Our containers arrived which means not only do we have equipment but, in our case, we now have a lab. One of my most asked questions is whether the labs are built in to the ship and the answer is yes, many are, but some are also built into shipping containers which can be mixed and matched according to the needs of the specific expedition. That called for a 0500 start today, which was easy due to the excitement we all felt at finally getting going, and all hands on deck (now I understand where that phrase comes from!) to get everything unpacked, set up and tied down to enable us to sail the next morning. The first order of business was building filtering rigs - we have 8 in total with capacity to process 45 samples at a time, which might seem excessive but is absolutely necessary to do everything we want to do. Most of our rigs are towers connected to a vacuum pump, which uses negative pressure to draw the samples through the filters, but I also brought a peristaltic pump to filter my cartridge filters in-line, which required some clever DIY rigging to stay stable and easy to use. We finished early and headed out for one final night in Rio - which was bittersweet as we'd come to miss it but we're absolutely ready to get going

Today was definitely the craziest day we've had so far - and on a Sunday too. Alarms went off at 02:45am, which is feeling pretty normal by now, and we headed down to the main lab to watch the CTD profile as it went down. Whilst my filters were filling (15L per line takes me about 1.5hours), we filtered samples for SEM, which will allow us to visualise, count and identify our plankton to get an idea of their diversity and community changes over physical gradients. Having snap frozen my samples, stored them down in the -80, and acid-rinsed my carboys, I finished up my first shift around 10:00am. Then it was time to convince my body to sleep again (in fairness, it had been working for 7 hours), to store up a few hours in anticipation of the next night shift. I resurfaced just before dinner (or is it breakfast? I've lost track) and checked in with the day shift crew as their evening CTD went down. Once it was back on deck it was time for the affectionately named ‘fishing trip’. This is the deployment of the Tow-Fish, the sampling device that will supply us with trace-metal clean surface seawater straight to our RN container, so we can do our 120-hour nutrient addition experiments. We took the Fish out of her box and re-applied the electrical tape we had so diligently removed (oops!), and before long it was time to get her up into the air and over the side. With the help of Tina, Paul P and Richie (NMF Techs and the real stars of the research cruise show), as well as crew members Burt and Andy, we managed to get the Fish in the water whilst the captain ramped the ship up to 5 knots to flush through the system. This was the most exciting moment of the cruise so far for me, and I'm so pleased to say the whole thing went off without a hitch in the end. We have an ongoing joke that this is our experiment when it’s going well, and my experiment when it’s not (or when it's keeping us up at unsociable hours), but all joking aside, this is really my baby and I am super excited to have it underway. We'll hopefully have three 5-day experiments over the next few weeks, which should give us temporal and spatial resolution of nutrient limitation and co-limitation across the South Atlantic. Once the Fish had flushed through for a few hours, it was time to get down to the business of filling bottles, taking T0 samples and spiking with nutrients, which took us 'til around 0200. Dropping the bottles into the on-deck incubator was an extremely satisfying moment. Add another few hours of filtering, fixing and freezing and it was a 0400 bedtime for us, making this a 25-hour shift in total (with a little lunchtime nap). I'll see you tomorrow, not too early though.

