TAUrin in Marine Ecosystems: Release and Prokaryotic USe (TAURUS)

The sources of carbon and energy sustaining microbial heterotrophic activity in marine ecosystems are not yet fully characterized. Amino acids and carbohydrates, mostly released by phytoplankton, have been frequently studied, while other compounds that also released in copious amounts by organisms into the ambient water, such as taurine, have been rarely studied. This research proposal represents a synergistic approach to characterize the taurine pathways in marine ecosystems, from its release by marine metazoans and its availability in different ecosystems to its utilization by prokaryotes. The taurine concentrations in marine systems will be assessed by well-established chemical methods (HPLC), which allow distinguishing taurine from amino acids. Taurine release by marine animals, especially crustaceous zooplankton, will be assessed via the increase in the concentration of taurine over time in incubation experiments with the zooplankton incubated in 0.2m filtered seawater hence, in the absence of prokaryotes. The determination of taurine concentrations in different ecosystems (coastal vs. open ocean, surface vs. deep waters) and during different seasons, together with the assessment of diel variations in the release rates by zooplankton will allow us characterizing the availability of taurine to microorganisms in its spatio-temporal variability. The potential utilization of taurine by members of the prokaryotic community will be assessed via molecular methods. The assessment of genes implicated in taurine uptake and degradation will be explored using a metagenomic approach in combination with single amplified genomes to determine the potential utilization of taurine, while a metaproteomics approach will be used to determine the fraction of the prokaryotic community actually exhibiting metabolic pathways for taurine. Additionally, the abundance of taurine-related gene and transcript abundance will be determined by q-PCR over different seasons in the coastal North Adriatic and different depth layers in the North Atlantic. This q-PCR approach will provide information on the biogeographic distribution of the presence and expression of this particular function. Taurine can provide energy, carbon, nitrogen and sulfur. Thus, it might play an important role in the biogeochemical cycles of these different compounds. Bulk uptake of taurine will be assessed by traditional methods (uptake of 14C-labeled taurine), while the differential use of the moieties of taurine by different prokaryotes will be assessed by micro-CARD-FISH and nanoSIMS, which allow quantifying the uptake of compounds by prokaryotes as well as to assess the phylogeny of the organisms taking up taurine. Respiration rates of taurine and its use as an additional energy source, e.g. for ammonia oxidation by Thaumarchaeota, will be investigated in specific experiments. Taken together, the information obtained by the different methods will provide a holistic view on the role of taurine in marine ecosystems and in the biogeochemical cycles of the ocean.

The ocean is inhabited by numerous organisms, many of them microscopic, such as the prokaryotes (single celled organisms without organelles in contrast to eukaryotes). Prokaryotes are not only very abundant, but also very versatile organisms in the types of substrates they can utilize, and thus, they play a major role in the cycles of elements on earth, including the carbon cycle linking to climate change. However, characterizing the chemical compounds that prokaryotes utilize from the multitude of compounds available in the ocean is not an easy matter, specially in the largely unexplored deep ocean waters. Taurine is an abundant amino acid like substance, present in animals including humans, particularly brain and heart, and frequently added to energy drinks. In the ocean, sea animals, such as fish, molluscs, or corals, as well as freely floating algal cells known as phytoplankton, produce taurine which they utilize for cell signalling, antioxidant, etc. However, a part of it is not utilized and it is released to the ambient water. Considering that taurine consists of sulfur, nitrogen and carbon, it is a valuable substrate for prokaryotes. Yet, its concentration and potential use by microorganisms, and its production by abundant sea organisms, had not been evaluated. In this project, we tried to assess the potential role of taurine in the open ocean and the seasonal dynamics in the coastal Adriatic sea. The release by small pelagic animals, zooplankton, and other benthic animals, such as mussels, amounted up to 9.5 mol g-1 C biomass h-1 in some zooplankton species. Zooplankton are the most abundant animals in open oceans, and they are known to migrate vertically during the day, from surface waters in the night, where they prey on phytoplankton, to deep waters (>1000 m depth) during the day, where they hide from their predators. Thus, they can supply taurine and other compounds to prokaryotes living in these deep waters, contributing to the so-called biological carbon pump that transfers carbon from the atmosphere to the deep ocean. However, due to the utilization by prokaryotes, taurine does not accumulate in the water, with maximum concentration in surface and coastal waters of ~15 nM and minimum concentration in the deep ocean of 0.2 nM. Our study indicates the important role of taurine in the global ocean, mediated by prokaryotes and contributes to our knowledge on the global marine carbon cycle.