In the face of the climate crisis, the ocean’s role as a carbon reservoir is coming under increasing scrutiny. In particular, the ‘biological carbon pump’ process, which consists of trapping carbon dioxide (CO₂) in the ocean depths, is at the heart of recent scientific research. A study by the Tasman Sea Institute and the Institut of Oceanography of Villefranche ( Sorbonne University) explores the little-known role of microbes in this process. Using an innovative device, C-RESPIRE, the researchers were able to analyse in detail the microbial activity responsible for carbon degradation, revealing the importance of these organisms in regulating deep-sea carbon.
by Laurie Henry
Once particles of organic carbon have fallen to the depths, they are partially degraded by microscopic organisms. The study by a team from the University of Tasmania and the Institute of Oceanography of Villefranche (Sorbonne University) sheds light on the mechanisms of this degradation by analysing the role of microbes in attenuating the flow of particulate carbon. Using C-RESPIRE, they were able to examine microbial processes at various depths, revealing marked regional and environmental disparities in the degradation of oceanic carbon.
The biological carbon pump, a key climate regulation mechanism
Every year, between 5 and 10 gigatonnes of carbon are captured by the oceans, reducing the concentration of CO2 in the atmosphere. In the euphotic zone, the upper layer of the ocean where light penetrates, marine organisms such as phytoplankton use sunlight to photosynthesise and produce organic matter. When this organic matter dies or decomposes, it forms carbon particles that sink to the depths, carrying the carbon away from the atmosphere.
Once the organic carbon particles have fallen to the depths, they are partially degraded by microscopic organisms. This transfer of carbon to the seabed, known as particulate organic carbon (POC) flux, is essential for long-term carbon storage. However, some of this carbon is degraded en route by marine organisms and various biological and physical processes, reducing the efficiency of the biological pump.
The research team, led by M. Bressac, focused on the factors that influence this attenuation of carbon fluxes, particularly at intermediate depths. In this new study, published in the journal Nature, they sought to gain a better understanding of the interactions between zooplankton and microbes, two key players in this degradation process. Unlike traditional models, such as the ‘Martin curve’ of the 1980s, which describes the attenuation of carbon flux with depth without including the role of microbes, this new study proposes an enriched approach, integrating microbial influences to refine predictive models of ocean carbon sequestration.
New technology to study microbes in depth
To study the role of microbes in the degradation of organic carbon at depth, the researchers used a novel device: the C-RESPIRE. This collector-incubator, designed specifically to observe microbial processes under natural conditions, captures and monitors the behaviour of organic particles that sink into the intermediate layers of the oceans, the ‘mesopelagic zone’.
Deployed in six highly contrasting ocean regions, from the nutrient-rich waters of the sub-Antarctic to the nutrient-poor waters of the subtropical gyre in the South Pacific, C-RESPIRE has been used to measure variations in microbial activity according to environmental characteristics such as nutrient availability and temperature.
Deployment of a drifting mooring line comprising a C-RESPIRE trap/incubator (top) and a sediment trap (bottom) © Hubert BatailleC-RESPIRE works by intercepting sinking organic carbon particles and confining them in incubation chambers that reproduce the pressure and temperature conditions at depth.
In this controlled environment, the device measures oxygen consumption, a key indicator of microbial activity, for extended periods ranging from a few days to several days. This monitoring makes it possible to calculate the rate of microbial remineralisation, i.e. the conversion of organic matter into dissolved inorganic carbon by the microbes, and to quantify the contribution of this degradation to the overall reduction in carbon flux.
This innovative methodology thus provides precise, in situ data on the mechanisms that contribute to carbon recycling in the oceans, in particular the relative share of microbial activity compared with that of other organisms such as zooplankton.
Contrasting and instructive results
Experiments conducted with C-RESPIRE show that microbial degradation of organic carbon varies considerably between ocean regions, with microbial contributions ranging from 7 to 29% of the total attenuation of particulate organic carbon (POC) flux. This rate depends largely on local conditions, and highlights the marked regional complexity of carbon sequestration mechanisms. In nutrient-rich areas close to Antarctica, the microbial contribution is relatively low. Conversely, in the nutrient-poor waters of the South Pacific, microbes are responsible for a significant proportion of POC degradation. These variations suggest that microbes adapt to local conditions, their activity being more intense when resources are scarce, particularly in oligotrophic regions.
Study sampling zones. © M. Bressac et al., 2024
In addition, the researchers observed that microbial degradation is influenced by temperature, but to varying degrees depending on the region. In tropical areas, where temperatures drop rapidly with depth, microbial activity also decreases, suggesting that microbes are highly sensitive to temperature. On the other hand, in temperate and cold regions where temperature gradients are less pronounced, microbial activity remains stable despite temperature variations.
Towards a revision of biological carbon pump models
The results thus highlight the fact that the regional and vertical variability of the microbial contribution means that models of the biological carbon pump need to be revised.
The authors stress the importance of incorporating regional characteristics such as temperature, particle chemistry and microbial diversity into the models. Indeed, microbial degradation is not solely influenced by temperature, as has long been assumed. It also depends on the specific characteristics of the organic carbon particles and the communities of microbes that colonise them. These regional differences and the adaptations of microbes to local conditions have a profound influence on the attenuation of POC flux. This is why a one-size-fits-all approach to sequestration models does not reflect the complexity of the biological and chemical interactions that affect deep carbon flux.
a. Microbes attached to particles undergo temperature variations (SPSG: high, MED: low) as they descend. b. Falling particles vary biochemically, influencing respiratory quotient (RQ). c. Thermal sensitivity of carbon utilisation depends on strain and substrate (arrow b to c). d. In the mesopelagic zone, zooplankton and disaggregation fragment the aggregates, encouraging microbial colonisation and remineralisation. © M. Bressac et al., 2024To refine our understanding of these mechanisms, the team proposes to deconstruct the ‘Martin curve’ into specific factors. By isolating parameters such as microbial degradation and particle fragmentation, they believe that we can ‘ better understand the underlying mechanisms of microbial degradation, thus offering a finer perspective on the global carbon cycle in the oceans ’.
This approach makes it possible to explore more precisely the role of microbes and zooplankton in attenuating carbon flux. By analysing these distinct contributions, the researchers aim to improve predictive models to better assess the impact of the oceans on future climate regulation, taking into account the specific characteristics of each marine ecosystem.
Source : Bressac, M., Laurenceau-Cornec, E. C., Kennedy, F. et al. “Decoding drivers of carbon flux attenuation in the oceanic biological pump”. Nature 633, 587–593 (2024).