Questions from undergraduates at UBC

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This figure pertains to Mak's answer to question 6 (see below). This is Figure 2 from: M.A. Saito, S.W. Chisholm, J.W. Moffett, J. Waterbury. 2002 Cobalt limitation and uptake in the marine cyanobacterium Prochlorococcus. Limnology and Oceanography 47(6):1629-1636. (Mak Saito)


Questions from November 12, 2006

Hi Guys,

...I imagine that you may have already collected water for incubations. I understand that there was some question about whether we should or should not ammend our semi-continuous bottles with Fe. This is a possibility for us, and we have successfully conducted low Fe work with this set up. However, my preference would be to repeat last year's design with modest Fe enirhcments in each bottles (~ 2 nM). The advantage of this approach is that it eliminates variable levels of contamination as a confounding factor. In addition, it will make the work more comparable to what we did last year. Last year's incubation was dominated by diatoms, and the co2-dependent changes we saw were manifested as shifts in the abundance of various diatoms species. According to Christina's counts, Phaeo started out about 10% of the total cell count, and it didn't change much during the experiment. This year, we're hoping to get good growth of Phaeo with some diatoms mixed in so we can look for co2 affects on these two groups. Dave's results from last year suggested that Phaeo should be most prevalent in Fe-rich waters. I believe that this is a further reason to add Fe to bottles. I'd be interested in hearing your thoughts.

Ok, that's it for now.

Hope that things are going smoothly.

- P

QUESTIONS FROM MY UNDERGRADS:

1) What is the role of ‘top-down’ controls (i.e. grazing) on phytoplankton community structure in the Ross Sea? How does grazing differentially affect diatoms vs. Phaeocystis? How do predators decide which is better to eat? What are the dominant types of grazers found in the Ross Sea?

2) Since diatoms need light to grow and photosynthesize, how do they grow when there is no daylight form months, and when there are thick ice sheets that cover the Ross Sea for much of the rest of the time?

3) Your research is during springtime in the Ross Sea. How will the ozone depletion which occurs over Antarctica each spring affect the results of your experiments?

4) Could differential viral lysis of diatoms and Phaeocystis be a factor contributing to their relative dominance?

5) Are public outreach programs crucial to modern-day science, or are they a nuisance to researchers who have to update blogs and answer questions while trying to work?

6) What determines whether Zn, Cd, or Co is utilized by the phytoplankton? Can any species use all three, or are there specific differences? What difference does using one or another make to phytoplankton productivity?

7) In what chemical form do you measure carbon, nitrogen, phosphorus, Iron and Cobalt in the Ross Sea?

8) What phytoplankton besides diatoms and Phaeocystis are dominant in the Ross Sea? How do you quantify the abundance of these other groups, and how might they affect your results?

9) How do PCO2, Fe, and light change over an annual cycle in the Ross Sea? What is the source of Fe in the Ross Sea?

10) Which in situ measurements will you be making using the underway pumping system?

11) How will this second cruise build upon the information gained from last year’s results?

12) What changes doe you expect to see in algal production rates over time with increasing levels of CO2 in the atmosphere? Is there any evidence of an adaptation by algae to increasing atmospheric CO2? - i.e. have they been found to be more efficient in their photosynthetic abilities compared to decades ago?

13) If there are long-term changes in phytoplankton community structure, do you expect to see changes in the food web structure?

14) Are there physiological adaptations that allow solitary Phaeocystis to survive in low nutrient conditions, while colonial phases do not do as well? Is the only difference surface area to volume ratio?

15) Why is the polynia region important to study? Why not work in a more accessible regions? Relative to the rest of the ocean, where does the Ross Sea rank in terms of primary productivity.

16) Do you think that Fe fertilization of the Southern Ocean is a good/feasible idea to counteract CO2 emmisions? Do you think that any current method of sequestering carbon dioxide looks promising? Do you think that carbon sequestration is an approach that we should even be considering when trying to combat global warming?

17) The katabatic winds will tend to be stronger on this CORSACS trip in November compared to last year’s trip. Will this cause a significant change in the depth of the mixed layer?

18) How many penquins have you seen? (Sorry, had to ask)



Responses from Rob Dunbar to Questions 13 & 14

13) If there are long-term changes in phytoplankton community structure, do you expect to see changes in the food web structure?

Long term changes in Ross Sea phytoplankton community structure will inevitably affect the community structure of all the higher trophic levels that depend on algal primary production.  It is a long-standing paradigm that diatom-dominated communities support the traditional “grazing food web” that leads to efficient production of fish, birds and marine mammals.  In the Ross Sea, this type of food web model might typically consist of diatoms to krill to penguins to leopard seals to orcas.  If future changes in the environment such as climate warming, rising CO2, and shifts on iron availability instead favor more dominance by Phaeocystis, the biomass of these large animals may decrease.  Phaeocystis is generally thought not to be efficiently grazed due to its colonial form, mucous production and possibly its synthesis of unpalatable chemical compounds.  If future drastic climate change shifts the community radically, it could even come to resemble the picoplankton dominated communities that are currently typical of the Subantarctic Southern Ocean.  Loss of sea ice and the associated diatom/Phaeocystis communities would also push the algal assemblage this way.  If so, the Ross Sea food web would probably shift strongly towards a microbial food web-dominated system that would not be able to support the current levels of higher trophic level production.  In this extreme scenario, penguins, seals and whales would probably virtually  disappear from the area. 

