Royal Netherlands Institute for Sea Research
Royal Netherlands
Institute for Sea Research
Phone number
+31 (0)222 36 9455
Ocean Systems (OCS)
Senior Scientist
  • Marine (bio)geochemistry
  • Nutrient cycling and redox chemistry
  • Past and present ocean deoxygenation
  • Micro-scale element and mineral characterization
  • Sampling under oxygen-free conditions
Read the interview with Peter Kraal in the NIOZ Annual Report 2020

Dr. Peter Kraal

Senior Scientist

‘There are critical gaps in our understanding of the phosphorus cycle’

Marine biogeochemist Peter Kraal studies the cycling of essential nutrients in the oceans. ‘My favourite element is phosphorus (P). This is a particularly relevant nutrient, because we are rapidly exhausting the world’s usable supplies only to eventually discharge it in diluted form into the seas and oceans, where it can cause huge environmental problems. In this sense, human activities have effectively “broken” the natural P cycle.’

Anoxic – breathless - sea

‘Phosphorus can give such a strong boost to algal growth in certain areas that the oxygen demand for breaking down all the dead and rotting algae is larger than the oxygen supply. Under the anoxic conditions that develop, P released from rotting algae is retained in the seafloor much less efficiently but rather recycled back into the water column to further boost algal growth. This process is a vicious cycle, which has given rise to prolonged periods of oxygen loss in the past. It still occurs in coastal seas such as the Baltic Sea, where anoxic ‘dead zones’ and harmful algal blooms are perpetuated by P recycled from the sediment, while the influx of phosphorus from agriculture to the surface water has decreased dramatically.’

Role for iron

‘The binding of phosphorus to other molecules is one of the keys in my research; it controls the mobility of phosphorus in aqueous ecosystems. Iron (Fe) minerals play an important role in P absorption and thereby in controlling P bio-availability in natural systems. Furthermore, formation of Fe-P phases is an important process for removing excess P in waste water treatment technology. Reusing P isolated from waste waters as fertilizer is an important component in research aiming to “repair” the P cycle.

Acidified cycle

‘I am currently also investigating the interactions between P and calcium carbonate (CaCO3). Though less reactive than Fe minerals, CaCO3 can also play an important role in binding P and this role is affected by human-induced environmental change such as ocean acidification and deoxygenation.  We do not yet fully understand the consequences of this. Furthermore, the oceanic P cycle is intimately coupled to the cycles of numerous other essential elements. I hope that with my research, I can contribute to a better understanding of nutrient cycling in the oceans and to solving environmental problems such as anoxic coastal seas.’

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Research Interests

I am a marine geochemist investigating the interplay between oxygen availability and the biogeochemical cycling of essential elements such as iron and phosphorus in marine waters and sediments. Globally increasing eutrophication and marine oxygen depletion, often caused or exacerbated by human activities, gets me worried but also scientifically excited.

Moreover, I have developed a strong interest in analytical techniques that push our understanding of this research field forward, such as X-ray absorption spectroscopy for particle characterization and polymer-based passive sampling techniques for high-resolution, two-dimensional porewater analysis.

To understand past and future impacts of marine deoxygenation I investigate the mineralogy, chemical composition and fluxes in modern surface sediments as well as the chemical signature of oxygen depletion in historical sediment records. A particular interest of mine is the impact of (changing) chemical conditions on the stability of authigenic minerals.

Sampling sediments from the Black Sea under oxygen-free conditions

Education and Experience

2015 - 2017 NWO-Veni postdoctoral fellow at Utrecht University - Department of Earth Sciences-Geochemistry; Iron mineral transformations and preservation: Effects of eutrophication-driven changes in sediment geochemistry

2013 - 2014 Postdoctoral researcher at Utrecht University - Department of Earth Sciences-Geochemistry; Coupled cycling of iron, phosphorus and manganese in the Black Sea

2011 - 2013 Postdoctoral researcher at Southern Cross University - Geosciences; Hyperaccumulations of monosulfidic sediments: Exploring a biogeochemical extreme to resolve fundamental sulfur biomineralisation pathways

2006  -2010 PhD student at Utrecht University - Department of Earth Sciences-Geochemistry; Redox-dependent phosphorus burial in modern and ancient marine sediments

Research expeditions

I have been involved in various research cruises, during which I collected and processed (anoxic) water column and sediment samples for on-board and on-shore analyses.

February 2019 | Chief scientist on the FUNAMOX (Biogeochemical functioning in Namibian oxygen-depleted waters) expedition with RV Pelagia to the Namibian shelf and slope, sampling water column and sediments to understand the impact of oxygen depletion on the biogeochemical cycling of essential elements such as N, P and trace metals.