'Drifting over the Lake District in an airship at a height of 3000 metres and trying to drop a rock onto a barn roof. At night' ~ Chief Scientist Professor Jonathan Sharples Those are the words used by the Principal Scientific Officer of research cruise JC275 (CarTRidge) to describe our expedition across the south Atlantic Ocean in the austral summer of 2025. In fact, he is referring not to the terrestrial mountain range in north England, but rather to the divergent plate boundary that runs down the middle of the Atlantic Ocean, separating the North and South American plates from the Eurasian and African plates. This, the mid-Atlantic ridge, forms part of the longest mountain range in the world, extending over 16,000 kilometres. And, rather than an airship, we were aboard the British Royal Research Ship James Cook, dropping not a rock, but state-of-the art oceanographic equipment to the ocean depths, in the hopes of answering fundamental questions about the impact of this ridge on our marine ecosystems and biogeochemical cycles. Between February and April 2025, we sampled 5000 litres of seawater, sailed 4000 nautical miles, lived for 47 days at sea, and undertook 35 experiments, with 21 scientists aboard working towards a unified goal: to understand the role phytoplankton in our surface ocean play in the global carbon cycle, their interaction with such geological features, and how this relationship will impact, and be impacted by, our changing climate. Figure 1. Team Plankton standing beside the CTD. (Left to right) Arianwen Herbert, Frieda Schlegel, Ben Fisher, Alex Poulton. The ocean is likened to many things—the ‘lungs’ of the planet, the earth’s ‘air conditioner’, an aquatic ‘rainforest’—but the bottom line is this: without a healthy ocean, human life could not be sustained on Earth. It absorbs 90% of the excess heat generated by human activity, produces oxygen for every second breath we breathe, and absorbs half of the carbon dioxide from our atmosphere, despite representing just 1% of photosynthetic biomass. Yet, these duties are taking their toll and with a rapidly changing climate, it can’t continue to do these things indefinitely. Ocean warming, acidification and pollution are just a few of the challenges facing our marine world today. And it’s crucial we understand how our ocean is changing in response to these things, so we can make accurate predictions on the future of our planet and its ecosystems. As such, our task on expedition JC275 was to look at how the specific environment on the mid-Atlantic ridge, with its internal tides altering the light, nutrient and mixing regimes experienced by its local phytoplankton communities, impacted the ability of these communities to capture carbon and export it to the ocean’s depth. Within the team of 21 scientists, we five members of ‘Team Plankton’ were responsible for interrogating the biological aspect of the picture. We approached this in two ways: by looking at the natural plankton communities and observing changes over a vertical gradient, from the surface to the deep chlorophyll maximum (DCM), and by conducting shipboard nutrient addition experiments, investigating the impacts of alleviating nutrient stress on phytoplankton community structure and function. In the pursuit of these answers to these questions, we awoke each day between the hours of midnight and 3am, rising before the sun so as not to shock our photosensitive deep photic plankton (if you’re curious to see what a day on the ship was like, click here !) We collected our samples using a Conductivity, Temperature, Depth (CTD) instrument which, as well as bringing back 24x 20L Niskin bottles full of seawater from various depths, provides real-time information about the temperature, fluorescence, salinity, turbidity and oxygen concentration of the water below us. Figure 2. Arianwen and Ben sample from the CTD. Thus began a flurry of filtering, fixing and analysis. We took our hundreds of litres of water back to light- and temperature-controlled laboratories, where we analysed the chlorophyll content of our samples, the photosynthetic efficiency of the plankton within (Fv/Fm) and bottled water with fixative for analysis with a ’flow cam’, a plankton imager which would enable us to observe directly the plankton in our samples. We filtered for scanning electron (SEM) and light microscopy, high performance liquid chromatography (HPLC) and fixed samples for flow cytometric analysis, all of which would allow us to interrogate the ‘who’s who’ of plankton at each point along a vertical (depth) and horizontal (longitude) gradient. Whilst we sampled CTDs most days, some days were extra special (and with an extra early alarm!). These