Wednesday 28 December 2016

Selective enrichment of saccharides and ions in sea spray aerosols



Sea spray aerosols (SSAs) are formed by bubble bursting at the ocean surface. These aerosols can reduce solar radiation and act as cloud condensation and ice nuclei. Rising bubbles scavenge surface-active organic matter from the surface micro layer (SML), thus leading to a different composition of SSAs compared to the SML or subsurface water. As microbes and viruses are the main regulators of organic carbon in the marine environment, it is thought that their activity might also change the composition of SSAs. In their paper, Jayarathne et al. (2016) investigated the enrichment of budding SSAs with organic carbon (OC), especially saccharides, and inorganic ions. Moreover, the paper also looked at the selective transfer from water to air and the influence of the microbial loop. 

The study was conducted with coastal ocean water in a wave flume at the University of California, two phytoplankton blooms occurred during the experiment. SSAs were generated by wave-breaking and samples were collected from the covered wave flume with quartz fibre filters. Furthermore, SSAs were divided into smaller and larger SSAs. Enrichment Factors were used to evaluate enrichment relative to Na+. Thermal optical analysers, ion-exchange-chromatography and high-performance anion-exchange-chromatography were used to detect OC, ions and saccharides respectively. 

The values and temporal variations of biological activity and dissolved organic carbon (DOC) were similar to data obtained in field studies. Both smaller and larger SSA were mostly composed of sodium (18 and 22%) and chloride (33 and 41%). While all particles showed the same components, larger SSA showed more inorganic ions and smaller SSAs were more enriched with OC (8 compared to 1.2%). 

SSAs were significantly enriched with OC, indicating a selective enrichment process. Overall, the enrichment factors did not change with biological activity. However, the authors suggest that differences in the influence of biological activity between different sized SSAs were undetectable in this experiment.

Saccharides formed 11 and 27% of OC on smaller and larger SSAs respectively, with glucose being the most abundant saccharide throughout the experiment. Enrichment factors for SSAs were higher for smaller aerosols but both factors were still significantly higher than for the SML. Smaller SSAs mainly contained saccharides used for energy storage and associated with DOM. However, larger SSAs also harboured structural saccharides which are associated with POM. This difference may arise during the formation of the SSAs. While smaller SSAs are formed by film drops, larger SSAs are formed by jet drops from older, less-enriched parts of the SML. Moreover, the abundance of saccharides also changed rapidly during the experiment, presumably due to changes in biological activity. 

SSAs were also significantly enriched with cations compared to the SML. Divalent cations such as, Ca2+and Mg2+ were more abundant than single charged cations. The observed selective enrichment of cations corresponded to the binding affinities of the cations to fatty acids and anionic surfactants. Thus, cations may be selectively transported to the SSAs when organic molecules are present. Furthermore, while the mechanisms of enrichment are not known, charged organic compounds may also transport anions to SSAs. Nitrate and sulfate were enriched in SSAs and also showed temporal variation, presumably due to changing biological activity. In contrast, chloride was not enriched on SSAs.

In conclusion, the study provides evidence that enrichment in SSAs is a dynamic process and is affected by biological activity. Moreover, the authors suggest that the established method of estimating salt concentrations using Na+ concentrations is biased because of the selective enrichment of divalent cations. The paper doesn’t name any potential ecological impacts, but in my opinion selective enrichment of SSAs could at the very least provide airborne microbes with important nutrients. 

Reference:

Jayarathne, T., Sultana, C. M., Lee, C., Malfatti, F., Cox, J. L., Pendergraft, M. A., ... & Bertram, T. H. (2016). Enrichment of Saccharides and Divalent Cations in Sea Spray Aerosol During Two Phytoplankton Blooms. Environmental Science & Technology. http://pubs.acs.org/doi/abs/10.1021/acs.est.6b02988
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6 comments:

  1. Unknown29 December 2016 at 12:51

    Hi Johanna,

    You said there was variations in saccharide abundance, do you think there could be a diurnal cycle, possibly linked with photosynthesis within the blooms? Do the authors consider this link? If not it could be an interesting aspect to consider in future research. I am also a little confused, I cannot picture how the nutrients are selectively transported to SSAs. But I really like your idea of providing nutrients to airborne microbes.

    Thanks,
    Chloe

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  2. Johanna Gebhard30 December 2016 at 09:54

    Hi Chloe,

    Thanks for your comment. The authors simply stated that there was variation in the concentration between the two phytoplankton blooms. Moreover, measurements were made once a day so the experimental set-up doesn't really allow for the detection of a diurnal cycle. However, I personally think a photosynthesis-influenced, diurnal cycle makes a lot of sense.
    I had to read through the paper a couple of times to understand, but basically the process has a lot to do with the mechanisms of bubble bursting and SSA formation. I couldn’t really explain the bubble bursting process without devoting a large part of my review on it or without explanatory figures, but if you are interested, I suggest you look at the figures in the paper or search for ‘jet drops’ and ‘film drops’.
    Ions that are bound to carbon compounds are transported to the SSA, therefore the binding affinities of different ions to carbon compounds determine the rate of transport.

    I hope this answers your questions,
    Johanna

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  3. Unknown31 December 2016 at 15:43

    Hi Johanna,

    Thank you for your review. You mention that microbes and viruses may change the composition of SSA's. Because of this, do you think that future studies across the globe may show patterns in the SSA compositions linking to the microbes and viruses found in that area? Do you know if any other studies like this have been carried out in other areas and have given evidence towards this already? I think it would be interesting to see if there was a connection between these.

    Thanks,

    Amy

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    Replies
    1. Johanna Gebhard2 January 2017 at 14:38

      Hi Amy,

      As far as I can tell this is the first study of this kind, but I think patterns of SSA composition are a distinct possibility. Hopefully, we’ll see some further research in this area.

      Thanks for your question,
      Johanna

      Delete
  4. Unknown1 January 2017 at 16:36

    Hi Johanna,

    I'ts interesting that different size SSA potentially determine whether Inorganic carbon of organic carbon is aerosolised. I agree with your idea about the SSA providing nutrients to airborne microbes, and this would be a really interesting research path to follow. It would also be intriguing to see whether this size related pattern has an effect in regions of the world that have Ice nucleating particles as a result of ocean surface bubble bursting; as it's already known that organic matter ( e.g algae exudates) have rapidly faster crystallising speeds than other exudates found in the SML (Wilson et al 2015).

    Thanks,

    Harriet

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    Replies
    1. Johanna Gebhard2 January 2017 at 14:45

      Hi Harriet,

      I didn’t think about ice-nucleating particles at all, thank you for bringing it to my attention.

      Johanna

      Delete

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