July 2018 | Co-chief scientist on NICO Leg 12 to the mid-Atlantic ridge with RV Pelagia to sample waters and sediments around the Rainbow hydrothermal vent field

September 2015 | "Fe Vici" cruise to the Black Sea with RV Pelagia, along a gradient from the oxic shelf to the sulfidic deep basin

September 2014 | Short cruise with RV Mare Nigrum to the deep northwestern Black Sea to collect sediment traps

June 2013 | "PhOxy" cruise to the Black Sea with RV Pelagia, along a gradient from the oxic shelf to the sulfidic deep basin

2011 - 2012 | Various samping trips to the shallow Peel-Harvey Estuary in Western Australia

May/June 2009 | "HYPER" cruise to the Baltic Sea with r/v Aranda to the Baltic Sea

August/September 2007 | Cruise to the Baltic Sea with r/v Skagerrak



Kraal, P., van Genuchten, C.M., Behrends, T., 2022. Phosphate coprecipitation affects reactivity of iron (oxyhydr)oxides towards dissolved iron and sulfide. Geochimica et Cosmochimica Acta (DOI link).

van Kemenade, Z.R., Villanueva, L., Hopmans, E.C., Kraal, P., Witte, H.J., Sinninghe Damsté, J.S., Rush, D., 2022. Bacteriohopanetetrol-x: constraining its application as a lipid biomarker for marine anammox using the water column oxygen gradient of the Benguela upwelling system. Biogeosciences, 19(1): 201-221 (DOI link).

He, Z., Clarkson, M.O., Andersen, M.B., Archer, C., Sweere, T.C., Kraal, P., Guthauser, A., Huang, F., Vance, D., 2021. Temporally and spatially dynamic redox conditions on an upwelling margin: The impact on coupled sedimentary Mo and U isotope systematics, and implications for the Mo-U paleoredox proxy. Geochimica et Cosmochimica Acta, 309: 251-271 (DOI link).

Kraal, P., van Genuchten, C.M., Lenstra, W.K., Behrends, T., 2020. Coprecipitation of phosphate and silicate affects environmental iron (oxyhydr)oxide transformations: A gel-based diffusive sampler approach. Environmental Science & Technology (DOI link).

Kraal, P., van Genuchten, C.M., Behrends, T., Rose, A.L., 2019. Sorption of phosphate and silicate alters dissolution kinetics of poorly crystalline iron (oxyhydr)oxide. Chemosphere 234, 690-701 (DOI link).

Kraal, P., Yücel, M., Slomp, C.P., 2019. Turbidite deposition and diagenesis in the southwestern Black Sea: Implications for biogeochemical cycling in an anoxic basin. Marine Chemistry, 209: 48-61 (DOI link)

Schoepfer, V.A., Burton, E.D., Johnston, S.G., Kraal, P., 2019. Phosphate loading alters schwertmannite transformation rates and pathways during microbial reduction. Science of The Total Environment, 657: 770-780 (DOI link)

Lenstra, W.K., Hermans, M., Séguret, M.J.M., Witbaard, R., Behrends, T., Dijkstra, N., van Helmond, N.A.G.M., Kraal, P., Laan, P., Rijkenberg, M.J.A., Severmann, S., Teacǎ, A., Slomp, C.P., 2019. The shelf-to-basin iron shuttle in the Black Sea revisited. Chemical Geology, 511: 314-341 (DOI link).

van Genuchten, C.M., Behrends, T., Kraal, P., Stipp, S.L.S., Dideriksen, K., 2018. Controls on the formation of Fe(II,III) (hydr)oxides by Fe(0) electrolysis. Electrochimica Acta, 286: 324-338 (DOI link).

Dijkstra, N., Kraal, P., Séguret, M.J.M., Flores, M.R., Gonzalez, S., Rijkenberg, M.J.A., Slomp, C.P., 2018. Phosphorus dynamics in and below the redoxcline in the Black Sea and implications for phosphorus burial. Geochimica et Cosmochimica Acta, 222(Supplement C): 685-703 (DOI link).

Kraal, P., Dijkstra, N., Behrends, T., Slomp, C.P., 2017. Phosphorus burial in sediments of the sulfidic deep Black Sea: key roles for adsorption by calcium carbonate and apatite authigenesis. Geochimica et Cosmochimica Acta, 204: 140-158 (DOI link).

Schoepfer, V.A., Burton, E.D., Johnston, S.G., Kraal, P., 2017. Phosphate-imposed constraints on schwertmannite stability under reducing conditions. Environmental Science & Technology, 51(17): 9739-9746 (DOI link).

Egger, M., Kraal, P., Jilbert, T., Sulu-Gambari, F., Sapart, C.J., Röckmann, T., Slomp, C.P., 2016. Anaerobic oxidation of methane alters sediment records of sulfur, iron and phosphorus in the Black Sea. Biogeosciences, 13(18): 5333-5355 (DOI link).