mornings saw us meeting in our trace-metal clean container lab (affectionately named ‘Steve’) just after midnight, to carry out our nutrient addition experiments. Each experiment lasted a week, sampling at the beginning, middle and end. During these experiments we sampled not from the CTD but surface water from a trace-metal free sampling device called a tow-fish, suspended over the ship’s side and dragged along with us. This allowed us to add macro- and micro- nutrients like nitrogen and iron, and observe the response of the natural plankton community in real time and high resolution, taking most of the same measurements as for our CTD sampling. This classic oceanographic experiment allows us to understand which nutrients are limiting at a given point in the ocean—here an on- vs off-ridge comparison—and how these communities respond to the alleviation of nutrient stress, both in terms of community structure (i.e. who wins and loses) but also the genetic basis of rapid adaptation to changing conditions. Figure 3. Arianwen working in the trace-metal free container laboratory. She is wearing full PPE: lab coat, hairnet and gloves. Perhaps most excitingly, over the course of the cruise I was able to filter thousands of litres of seawater for genetic analysis. Over the next year I will work with collaborators around the country to carry out metabarcoding and metatranscriptomics analysis of the samples we’ve collected over the last months. This is an exciting avenue, allowing us to investigate these communities in unparalleled resolution, unpicking the genes and gene pathways involved in adaptation to life under rapidly changing conditions. The data will produce a novel genetic database of the south Atlantic picoplankton community, show patterns of nutrient limitation and reveal the functional responses in community structure, trophic mode and gene expression of natural phytoplankton communities to changing nutrient, light and mixing regimes. Figure 4. Sunset over the aft deck of the RRS James Cook. Life at sea is not for the faint of heart. The hours are long, the work is both physically and mentally challenging and evenings and weekends aren’t in the question. And yet, I’m never happier than I am at sea. The total immersion in the subject I love, seeing nothing but the ocean for miles, and being surrounded by inspiring people who are passionate about the same things as me is a life I feel privileged to live, and will do everything I can to continue. Moreover, for an early career scientist, sitting bleary-eyed at breakfast (which was, for us, more like lunch!), eating toast across from some of the most eminent names in oceanography is a career-defining experience. My deepest thanks to Professor Alex Poulton for inviting me to take part in this expedition and his support throughout the planning and work, and to Frieda Schlegel, Ben Fisher and Barbara Duckworth of Team Plankton for being wonderful colleagues and friends. Thanks to Professor Jonathan Sharples and the scientists, technicians and crew of JC275 for their hard work and camaraderie throughout. And to my funders, the UKRI BBSRC, St John’s College, Oxford, and the Challenger Society for Marine Science for facilitating my partaking in this expedition and research.

The first of March brings with it an Aquarius moon (according to my moon journal!), the beginning of  meteorological spring, and, for these eager scientists - our first pre-dawn CTD!! Standing in the main lab at 02:59, we watched the profile as it descended to about 300m (a veritable 'dip' compared to its 6km capability) and returned with its bounty - 24 20L Niskin bottles filled with water from various depths through the profile. Not only this, but we were treated to a display of a shoal of squid hunting a marlin in the spotlight just off the aft deck - which stopped play on the CTD sampling for a minute but was totally worth it. Once it was back on deck I set about filtering for my genetics and fixing samples for flow cam (and had a slight mishap with some Lugols which, for the uninitiated, is essentially straight iodine!). And, just like that, I had my first complete set of genetics samples from a real pre-dawn CTD. I decided to abandon my vacuum filtering rig and run all my samples through Sterivex filters off a peristaltic pump which worked an absolute charm - my deepest thanks go to Prof Mark Moore for his generous donation of the filters, which were a luxury cost that I would have struggled to justify on a PhD budget. Around lunchtime we were able to catch view of a couple of marine snow catcher deployments - a fan favourite of mine. These snowcatchers enable us to see what the particles in the water are made of, their carbon content, size and sinking speed. This goes back to our theory of the biological matter in the surface ocean sinking to the seafloor, locking carbon out of the atmosphere. The hypothesis here is that we should see more large particles over the mid-Atlantic ridge due to internal waves pushing more nutrients, which leads to bigger particles, which sink faster, locking away more carbon. You know the drill by now. In the afternoon we had to start filtering another set of incubation experiments from the previous day. These required filtering in the dark which meant it was time to don the red headtorches

