a coordinated effort between Trichodesmium and their epibionts?

Question 3 - What is the role of quorum sensing in affecting the acquisition and partitioning of phosphorus between Trichodesmium and their epibionts?

Question 4 - What is the role of coordination and quorum sensing in affecting physiological phosphorus stress in Trichodesmium and their rates of N₂ fixation?