Kraal, P., Bostick, B.C., Behrends, T., Reichart, G.-J., Slomp, C.P., 2015a. Characterization of phosphorus species in sediments from the Arabian Sea oxygen minimum zone: Combining sequential extractions and X-ray spectroscopy. Marine Chemistry, 168(0): 1-8 (DOI link).

Kraal, P., Burton, E.D., Rose, A.L., Kocar, B.D., Lockhart, R.S., Grice, K., Bush, R.T., Tan, E., Webb, S.M., 2015b. Sedimentary iron–phosphorus cycling under contrasting redox conditions in a eutrophic estuary. Chemical Geology, 392(0): 19-31 (DOI link).

Chen, A.P., Berounsky, V.M., Chan, M.K., Blackford, M.G., Cady, C., Moskowitz, B.M., Kraal, P., Lima, E.A., Kopp, R.E., Lumpkin, G.R., Weiss, B.P., Hesse, P., Vella, N.G.F., 2014. Magnetic properties of uncultivated magnetotactic bacteria and their contribution to a stratified estuary iron cycle. Nat Commun, 5 (DOI link).

Dijkstra, N., Kraal, P., Kuypers, M.M.M., Schnetger, B., Slomp, C.P., 2014. Are iron-phosphate minerals a sink for phosphorus in anoxic Black Sea sediments? PLoS ONE, 9(7): e101139 (DOI link).

Kraal, P., Slomp, C.P., 2014. Rapid and extensive alteration of phosphorus speciation during oxic storage of wet sediment samples. PLoS ONE, 9(5): e96859 (DOI link).

Burton, E.D., Johnston, S.G., Kraal, P., Bush, R.T., Claff, S., 2013. Sulfate availability drives divergent evolution of arsenic speciation duringmicrobially mediated reductive transformation of schwertmannite. Environmental Science & Technology, 47(5): 2221-2229 (DOI link).

Kraal, P., Burton, E.D., Bush, R.T., 2013a. Iron monosulfide accumulation and pyrite formation in eutrophic estuarine sediments. Geochimica et Cosmochimica Acta, 122(0): 75-88 (DOI link).

Kraal, P., Burton, E.D., Rose, A.L., Cheetham, M.D., Bush, R.T., Sullivan, L.A., 2013b. Decoupling between water column oxygenation and benthic phosphate dynamics in a shallow eutrophic estuary. Environmental Science & Technology, 47: 3114-3121 (DOI link).

Lockhart, R.S., Berwick, L.J., Greenwood, P., Grice, K., Kraal, P., Bush, R., 2013. Analytical pyrolysis for determining the molecular composition of contemporary monosulfidic black ooze. Journal of Analytical and Applied Pyrolysis, 104(0): 640-652 (DOI link).

Kaal, J., Nierop, K.G.J., Kraal, P., Preston, C.M., 2012. A first step towards identification of tannin-derived black carbon: Conventional pyrolysis (Py–GC–MS) and thermally assisted hydrolysis and methylation (THM–GC–MS) of charred condensed tannins. Organic Geochemistry, 47(0): 99-108 (DOI link).

Kraal, P., Slomp, C.P., Reed, D.C., Reichart, G.J., Poulton, S.W., 2012. Sedimentary phosphorus and iron cycling in and below the oxygen minimum zone of the northern Arabian Sea. Biogeosciences, 9(7): 2603-2624 (DOI link).

Kraal, P., Slomp, C.P., de Lange, G.J., 2010a. Sedimentary organic carbon to phosphorus ratios as a redox proxy in Quaternary records from the Mediterranean. Chemical Geology, 277(1-2): 167-177 (DOI link).

Kraal, P., Slomp, C.P., Forster, A., Kuypers, M.M.M., 2010b. Phosphorus cycling from the margin to abyssal depths in the proto-Atlantic during oceanic anoxic event 2. Palaeogeography, Palaeoclimatology, Palaeoecology, 295(1-2): 42-54 (DOI link).

Kraal, P., Nierop, K.G.J., Kaal, J., Tietema, A., 2009a. Carbon respiration and nitrogen dynamics in Corsican pine litter amended with aluminium and tannins. Soil Biology and Biochemistry, 41(11): 2318-2327 (DOI link).

Kraal, P., Slomp, C.P., Forster, A., Kuypers, M.M.M., Sluijs, A., 2009b. Pyrite oxidation during sample storage determines phosphorus fractionation in carbonate-poor anoxic sediments. Geochimica et Cosmochimica Acta, 73(11): 3277-3290 (DOI link).

Kraal, P., Jansen, B., Nierop, K.G., Verstraten, J.M., 2006. Copper complexation by tannic acid in aqueous solution. Chemosphere, 65(11): 2193-8 (DOI link).

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Linked projects

UUNIOZ_The role of Marine Flavobacteria
Pierre Offre
Utrecht University
Project duration
1 Jan 2021 - 31 Dec 2025
Peter Kraal
Utrecht University
Project duration
1 Jan 2021 - 31 Dec 2025