Yesterday we transited to our ‘ridge’ site – almost time to test our theory of increased productivity on the mid-Atlantic ridge! We used the afternoon to host another mini-conference of science talks – and today was my turn to present. It was a really supportive and curious atmosphere and, though it was only a 10/15 minute talk, sparked much discussion afterwards. While we’re here, I’ll take the opportunity to tell you a little about my PhD. My focus is marine phytoplankton and their role in the carbon cycle. Phytoplankton are key players in the biological pump, that is, the flow of carbon from the air, to our oceans, to the deep ocean. This is a key process as it locks carbon out of our atmosphere, reducing the effect of CO2 in warming our planet. However, as our planet is warming and the ocean is absorbing that heat, its surface is increasing in temperature. This causes an increase in the temperature gradient between the surface, low nutrient, high plankton layers, and the lower, high nutrient, low plankton layer. The plankton in the surface rely on mixing between these layers to get the nutrients they need to grow. As the stratification of the ocean increases, there is less mixing, and less nutrients are delivered to the surface. We think this will alter the community structure of phytoplankton in the surface ocean, as a result altering the capacity of the ocean to absorb carbon dioxide. One theory is that this will shift phytoplankton communities towards smaller phytoplankton, which is where my PhD comes in. With my fieldwork and lab-based study, I am seeking to understand a) how is community structure of phytoplankton in the surface ocean likely to change in response to changing nutrient supply?, b) how and why are picoeukaryotes (particularly a few groups) so well adapted to these conditions (physiological and biochemical mechanisms)? and c) what does photosynthesis and carbon cycling really look like in these organisms - and can we optimise this? As part of this, I’m also looking at specifically-adapted communities, such as those in the deep chlorophyll maxima, and the effect of different nutrient and light regimes, such as those on and off the mid-Atlantic ridge. Left: my peristaltic pump filtering rig. Middle: fitting 0.2 micron Sterivex filters to the tubing ends. Right: running chlorophyll samples on the spectrophotometer. Having transited overnight, we arrived around 0400 and immediately set about getting the CTD in the water. Unfortunately we didn’t manage to get it out before dawn but it was a good day to have a bit of a play with methods - I was able to try out my new filtering rig, with the consensus being that you cannot filter 5L of seawater through a 0.2um filter under vacuum (at least, not in less than 5 hours!). So, lesson learned (the long way). Filtering in hand, it was time to measure some chlorophylls - we were measuring total and size-fractionated chlorophyll, to enable us to understand the structure of the phytoplankton community in the water. We measure this with a spectrophotometer, once the filters have been in acetone for around a day after sampling. This measures the absorbance of light through the sample, from which we can understand how much of the chlorophyll pigment was present in the original filter. Today we also dropped our second mooring – this time on the ridge. This works much the same as the first (see 25th Feb entry) except now we’re trying to drop the anchor on a specific location on this underwater mountain range. To quote our chief scientist, Jonathan, ‘it’s a bit like drifting over the Lake District in an airship at a height of 3000 metres trying to drop a rock onto a barn roof. At night.’ These two sets of moorings, together, will enable us to observe differences between the two sites – on and off the ridge. Sunset over the aft deck during mooring deployments. In the evening (or, our version of evening, which is essentially 4-5pm) we were lucky enough to witness a part of the planetary parade - seeing Venus, Jupiter and Mars while the wirewalkers were deployed over the side.

