The deep ocean is a large ecosystem on this
earth. It is characterized by low temperature, high pressure and the absence of
light. Thus, no photosynthesis can occur in the bathypelagic zone (1000 – 4000 m
depth) and is therefore dependent on organic matter from upper layers and
prokaryotic production. Heterotrophic protists (HP) are suggested the main
consumers of prokaryotic production in the deep ocean. It is known that HP are
important grazers in surface waters but not much is known about their role in deep
waters. Therefore, Pernice et al.
(2015) investigated the abundance of HP in the world’s oceans during the
Malaspina 2010 Expedition (Dec 2010 – Jul 2011) where they took water
samples from the meso- and bathypelagic (between 200 and 4000 m) from 116
stations, not including the polar regions.
To estimate the abundance of HP they used
epifluorescence microscopy by DAPI staining and TSA-FISH and were the first to
establish the flow cytometry (FC) to investigate the abundance of HP.
Pernice et
al. found that the HP abundance decreased with increasing depth and that
there were no significant differences between the Atlantic, Pacific and Indian
Ocean. The highest abundance was recorded in the mesopelagic and bathypelagic
of the Equatorial Pacific. A multiple regression model was used to examine if
the HP abundance can be predicted by using abiotic and biotic factors. Therefore,
depth, temperature, oxygen, salinity and the abundance of prokaryotes and large
viruses was tested. A significant effect of depth, oxygen and prokaryote was
verified. The ratio between prokaryotes and HP was lower in mesopelagic waters
than in bathypelagic. However, significant differences between the oceans with
minimal ratios in the South Atlantic and maximal rations in the Great
Australian Bight, were shown. Again, the Equatorial Pacific did not fit in the
results with no significant raltionship between HP and prokaryote abundances.
Pernice et al. conclude that the
variation in HP abundance is mostly related to prokaryote abundance.
Most of the HP were found in the size classes
10 and 15 µm3. The number of small cells (defined as diameter < 3 µm)
decreased with depth in contrast to a previous study that showed a decrease in
larger cell in the subarctic Pacific. This study shows that HP from the
bathypelagic are slightly larger than in the mesopelagic. Furthermore, the
theory that the HP biomass will decrease with increasing depth was proven. A
faster decrease of HP bio than decrease in prokaryote biomass with increasing
depth was detected and raised the question of the importance of the HP grazing
pressure on prokaryotes in deeper bathypelagic waters.
Apparently prokaryotes
are not the single carbon source for HP in deep waters. Another study of the
Malaspina 2010 Expedition showed that in areas with a high ratio of
prokaryotes : HP a significant contribution of fungi was detected. The
researchers suggest HP as important in the deep ocean and their study provides
a global base for further research.
In my opinion this is a good and interesting
paper that differs from other studies, because it compares sample sites from
all over the world (missing the polar regions) and is therefore unique and very
important and useful for further research. It also provides FC as method to
investigate the HP abundance which is faster than microscopy with DAPI staining
or other methods and gets the same results. Together with epiflourescence microscopy
it seem to be very efficient and accurate. Further research should be done,
e.g. on the question why the Equatorial Pacific is so different to the other
sample sites? Maybe the temperature plays an important role in mesopelagic
waters? But what about the bathypelagic? The temperature there is quite
homogenous so it should not have a high influence.
The authors lack to answer the question why
their results are different from a previous study in the subarctic Pacific. I
am wondering what the differences between the Malaspina sample sites and the
sample sites in the subarctic region might be. It could be possible that predators
or the prokaryote ratio play a role. It would be interesting to see if the
polar regions in general differ from the results of the Malaspina Expedition. All
in all this study is a really good base for further research and for global comparison.
Reviewed paper:
Pernice, M. C., Forn, I., Gomes, A., Lara, E.,
Alonso-Sáez, L., Arrieta, J. M., ... & Sintes, E. (2015). Global abundance
of planktonic heterotrophic protists in the deep ocean. The ISME journal, 9(3), 782-792.
http://www.nature.com/ismej/journal/v9/n3/pdf/ismej2014168a.pdf
Hi Eleni,
ReplyDeleteThank you for another great review – this is undoubtedly an important topic. Due to the obvious biogeochemical impact of photoautotrophic primary production by marine microbes, HP’s can often be overlooked. I am confused as to how the authors ensured that the protists they were enumerating were indeed heterotrophic. Having glanced over the paper myself, it appears they used a pan-eukaryotic FISH probe to enumerate all protists with the assumption that only the HP’s would be present in the water sample as it is in darkness? Perhaps I have misunderstood, was this your interpretation? I only ask as recent work by Agusti et al, 2015 has shown that large, healthy diatom populations (and other photosynthetic microbes) can exist in the dark oceans down to 4,000, so could such non-HP ‘contaminants’ influence these counts in your opinion?
Thanks,
Davis
Agusti, S., González-Gordillo, J. I., Vaqué, D., Estrada, M., Cerezo, M. I., Salazar, G., ... & Duarte, C. M. (2015). Ubiquitous healthy diatoms in the deep sea confirm deep carbon injection by the biological pump. Nature communications, 6. http://www.nature.com/articles/ncomms8608
Hi Davis,
Deletesorry for not making this point clear. They used an oligonucleotide probe (EUK502) which targets all eukaryotes, carried out a TSA and stained the probes with DAPI to enumerate them.
So if I understood it right they targeted all the eukaryotes. I'm not entirely sure if these targeted eukaryotes were washed in the following steps before the TSA or if they simply are not visible or obviously different from the HP after the DAPI staining and therefore the HP could be enumerated.
I am sorry that I cannot clearly answer your last question about the non-HP 'contaminants'; I don't know enough about the methods and how they exactly work.
Eleni
Hi Eleni,
ReplyDeleteThanks for your review. It seems to me that the next logical thing to do would be to take the polar regions into account as they seem to be a major area that has been left out. I was just wondered if the authors had suggested from their own results what patterns they might find in the polar regions? Or if perhaps you yourself had any ideas as to what may be seen in the polar regions from the data collected in this study?
Thanks,
Amy
Hi Amy,
Deletethe authors only mentioned the findings from a previous study (Fukuda et al. (2007)) that was conducted in the subarctic Pacific (as I already mentioned in the review). By looking over the paper by Fukuda et al. I found that the decrease in larger cells with increasing depth is consistent with other studies from the NW Mediterranean and western Northern Pacific.
These findings are really different to the data Pernice et al. found.
I don't know the reasons for it but it would be interesting to figure this out. The first difference in their methods is that the samples from Fukuda et al. were taken in July - September (summer months) and not in the winter, spring and early summer months. Maybe there is a difference in the behaviour of HP.
I am not sure if nanoflagellates (investigated by Fukuda et al.) are the same as the HP so maybe this could also be a reason for the different results. On the other hand it would be weird to refer to data from a paper that is not studying the same organisms.
Therefore, I find it really hard to say how the situation will be in polar regions.
Eleni
This is the paper from Fukuda et al. :
DeleteFukuda, H., Sohrin, R., Nagata, T., & Koike, I. (2007). Size distribution and biomass of nanoflagellates in meso-and bathypelagic layers of the subarctic Pacific. Aquatic Microbial Ecology, 46(2), 203-207.