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NIOZ oceanographer Darci Rush about ‘the best cruise ever’

Organisms that consume methane are extremely important
On 24 May, the research vessel Pelagia docked at the NIOZ home base on Texel after an 8-month expedition.
NIOZ researcher Darci Rush (organic geochemist) went on board during the seventh leg of a 12-leg voyage. Aboard the Pelagia, she studied the behaviour of microorganisms in ocean areas with low oxygen levels.

Like many other scientists, Rush had put forward subjects for research on the expedition route of the Pelagia: the voyage was from Texel to Gran Canaria, across the Atlantic Ocean to the Caribbean area and then, via Ireland, the North Sea and the Azores, back to Texel. Rush managed to convince the organisers of the expedition to make a detour via the Gulf of Mexico. ‘That loop was not originally planned.’

Less oxygen and more methane
Why did Rush and her NIOZ colleague Sigrid van Grinsven want to go to the Gulf of Mexico? Rush: ‘The Mississippi Delta is characterised by oxygen-deficient areas; furthermore, many substances flow into the Gulf from the arable fields in Louisiana. My NIOZ research focuses on the nutrient cycle of microorganisms in a water column. Algae live from inflowing nutrients. When they die, the remnants sink and provide food for bacteria and microorganisms. As a result of this process, the oxygen level in the water just below the surface decreases. This is colloquially known as a dead-zone. I wanted to investigate whether the microorganisms in the Gulf of Mexico produce methane, and whether they use methane that is released from the seafloor for their metabolism. Little is known at present about this methane cycle. We didn’t manage to be in the area to catch the dead-zone (it usually forms, and is most intense, at the end of spring). However, we left behind a trap in the water column to catch sinking particles. Using the samples collected by the trap, we can then study the development of the dead-zone as it forms and terminates over time. This trap will be collected by a NIOZ employee mid-September.’

‘As a result of climate change, the oceans are warming up and contain less oxygen and more methane. Organisms that can consume methane are therefore becoming increasingly important.’ She describes the parallel with the Cretaceous and Jurassic periods when large parts of the oceans contained no oxygen and huge numbers of animal species like the dinosaurs became extinct. Rush: ‘In the sediment, we can find biomarkers of biogeochemical processes that occurred in the past. My goal is to identify biomarkers we can use to mark the influence of microorganisms, so that we can better predict what will happen in the future.’

Delays and the sampling of a brine pool
However, the expedition of Rush started with a hiccup. Rush: ‘Our group consisted of ten researchers: from NIOZ, University of Amsterdam and Utrecht University. When we came together at the embarking point on St Martin, we discovered that we would first have to wait five days for the research equipment that had been forwarded separately in advance. These days we slept on the boat, that was alternately in the harbour or anchored out at sea. This was a frustrating delay because the number of research days is limited. Nevertheless, nobody was grumpy and, in good consultation, we discussed what experiments could be changed so that everybody could still carry out valuable research.’

Besides this initial hiccup, Rush’s research on the Pelagia subsequently took a different course than planned. Measurements revealed that the water was not sufficiently oxygen-deficient. ‘There was no time to localise the zones without oxygen', she explains. Then Rush and Van Grinsven accidentally stumbled across a brine pool of 400 km² at a depth of 2 km below the sea surface. These brine pools occur elsewhere as well, but the conditions in these are rare: the extremely salty water has stood still for thousands of years, and consequently, there is a virgin closed system with its own biodiversity. In the samples from the brine pool, Rush hopes to find remnants of microorganisms that live on methane.

The best cruise ever
Recalling her expedition on the Pelagia, Rush simply states: ‘The best cruise ever. As we were delayed, we wanted to do research 24/7. This also meant that the support crew had to work much longer hours. Partly due to their efforts, the atmosphere on board was incredibly pleasant.’ Rush will hire a PhD student to work on the samples she took during the expedition and hopes to publish interesting research results soon. Furthermore she wants to take part in another expedition in February 2019: hopefully, the Pelagia will then cruise along the coast of Namibia where low-oxygen zones are also found.

Rush and NIOZ
The Canadian Rush (34) says that, after a master’s in oceanography at University Aix-Marseille II, she came to NIOZ to obtain her doctorate, even though she had never heard of Texel. After her doctorate, she did a postdoc in Newcastle. ‘There, I realised how good NIOZ is: the labs and the Department of Marine Microbiology and Biogeochemistry are the best in the world.’ She returned to NIOZ as a tenure-track researcher. When asked if Texel is an attractive place to live, Rush is clear: ‘Rain or shine, I cycle to work. I like the landscape and the fantastic skies. In ten years’ time, I hope to be here still with my own research group.’