Another dawn CTD, but this time a full run-through, enabled us a dress rehearsal for our long-anticipated pre-dawn. Sampling went off without a hitch (surprisingly there is not a huge amount of competition for sampling at 0400!), and while my samples were running I decided to embark on a DIY project to see if I could optimise my filtering set up. As a minimum I have 12 samples to do at a time, and only 6 lines on my inline peristaltic pump set up, so I wanted to try my hand at constructing a manifold vacuum filtering rig, which would give me an additional 6 sample slots. With some help from a power drill, my trusty green tape and lots of parafilm (if you know, you know), I had a pretty neat-looking rig assembled. Happy to see my filters turning green, I soon had my first full set of genetics samples safely tucked away in the -80. Happy scientist. Left: a pre-dawn (0300) CTD deployment. Middle: me sampling the CTD. Right: my filters collecting lots of phytoplankton from the 10 litres of seawater I filter through them. Today I also had a couple of ‘phone home’s, which was very comforting. It can be hard watching your life carry on without you. Another call was made to discuss sample processing and analysis; a collaboration with one of my lab postdocs for whom I’m collecting samples. Finally I spoke with one of my supervisors so they could live vicariously through my porthole view from the chemistry lab! Most gratifying as well as encouraging. Choose your supervisors with care, a supportive one is priceless. In the evening we were able to watch another couple of deployments - the wirewalker and glider. The wirewalker contains similar sensors to the CTD, and it works by allowing the main body to be ratcheted up and down the wire by wave action on a surface float attached to it, measuring a number of parameters including the temperature and salinity of the water as well as the chlorophyll in the water. We'll leave it out here, profiling about twice an hour, and collect it in around 3 weeks. After the wirewalker we deployed a glider - another of my favourite 'yellow toys' as they are affectionately called (think - Boaty McBoatface!). The gliders are free-moving - not attached to a chain or buoy - and have much more control over their movement. They can alter their buoyancy to change their position in the water, and move forward and backwards. They also measure temperature, salinity and chlorophyll, amongst other things, sending some of this data by satellite, and keeping some of it locally for us to unpack upon recovery.

Today was an exciting day because it was the inaugural meeting of ‘espresso club’ – aka, Team Plankton meeting outside the ship’s coffee room at 0400 to begin the gradual shift to our 0300 starts (we still had a couple of time zones to go at this point). Making the most of the bonus hours in our day, we decided to take the opportunity to get our FIRST DATA – even if not from this cruise. The JC273 team had left us a couple of trays of chlorophylls in the hold, which we measured concentrations of. This felt like a momentous event, given how long we’d waited to start doing science. Powered on by a gorgeous sunrise from the bridge and a run on the ship’s treadmill (god bless dynamic positioning), we spent the afternoon in our RN container (henceforth named and referred to as ‘Steve’) making up nutrient spikes for our amendment experiments. In the afternoon we deployed a further two Argo floats, before the crew spoiled us with an incredible spread at a barbeque on the aft deck – complete with a spectacular cake to celebrate Marika’s birthday. Left: Team Plankton members outside the coffee shop. Middle: vaccum filtering rig set up. Right: Marika and chef on her birthday! Another job on the to-do list was cleaning Steve. Steve is a metal-free laboratory, fit for doing experiments with trace metals like iron. This means Steve himself is free from contaminating metals – hinges and screws are covered, benches and worktops are made from plastic – but we need to keep him absolutely clear of dust or grease: anything that might carry trace metals and elements from the outside world. Perhaps counterintuitively, we don’t clean with cleaning solution – deionised water or 0.5% HCl solution will do for our purposes. So we donned our lab coats, hair nets and nitrile gloves, and attacked with our cloths and blue roll. A couple of hours and much AC/DC later (Alex’s contribution to the group playlist), Steve was clean and ready to be connected to his trace-metal clean water supply. Unfortunately we had some problems with the deployment, so I’ll leave that to another day (edit: head to the 2nd March  for successful deployment!). Left: Frieda and I dutifully donned our clean lab outfits for the first of many times to clean our trace-metal laboratory, Steve. Right: chief scientist Jonathan Sharples gives us the run-down on internal tidal waves. In the afternoon Jonathan held a science talk for the crew, which many of us attended. The next day also brought the first instalment of science mini talks, an initiative organised by Ric Williams, as an opportunity for us all to talk about previous or ongoing work we’d done outside of the cruise, which was a thoroughly enjoyable afternoon. It’s really valuable for everyone to understand the aims and objectives of the cruise, especially when we are from such a broad range of scientific backgrounds.

