The idea of being able to decrease atmospheric
CO2 by boosting phytoplankton seemed promising when the idea of iron
fertilization first emerged in the 1990s. Several in situ experiments have shown since then that iron is the limiting
factor for phytoplankton growth in high nutrient, low chlorophyll regions
(HNLC) and that the addition of iron results in vast plankton blooms. However,
a concomitant controversy on the efficiency as carbon sink and on the
ecological consequences was unavoidable.
Thiele et
al. (2012) focused on the very central part of the biological pump, namely
the microbes, and monitored the response of microbial communities to iron
fertilization, by looking especially for the characteristic succession patterns
observed during natural plankton blooms. It is unquestionable that this
knowledge is essential to understand the fate of fixed carbon following iron
fertilization.
Water samples from different depths, at
time intervals from 4-5 days were taken using Niskin bottles from inside and
outside the fertilization area during the LOHAFEX experiment in the Southern
Atlantic Ocean. Thymidine and leucine incorporation rates, into DNA or proteins
respectively, were used to assess microbial productivity, whereas CARD-FISH
(remember that is the enzyme catalysed version of fluorescence in situ hybridization) allowed to
quantify and identify community members, using both general and clade-specific
probes.
The iron addition caused a phytoplankton
bloom stretching over 300 km2 mainly composed of Prymnesiophytes (containing the maybe
better known group coccolithophorids) and not of diatoms, because of silicate
depleted waters. The data analysis
showed a significant increase in total microbial cell number inside the
fertilization patch, along with significant increases in thymidine and leucine
uptakes. Moreover the CARD-FISH revealed that the Bacteria, rather than the Crenarchaea,
were responsible for the significant increase in cell numbers within the
fertilized patch. In fact, the SAR11 clade accounted for 50 % of total cell
counts and increased significantly at day 18, then remained stable. Also Roseobacter and Bacteroidetes were significantly more abundant within the fertilized
area than outside. However, no changes were observed for Gammaproteobacteria.
Three different community richness indices
were applied: whereas Chao-1 values decreased till day 9 before increasing till
the end of the experiment, this trend was not reflected by Shannon and Simpson
indices.
So far, so good. But then I read their discussion, where after
having listed innumerous results of significant increases, the authors conclude
“total cell numbers of bacterioplankton and of the major clades are rather
constant”. What does “rather constant” even mean, relative to what!? Moreover,
they state that their results are concordant with similar studies showing that
iron fertilization is not followed by a change in microbial communities. If
they had made reference to the usual scale of community shifts during other
plankton blooms, the reader would maybe be able to come to the same conclusion.
Instead, the three diversity indices are not even mentioned again in the
conclusion, nor a possible explanation for the observed trend in the Chao-1
values. The increase in cell numbers
might have been small, but it was statistically significant, still no
biological explanations are proposed. Instead the authors relate the apparently
constant numbers of cells to flagellate grazing and top-down control, an idea
that had emerged from previous iron fertilization experiments.
By no means am I saying that the results
and the conclusion of this study are wrong, but I think the paper is lacking
some essential biological information to be able to come to the same
conclusion, and too many assumptions were made (suddenly an hypothesised
pre-experiment plankton bloom makes its appearance in the discussion to explain
inconvenient results) . In my opinion, the authors should have made an advance
and tested the hypothesis of a negative correlation between bacterial abundance
and abundance of heterotrophic nanoflagellates “hinted” by previous studies, instead
of basing their results on the same assumptions as their colleagues.
Thiele, S., Fuchs, B., Ramaiah,
N., Amann, R., 2012. Microbial community response during the iron fertilization
experiment LOHAFEX. Applied and environmental microbiology, (October).
Available at: http://www.ncbi.nlm.nih.gov/pubmed/23064339
Hi Anna,
ReplyDeleteWhen I've read about iron fertilization before I'd found that most studies reported an initial increase in cells but found that this bloom event was not actually sequestering carbon in the long run as it was not a sustainable growth. Do you think that this was meant by their results being constant?
Robyn
Hi Robyn, they actually don't link their results to possible consequences for carbon sequestration. Azam et al. 1983 showed that there is a positive correlation between phytoplankton concentration and bacterial biomass, which utilise 10-50% of the carbon fixed by photosynthesis. It has also been shown that phytoplankton community diversity is related to shifts in bacterial community composition. For instance, Kent et al. (2004) studied the bacterioplankton composition changing with annual succession patterns of phytoplankton and zooplankton and showed that there was a noted synchrony between these population and species shifts. So in my opinion, Thiele et al. could have at least made reference to these results and should have calculated whether there was a positive correlation between phytoplankton and bacterioplankton. That, in addition to monitoring the changes in the heterotrophic flagellates abundance, would have been a neat explanation for the absence of typical succession patterns of blooms and the "rather constant" results.
ReplyDeleteHi Anna, whilst this isn’t a blog on how to write science, it’s nice to see you highlight some of the ambiguous language used in primary, peer-reviewed literature! I wonder how many times this paper and its “rather constant” results will be cited as imperative evidence…
ReplyDeleteOh, that's really interesting that they don't mention what I would have thought to be the biggest "wider significance" implication of the study at all! Given the conclusions (or lack of) that seem to have been drawn from the paper (at least in your review, I haven't actually read the paper myself) I am left wondering what the actual point of the paper is?
ReplyDeleteAs stated in the conclusion, their results are similar to findings from the EisenEx experiment where no changes in the microbial community were found. It is interesting how from significant increases in the results, they go to "rather constant" then to "no changes"... So I agree with Vicky that when looking only at their conclusion one is tempted to take it as imperative evidence.
ReplyDeleteBut overall their point is that compared to the magnitude of other blooms, the bloom caused by the addition of iron is more like a coastal late spring/summer bloom, rather than a proper spring bloom and that any change in the microbial community is tightly controlled by flagellate grazing. So basically instead of producing a massive bloom that would successfully sequester carbon into the deep ocean, increased grazing prevents the bloom to "flourish".