The sea surface microlayer (SML) forms an important boundary
between the sub surface layer (SSL) and the atmosphere, and may have an
important function in the cycling of different nutrients. Atmospheric
deposition is one of the main external sources of nutrients, and the SML is the
first point of contact for these aerosols. The south east Mediterranean Sea
(SEMS) is a low nutrient area dominated by small sized oligotrophs. Previous
studies have shown the importance of atmospheric deposition in supplying the
SEMS with nutrients, but none have looked at whether this input of nutrients
affects the SML and SSL differently. Astrahan et al have attempted to remedy this using a mesocosm set up to look
at biomass and activity of various groups in the neuston.
Water samples of the SML and SSL were collected, as were
aerosols from a dust storm. Samples were incubated in outdoor pools either with
aerosol addition or without. Samples from each replicate were taken at various
intervals over the next 44 hours, and primary production and bacterial
production were measured. Before aerosol addition, heterotrophic bacterial
abundance was similar in both the SML and SSL, but bacterial production was
roughly 50% higher in the SML. Following the addition of aerosol to the
samples, heterotrophic bacterial abundance increased 2-fold in both the SML and
SSL water, but the increase in bacterial production was much higher in SML than
the SSL. The authors suggest that the added nutrients may have been used to fulfil
metabolic needs rather than for growth and cell division, which explains the larger
increase in production rather than abundance.
A mesocosm setup can only go so far in showing us how
different processes occur in the ocean, especially when only considering a very
small 44 hour window post aerosol deposition. The authors did not look at the
diversity of bacterial groups, but I think this would have been really
interesting to see if the bacterial community changed after aerosol addition
and how quickly this change occurred. I also think it would have been better to
increase the length of the study, as the first 44 hours shows us the initial
changes, but gives us no indication of how long this enrichment lasts or when
the community abundance and production returns to normal.
Astrahan P., Herut B., Paytan A., Rahav E.(2016) The Impact
of Dry Atmospheric Deposition on the Sea-Surface Microlayer in the SE
Mediterranean Sea: An Experimental Approach. Frontiers in Marine Science. 3:222
Hi Tabby,
ReplyDeleteI found this to be very interesting review! You have mentioned the authors looked at the heterotrophic bacteria in both the SSL and SML layers and I wondered whether the authors looked into the activity and abundance of autotrophic microbes within these two layers too? It would definitely be interesting to see how they would be affected by the aerosol addition and to see how it would influence the bacterial production rates. As the heterotrophic bacteria abundance is similar in both layers, however the bacterial production was much higher in the SML, would the autotrophic bacteria potentially be responsible for this difference in production rates?
Thanks!
Faye.
Hi Faye,
DeleteYes they did also look at the biomass and productivity of pico-phytoplankton. The aerosol addition caused a moderate increase in primary production in both the SML and SSL. Before the addition of the aerosol there was a very slight dominance of autotrophs over heterotrophs in both layers, but the authors suggest that this flips after aerosol addition. It would be interesting to see whether this is directly caused by the increase in bacterial production, or whether its another factor. The authors did say that adsorbtion of some nutrients onto the dust particles may be one cause - as this will remove available nutrients and favour heterotrophic feeding regimes.
I hope this answers your question,
Tabby
Hi Tabby,
ReplyDeleteyou have mentioned that the abundance of heterotrophic bacteria in both the SML and the SSL was the same before adding the aerosol. Do you know if this is a 'normal' condition that can occur in the oceanic waters? I thought that the SML shows a higher abundance of bacteria than the SSL. Did the researchers homogenise both layers to obtain results that are comparable?
Thanks,
Eleni
Hi Eleni,
DeleteSimilar bacterial abundance in the SML and SSL has been found in some papers (Cunliffe et al 2009,2010), but in others, the bacterial abundance is higher in the SML (MacIntyre 1974, Hardy 1982). It seems that abundance may be quite variable in the SML, so I think rather than abundance, it may be better to look at the types of bacteria, as well as other groups of microbe, in the SML and SSL in order to provide a better comparison.
Thanks, Tabby
MacIntyre,F.(1974).Thetopmillimeteroftheocean. Sci.Am. 230,62–77.doi:
10.1038/scientificamerican0574-62
Hardy,J.T.(1982).Theseasurfacemicrolayer:biology,chemistry and
anthropogenicenrichment. Progr.Oceanogr. 11,307–328.doi:10.1016/0079-
6611(82)90001-5
Cunliffe,M.,Salter,M.,Mann,P.J.,Whiteley,A.S.,Upstill-Goddard,R.C.,and
Murrell,J.C.(2009).Dissolvedorganiccarbonandbacterialpopulationsinthe
gelatinoussurfacemicrolayerofaNorwegianfjordmesocosm. FEMSMicrobiol.
Lett. 299,248–254.doi:10.1111/j.1574-6968.2009.01751.x
Cunliffe,M.,Upstill-goddard,R.C.,andMurrell,J.C.(2010).Microbiology
ofaquaticsurfacemicrolayers. FEMSMicrobiol.
Hi Tabby,
ReplyDeleteI'm definitely in agreement with you in terms of the experiment being a little too short. If they had prolonged it for, perhaps, a week; interesting results could have been yielded, especially as this is still a fairly new area of microbial research and there is still much to be discovered. It is also a shame that there was no taxonomy performed, as I imagine there would be differences in bacteria both between the SML AND SSL before and after the addition of the dust.
Thanks,
Harriet
Hi Harriet,
DeleteI think if they had identified the bacterial groups they could have shown some really interesting results, as the abundance of heterotrophic bacteria may remain the same yet its constituents might vary considerably between the SML and SSL. I think that if they had done so, this study might have had wider applicability as the changes in bacterial groups in the SEMS could have been compared to other low nutrient areas of the ocean.
Thanks, Tabby