CORDIS - Forschungsergebnisse der EU

Biological Understanding of the CO2 and O2 LeveL in the ocEan

Periodic Reporting for period 1 - BULLE (Biological Understanding of the CO2 and O2 LeveL in the ocEan)

Berichtszeitraum: 2020-12-01 bis 2022-11-30

• What is the problem/issue being addressed?

The problem being addressed revolves around accurately estimating net community production (NCP) and understanding its implications for the ocean's carbon cycle and climate change. Bacterial respiration is a crucial process in the ocean's carbon cycle, contributing up to 90% of oceanic respiration and playing a significant role in nutrient recycling and CO2 production. However, current methodologies for measuring bacterial respiration introduce uncertainties that can lead to imbalances in carbon flux budgets. The use of conversion factors, particularly the respiratory quotient (RQ), to convert O2 consumption to CO2 production is controversial and lacks environmental regulation data. This uncertainty hinders our understanding of the ocean's capacity to sequester CO2. Additionally, the hypothesis of alternative metabolic pathways reducing RQ under Fe-limited conditions (Fourquez et al., 2014) raises questions about current estimates of CO2 produced by marine bacteria in vast oceanic areas. Therefore, accurately determining RQ and its variability in different environmental conditions is crucial for comprehending the role of biological processes in the global carbon cycle.

• Why is it important for society?
This research topic is important for society because it addresses the crucial issue of the ocean's carbon cycle and its implications for global climate change. Improving the accuracy of estimates of bacterial respiration is essential for accurately determining the ocean's capacity to sequester atmospheric CO2 and to better understand the role of biological processes in the global carbon cycle. This research has important implications for policymakers, scientists, and the public, as it can inform strategies for mitigating the effects of climate change and promote sustainable environmental practices.

• What are the overall objectives?
The overall objectives of this research project are to measure bacterial respiration, both as O2 consumption and CO2 production, at a cellular level, and to determine the factors that control RQ in situ by developing a new method. The main goal of the project is to gain a better understanding of the environmental conditions that drive RQ. Among the wide range of parameters that could potentially affect this coefficient, my hypothesis is that the metabolic pathway taken by carbon within a cell, in relation to stoichiometry and nutrient bioavailability, is a key factor.
The work performed during the BULLE's project encompasses methodology development, controlled-lab experiments, and fieldwork. Overall, I validated the use of membrane inlet mass spectrometry in measuring RQ in marine bacterial cultures. I demonstrated that RQ changes in response to Fe-limitation in the lab.
At the global scale, the Southern Ocean represents the largest Fe-limited region and is also the largest oceanic region for CO2 sequestration. Therefore, any carbon budgets established for the Southern Ocean are critical in climate models.
As part of the BULLE project, I aimed to determine the RQ in natural bacterial assemblages during the SWINGS (South West Indian Geotraces Section) expedition in the Indian Sector of the Southern Ocean, using a combination of cutting-edge methods with stable isotopes (e.g. NanoSIMS). The results confirmed that RQ varies from site to site, which ultimately influences further net community productivity calculations.
At the start of the project, it was only hypothesized that Fe limitation and metabolic plasticity in general may influence the respiratory quotient (RQ) during bacterial respiration (Fourquez et al. 2014). The work conducted thus far has confirmed this hypothesis: Fe limitation decreases the RQ. This result has significant implications, as Fe is the limiting element for growth in the Southern Ocean, which represents 40% of all oceanic uptake of anthropogenic CO2. Therefore, this discovery directly impacts estimates of net primary production in this climatically important region, which is the difference between the amount of CO2 uptake during photosynthesis and the amount of CO2 released during respiration.

Advanced technologies were combined to support the validity of this hypothesis. At the cellular level, membrane inlet mass spectrometry (MIMS) enabled continuous and simultaneous recording of CO2 and O2 levels in a controlled experiment. This, combined with pH and alkalinity measurements, allowed for the derivation of total dissolved inorganic carbon produced during cellular respiration. At the community level, the deployment of MIMS technology to measure bacterial respiration is challenging. This is due to the lower density and activity of bacteria in natural environments compared to laboratory cultures. Although the technology itself is sensitive enough to measure respiration in these environments, the experimental setup requires no exchange with the atmosphere, which can rapidly lead to anoxic conditions before any measurement can be made. During the project, I conducted numerous tests, including the use of new pCO2 sensors, to measure RQ directly in the field. I discovered a promising methodology that involved using stable isotopes (specifically 13C) to track the journey of an organic molecule, glucose, into (by NanoSIMS technology) and out (by Gasbench mass spectrometry) of the cell. This new approach to addressing the question of RQ in situ confirmed the results obtained in the lab, showing that RQ varied and was lower in areas where iron (Fe) was limited.

Overall, the results will help to implement the carbon budget at a large scale and, importantly, to reconsider the parameters used in modeling to define bacterial respiration. Nowadays, having an accurate estimate of the ocean's capacity to sequester CO2 is of fundamental importance at both the scientific and societal levels.
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