Micro-heterogeneity of carbon mineralization and metal mobilisation in marine sediments

Lead Research Organisation: Scottish Association For Marine Science
Department Name: Scottish Association For Marine Science

Abstract

The oceans are major sinks of carbon dioxide. It is removed from the atmosphere during photosynthesis by microscopic phytoplankton, which are grazed by zooplankton. As they die, a proportion of the plankton sink to the sediment where they encounter large microbial populations that actively decompose the organic material in a process known as mineralisation. Oxygen is consumed in this process so that its concentration within the sediment is virtually zero. Other oxidants, including iron and manganese oxides are then used. Iron and manganese are transformed into their soluble, reduced forms and simultaneously trace metals that were bound to the oxides are released. We have traditionally studied these processes by analysing the sediments in a series of vertical layers and considering the vertical transport and reactions of the various components. New techniques that can provide two dimensional (2-D) images of oxygen, carbon dioxide and pH have recently become available. They use a digital camera to record a fluorescent signal triggered by the measured component and are known as planar optodes. They have been used to show that mineralization is not uniformly distributed in a layer of sediment. Rather it occurs most efficiently at discrete, mm or sub-mm sized sites, known as microniches. They have also shown that the organisms within the sediment, such as specialised worms, introduce localised structure. Another new technique is diffusive gradients in thin-films (DGT), which measures metal mobilisation by accumulating the metal in a thin binding layer after it has passed through a layer of hydrogel (like a contact lens). Images of mobilised metals in 2-D obtained using DGT show that metals are mobilised at microniches. This new evidence indicates that this important part of the carbon cycle is not as simple as previously thought. Systematic and careful studies need to be undertaken to find out whether consideration of the processes on this smaller scale in three dimensions will affect our understanding of process rates and our ability to model and predict mineralisation and metal mobilisation. To do that we need to be able to study individual microniches, which means that we must be able to make all measurements at exactly the same location. We have undertaken preliminary work which has combined DGT and planar optodes. We wish to use this combined 2-D probe to investigate the relationship between mineralization and metal mobilisation. A systematic series of measurements will be made in experimental systems that replicate microniches and worm burrows. The results will be interpreted using a newly constructed model of transport and reaction occurring in three dimensions. We will then perform measurements using optodes and DGT directly in the field, using the new understanding gained from the laboratory studies to interpret the findings. The work will advance understanding of mineralisation and metal mobilisation and the linkages between the two, including quantification of rates occurring in a 3-D framework. It will inform studies of the global carbon cycle and of trace metal mobilisation. The latter relates to exchange of metals with the overlying water and the effect of microniches on mineral formation processes.

Publications

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Jovanovic Z (2015) Rhizosphere O2 dynamics in young Zostera marina and Ruppia maritima in Marine Ecology Progress Series

 
Description In short, we have developed and optimized novel sensing approaches for combined two-dimensional measurements of oxygen and pH in complex systems - like sediments. We have combined this with microbial sampling to investigate how bacteria and virus activity interrelate to mineralization of organic material and regeneration of nutrients. The work has demonstrated that marine sediments are far more dynamic than previously anticipated and that this has to be acknowledged if their biogeochemical functions are to be fully understood.
Exploitation Route Our findings have so far been presented in more than 15 peer-reviewed manuscripts, and several international conferences. Several international research teams have contacted us and received training in these new measuring approaches. This has also initiated new collaborations and research projects.
Sectors Environment

 
Description The work has been included in teaching activities on undergraduate and graduate level
First Year Of Impact 2009
Sector Education,Environment
Impact Types Societal

 
Description Danskernes Akademi 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Presentation entitled "The seabed; a microbial reactor with impact on the CO2 and O2 balance on earth" within the TV program "Danskernes Akademi" airing on the national Danish TV channel 2 (DR2). The presentations on the program aimed at informing the general public on globally relevant scientific topics, e.g. global warming, nutrient cycling and the associated research and research fields.
Year(s) Of Engagement Activity 2012
 
Description National History Museum 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact This was an arrangement in National History Museum. The lecture with the title "The importance of bacteria communities in the deep sea" (30min was meant for an audience of 150 high-school students. One of the objectives was to rise awareness of the role on bacteria communities in deep-sea habitats using example from past and ongoing research.
Year(s) Of Engagement Activity 2013