“It's one big ocean and it covers 70% of the planet, so it should be “planet Ocean” rather than planet Earth.”
And Planet Ocean has been at the heart of Professor Peter Croot's (FRSC) work, contributing to clarifying the origin of the organic matter in the oceans. Professor Peter Croot FRSC, a Marine Biogeochemist at the University of Galway with a background in analytical and environmental chemistry, has dedicated his career to the marine environment, predominantly working in the open ocean. Prof. Croot’s global research has taken him to the Southern Ocean and Antarctica, the Atlantic Ocean, the Indian Ocean, and the Pacific, with a focus on biogeochemical cycles, including the cycling of carbon and metals, and their speciation: the determination of the chemical forms with which these elements are present, and the kinetics of interchange between one form and another. All these are strictly connected with what limits primary productivity in the ocean and the stresses the environment is affected by, such as climate change, global warming, ocean acidification, and ocean deoxygenation.
“Primary productivity is the uptake of CO2 into organic matter by phytoplankton. As this could be limited by light, we also make bio-optical measurements. But it could also be limited by nitrogen (N), phosphorus (P) and/or iron (Fe), so a lot of my research looks at what exactly is controlling primary productivity. This involves overcoming analytical challenges in measuring the different components, to establish if they're important for primary productivity or not. This also raises many analytical questions about the cycling of trace metals, and the cycling of carbon and nitrogen. These might not directly lead to changes in primary productivity, but it is still important in understanding the cycling of the elements and the inputs from the land to the ocean. 1"
Metrology forms a large part of Prof. Croot’s research with measurements approached from both an experimental and theoretical angle, exploring what existing analytical techniques are available, improving them where required, and developing new ones. All this feeds into an overall contribution to knowledge and society.
“In terms of climate change, the work that we are doing provides good baseline data to compare going forward, and there are numerous topics in and around that, such as ocean acidification, global warming, and ocean deoxygenation. Our work contributes new evidence about the impacts of stresses on the environment and this can be fed back, for example, into the Intergovernmental Panel on Climate Change (IPCC) reports on a global level, or, on a smaller scale, into regional assessments, informing policy, adaptation, and mitigation measures 1.
Another contribution is about pollutants, such as plastic, and how this is changing and impacting biogeochemical cycles. This not only provides data that can be used in an academic setting, but this research might also show that there could be economic, societal, or cultural benefits for maintaining blue and green oceans. Further, in the last 20 years, there has been more of a focus on the environmental field, addressing the life cycles of all sorts of chemicals and how they fit into natural and anthropogenic biogeochemical cycles. We are now seeing steps towards an international plastics treaty and a chemical equivalent to the IPCC. These are big, unprecedented changes in the chemical aspect of things. Finally, there has been more recognition regarding monitoring. Often this was seen as the lesser part of doing research, why would you be interested in doing lots of repeated measurements? But now we know there is incredible value in a long-term series of measurements. It’s a fundamental shift.”
Prof. Croot’s work is also contributing to clarifying the origin of the organic matter in the oceans. There are two major theories, one that supports the idea that all the humic components are the result of in-situ phytoplankton or zooplankton activity, and the other for which all the humic material has terrestrial origin and is then diluted into the ocean and destroyed through photochemistry on the surface.
“Something we look at, as a diagnostic marker, are the proteins tyrosine and tryptophan. They give us indicators of zooplankton activities and show that many proteins have come from grazing, giving us an idea of where organisms are actively being eaten. These waste products in the coastal ocean might indicate these processes, as well as what is being supplied by rivers, such as runoff from agriculture 2."
Figure 1. Aqualog in an onboard laboratory on the RV Sonne in the South Pacific in Dec 2015
Identifying the right tools for the job is a fundamental part of any research and A-TEEM (Absorbance Transmission Excitation-Emission Matrix) fluorescence spectroscopy and the Aqualog has proved to be the most appropriate technique and instrument for gathering data.
“The Aqualog has been a wonderful instrument. We gradually moved from doing a simple single emission spectrum to measure aluminium to a lot of fluorescence excitation-emission matrices (EEMs), finding the Aqualog to be more sensitive, and performing analysis faster than the fluorometer we had at that time. This was a big incentive. We have taken it out to the South Pacific where you will find optically the clearest water in the world. There is almost no signal for the fluorescence in the surface waters, having been bleached by the sun, but when you go deeper, you find more organic matter. The Aqualog has worked really well for this.