Today began encouragingly with a rainbow over the back deck, and the first order of business: building deck incubators. A deck incubator is essentially a paddling pool for sample bottles – we cover them with blue light filters (like you might find in a theatre) and pipe through underway water from below the ship – thus maintaining ambient light and temperature conditions you might find a few metres below the water’s surface. With the help of Grant (Chief Petty Officer, Science), we constructed some state-of-the-art deck incubators that only overflowed a little (!). We finished off the morning making up chemicals from stocks (solvents for chlorophyll measurements, acids and nutrients for our amendment experiments) and tidying up our filtering rigs, before it was time to settle down to some arts and crafts: aka, blacking out 24 carboys with rubble sacks. This is so when we bring back samples from the deep water we don’t shock them with the light levels and the surface, which is important when we’re measuring things like photophysiology. The afternoon’s excitement was a float deployment. These floats are a part of the  UK Argo  project, measuring temperature, salinity and depth. Once we drop them in they adjust their buoyancy to sink down to around 2km, from where they are free to move with the ocean currents for about 10 days, gathering information about the movement of the water masses they pass through, before they resurface. On the way up they continually measure salinity and temperature to give density values, and as they surface they send back data by satellite to scientists ashore. We have a number of floats to deploy on this expedition, which will enable us to build a nice picture of what’s going on at a number of different locations.

Not a huge amount of science was done today - everyone gets a day of grace to adjust to the ship's movement, which leaves most feeling at least a little groggy - but let me tell you a little about the experience of leaving port. We headed out to the front deck where we had the optimum view. When you leave a big port like Rio you get a 'Pilot' - someone who knows the way and comes up to the bridge to guide you out. He came on around 0900 and we were off. We sailed past many of Rio’s landmarks – Christ the Redeemer, Sugarloaf mountain, and, breaching the mouth of Guanabara bay, we were out into the open ocean! It's quite an indescribable feeling really, knowing you won’t see land again for almost two months. The biggest excitement of the day, apart from three or four planning meetings, was the muster and lifeboat drill. This saw us all gathering in the hangar with our life vests and hard hats before filing up to the lifeboats. We all filed into the boats and sat while we were briefed on what would happen in the event! We spent the evening scheming in anticipation of science beginning in the next few days, and were treated to the most serene first night sunset. I sent a picture home to my supervisor, who replied ‘the happiest place on earth – the aft deck of a research ship’. I’m inclined to agree!

Our supply containers have been delayed so, while we wait, let me tell you about the different experiments we have planned, and what sort of data we hope to get from them. There are two main types of experiments that correlate with sampling methods; those  from the CTD, and those from the Tow-Fish. The CTD, a large Rosette device, has 24 sampling bottles and can go down to a depth of 6km before ascending and collecting samples from different depths. The CTD also measures the physical parameters in terms of water: conductivity, temperature and depth (from where it gets its name). The Tow-Fish is a much smaller deployment that is suspended over the side of the ship and pumps water from the surface straight into our metal-free container. The Fish collects water from a distance away from the ship so as not to be contaminated by the ship itself. The samples it collects are trace-metal free and so when looking at things like Iron (a trace metal in seawater) there is no interference from the ship environment. An important concept in this research is the Deep Chlorophyll Maxima, essentially a region around 100m below the surface of the ocean where light is so low (around 1% of surface irradiance) that the phytoplankton community there massively upregulates the concentration of chlorophyll (the green pigment in plants and algae) to allow them to continue to photosynthesise. We are interested in understanding what that community is and what is going on on a genetic level to observe what genes are switched on or off under what conditions. The samples collected by the Tow-Fish will enable us to look at nutrient limitation. By adding macro- and micro-nutrients to the natural community and incubating them for a period of time we can observe the community response in terms of how the community changes, and what type of genetic responses individual phytoplankton have. In addition to the genetic analysis, we'll be measuring and sampling for many other things: chlorophyll, size-fractionated chlorophyll, scanning electron microscopy, flow cytometry, flow-cam, and more. This will combine to provide a more  complete dataset to help understand the current status of the ocean. Shift patterns will not be set but generally samples need to be taken before the sunrises at dawn. This means that most days will begin at 0300 hours.