The last months were crazy busy getting all set for implementing our very first Saharan wet dust fertilization experiment on a monoculture of Emiliania huxleyi, probably the most opportunistic and cosmopolitan of the coccolithophores. Here are some updates regarding what we were looking for with this experiment!
Why Emiliania huxleyi?
Firstly, E. huxleyi is an important and highly productive species (Winter et al., 1994) that greatly contributes for a large part of the suspended CaCO3 in the ocean through the occurrence of blooms spanning entire ocean basins (Balch et al., 2017). Secondly, we have recently also found what seemed to be a drastic increase in the coccolith fluxes of this species in response to dust-driven nutrients in open tropical North Atlantic, right underneath the largest dust plume originated from the Saharan desert Guerreiro et al. (2017).
What is Wet Dust Deposition and why is it so important?
Wet dust deposition is basically when dust is removed from the atmosphere by rain, instead of mostly by the force of gravity (aka Dry Dust Deposition). This is important because the uptake of secondary acids (e.g. H2SO4, HNO3) by dust particles during their transport in the atmosphere is probably the main factor for enhancing the solubility of dust-born nutrients and hence, their bioavailability (e.g. Jickells et al., 2005). The process involves a series of condensation/evaporation cycles to which aerosol particles are submitted through clouds during their atmospheric transport (Desboeufs et al., 2001 and refs. therein) before being washed from the atmosphere by rainwater.
How did we simulate dust deposition in the lab?
Following a lab procedure adapted from Korte et al. (2018, in review) and Desboeufs et al. (2001), we have used a certain concentration of Saharan dust (only the finer fraction < 32 mm) previously leached/processed for 24h in artificially acidified cloud water (using H2SO4 and HNO3 to obtain an end-pH2). This acid solution was subsequently diluted in Milli-Q water for mimicking wet deposition by rainwater (pH5) (see also Desboeufs et al., 2001; 2014; Guieu et al., 2010) over an artificial ocean represented by our culture medium containing cells of E. huxleyi (pH 8). This procedure aimed at simulating dust concentrations in the upper 1 m of the water column after a strong Saharan aerosol deposition event over the tropical Atlantic.
How did the experiment go and how long did it take?
Our experiment involved 12 culture flasks containing 400 ml of culture oligo-ESAW medium [4 Treatments x 3 replicates] that were kept under high light intensity, 24º C and low nutrient concentrations for mimicking the environment conditions in the tropical Atlantic. Twice per day (morning and evening) for 22 days, the flasks were gently shaken to ensure that all the cells were kept in suspension, allowing them to be exposed to light, preventing thermal stratification and keeping nutrients uniformly distributed. On the 5thday, we have introduced our “dusty-solution” in some of the flasks to investigate the response of E. huxleyi to wet dust deposition.
Who was involved?
Even a “tiny” 12-bottle experiment involved quite a large group of enthusiastic people in order to make it work! Amongst the main participants were our “dusty-partners” Laura Korte (Royal NIOZ) and Karine Desboeufs (Laboratoire Interuniversitaire des Systèmes Atmosphériques), who helped us define the best procedure for simulating Wet Dust Deposition. At the University of Lisbon, Rogério Tenreiro and Ana Tenreiro have opened the doors of their amazing Bugworkers’ lab at BioISI/TecLabs where we could run our experiment. Ana is a microbiologist specialized on the application of multiparametric flow cytometry, which she used for monitoring the cells’ growth evolution during the experiment. At the same time, Bernardo Vicente was doing the cells’ counts using the “traditional way” on the microscope at ALGOTECA. Bernardo is a young researcher from MARE-UL who is now about to finish is MSc thesis on the role of SiO2 for cell production and calcification of Coccolithus pelagicus.
Running this experiment was a hell of ride, especially for some of us who are not used to deal with “living cells”, nor with micropipettes, strong acids, and really, reaaaaaally small volumes :-) ! In the end, planning such an experiment that aims mimicking – in a simplified but yet realistic manner – what really happens in the ocean, is a tremendous exercise of forcing you to think about all the processes involved and how they link to each other. Having the support of a multidisciplinary team of experts is absolutely fundamental. And it is precisely when you get out of your comfort zone that you learn the most about natural processes.
Over the next weeks/months we hope to tell you fresh news about our observations! Stay tuned :-)