Figure 2. CTD-Rosette sampler being deployed in the ultra blue waters of the South Pacific Gyre in Dec 2015
“We do a lot of EEMs with Parallel Factors Analysis (PARAFAC) to look at the organic components in seawater. We gradually moved closer to shore, working our way up through the estuary and, as part of that, we eventually started working on peat bogs. The Aqualog had to go from counting with a very long integration time, to measure virtually nothing in the South Pacific, to us having to dilute samples because they are the color of black tea. Here we are trying to identify different types of organic matter, the humics or proteins, to see if we can follow those from peat bogs, through the river system, and then out into the ocean. This will give us clues about the carbon cycling that feeds into everything else.
It's still relatively unique that the Aqualog captures absorbance and fluorescence at the same time. This has taken us back to the original reason why we were using fluorescence, to look at elements’ speciation. One of the issues with this is that in river water and peat bogs, there is a lot of aluminium, which is toxic for some organisms. Aluminum is mostly removed as it mixes with the ocean. We have been doing kinetic work with the Aqualog to understand how fast the aluminum is transferred from one chemical species to another in river water or the estuaries.”
The 3-in-1 measurement capability of the Aqualog, fluorescence, and absorbance, with EEMs has enabled Prof. Croot to detect both the fluorescent and non-fluorescent complexes resulting from the lumogallion method for the detection of different aluminium species.
“We can use this information to look at the abundance of both those species as well as correct for the natural fluorescence from the organic matter. This is one of the big advantages of using the Aqualog; it lets you do what you can’t do with a traditional fluorometer. It lets you follow the whole story, he said.”
But with the Aqualog Prof. Croot can investigate much more and carry out all sorts of fluorescence assays: it has been used to look at reactive oxygen species and how this can alter the fluorescence of organic matter.
“We haven’t found anything else that will enable us to do all these things. Historically, we would have had two systems set up to measure in parallel.”
When working in such diverse research settings, from the crystal clear South Pacific waters to the Irish peat bogs, the Aqualog sensitivity and dynamic range and the ability to apply the inner filter effect (IFE) correction become crucial, reducing the need to manipulate the sample too much.
In light of evolving applications and methodologies, updating research techniques can broaden the potential for improved results, knowledge, and understanding. More and more groups are performing EEMs, which is an easy measurement to do and that provides information about the DOM in the water that can be related to different water masses.
“Even if it does not provide individual components, it is a big improvement on how we can categorise DOM. It’s a trade-off - you could get a lot of data using an Aqualog or using very expensive and time-consuming techniques to get the individual components, which would mean you gather less data. A lot of the time it is preferable to get as much spatial and temporal coverage as possible with the Aqualog than focus on identifying all the individual components that are contributing to the signal. There are a few labs that are doing it, and they are comparing them to what the Aqualog returns so that we can begin to draw more information out of the 3D fluorescence data.
I think that the more it will become commonplace in both the marine and in the freshwater research, the more it can be used as a diagnostic tool for people that are managing water quality of marine water and fresh water.”
According to Prof. Croot, this would apply to cases such as leakages from septic tanks or stormwater overflow/wastewater overflow, looking at impacts of how extreme weather events might have an influence on other aspects of the carbon cycle, but also removal techniques of organics in local water schemes1. But this novel technique also allows us to update some of the historical data and past research, improving our understanding of what is really going on.
“When you look at some of the older research papers, you know it was obviously measured on one machine and then another; it wasn't simultaneous. There is a lot of clever interpretation going on but there isn’t a lot of data.”
Prof. Croot and his team have gathered valuable data and contributed significantly to understanding biogeochemical cycles, speciation, and primary productivity in the ocean. The burning question now is, what’s next? Alongside continuing the work on Colored Dissolved Organic Matter (CDOM) and extending this to lake rivers and coastal water, Prof. Croot has recently started to work on photosynthetic pigments.
“Recently we have been using the Aqualog to look at photosynthetic pigments, extracting and measuring chlorophyll and other accessory pigments with both absorbance or fluorescence spectroscopy. The nice thing with the Aqualog is that you can do both and compare whether the two analytical measurements are giving you consistent results.”
Measuring the pigments’ contribution helps to obtain the so-called “optical closure”. In fact, the absorbance of each sample is the result of the contribution of the CDOM from the water, the pigments from phytoplankton and the inorganic components.
“Assigning how much light is absorbed and its backscatter throughout the water column is a really useful measure to have when looking at primary productivity.”
1 O’Driscoll, Connie, et al. "Tracing sources of natural organic matter, trihalomethanes and metals in groundwater from a karst region." Environmental Science and Pollution Research 27 (2020): 12587-12600.
2 Shi, Lin, et al. "Nutrient recovery from pig manure digestate using electrodialysis reversal: Membrane fouling and feasibility of long-term operation." Journal of Membrane Science 573 (2019): 560-569.https://doi.org/10.1016/j.memsci.2018.12.037.
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