14) Are there physiological adaptations that allow solitary Phaeocystis to survive in low nutrient conditions, while colonial phases do not do as well? Is the only difference surface area to volume ratio?

The obvious advantage of solitary Phaeocystis over the colonial form is a much thinner diffusion boundary layer thickness, due to their much smaller size.  This will greatly facilitate uptake of any nutrient, including iron, nitrogen, and CO2.  The other potential nutrient acquisition advantages of each life stage are presently unknown, but we can speculate about what they might be.  Single flagellated cells may have an advantage in being motile- swimming could allow them to react to and take advantage of small-scale heterogeneity in nutrient concentrations (for instance, by seeking out transient patches of regenerated nutrients).  Some larger motile algae such as dinoflagellates can even migrate down to the main nutricline and back to obtain nutrients, although I don’t think this has ever been demonstrated for Phaeocystis flagellates.  The colonial form could potentially scavenge and sequester particle-reactive nutrients like iron on mucous.  In addition, very elevated pH levels (over 9) have been measured inside the hollow matrix of colonies; these pHs are basic enough to precipitate trace metals like manganese and iron (although they could be problematic for CO2 fixation!).  As far as I know, though, the real differences in nutrient uptake strategies and capabilities between flagellated and colonial Phaeocystis remain to be determined. 



Answers to #7, 9 & 16 from Pete Sedwick

 7) In what chemical form do you measure carbon, nitrogen, phosphorus, Iron and Cobalt in the Ross Sea?

For Fe, we are measuring 'dissolved iron' (operationally defined as that passing through a 0.2 µm filter) using flow injection analysis with preconcentration on resin-immobilized 8-hydroxyquinoline. This method is assumed to measure both inorganic and organic forms of Fe, in both the +2 and +3 oxidation states, and it will include Fe in both the truly dissolved and colloidal size ranges. In addition, we are using cathodic stripping voltammetry to measure the concentration of 'dissolved' ligand-bound iron and iron-binding ligands, from which we can calculate the concentration of 'dissolved' inorganic iron (assuming that the ligands are organic species).

9) How do PCO2, Fe, and light change over an annual cycle in the Ross Sea? What is the source of Fe in the Ross Sea?

With regard to the annual cycle of dissolved Fe (see answer to question 7), field measurements made in spring and summer suggest that surface waters of the Ross Sea are seasonally Fe-limited: dissolved Fe levels appear to be relatively high (ca. 0.5-1 nM) in early spring, due to vertical resupply during winter mixing and additions from spring ice melt, and are then drawn down to low concentrations (ca. 0.1 nM or less) in the open-water areas during the summer, as a result of biological uptake, scavenging and vertical export. Presumably, dissolved Fe is resupplied to the upper water column during the fall, when vertical mixing increases and the sea ice begins to form.

16) Do you think that Fe fertilization of the Southern Ocean is a good/feasible idea to counteract CO2 emmisions? Do you think that any current method of sequestering carbon dioxide looks promising? Do you think that carbon sequestration is an approach that we should even be considering when trying to combat global warming?

Soon after John Martin's suggestion that iron addition to iron-poor high-nutrient Southern Ocean surface waters ('iron fertilization') might present a way to remove carbon dioxide from the atmosphere into the deep ocean, ocean biogeochemical modelers began to investigate the feasibility of this idea using numerical simulations (e.g., Kurz and Maier-Reimer, 1993). The application of increasingly sophisticated numerical models to this question during the past 15 years consistently provides the same answer: sustained, large-scale iron fertilization of the Southern Ocean would be expected to result in a very small drawdown in atmospheric CO2 (on the order of tens of ppm), relative to predicted anthropogenic inputs over the coming decades (on the order of hundreds of ppm). In addition, a recent study suggests that such large-scale ocean iron fertilization would be both logistically and economically untenable. On this basis, iron fertilization of the Southern Ocean is probably not a viable method of sequestering fossil-fuel CO2 from the atmosphere. Other methods of sequestering atmospheric CO2 are being actively investigated/used -- e.g., injection into geological reservoirs -- although there appears to be no 'magic bullet' that can rapidly remove a large proportion of CO2 from the atmosphere.



Response from Mak to Question #6

6) What determines whether Zn, Cd, or Co is utilized by the phytoplankton?  Can any species use all three, or are there specific differences?  What difference does using one or another make to phytoplankton productivity?

Thanks for your question.Biology uses many metals in key enzymatic and structural functions, but the discovery of the importance of metals, especially of those beyond iron is relatively new.You likely have heard about how people can become anemic, meaning they lack enough iron in their diet to be healthy.And in the past few years, the importance of zinc nutrition to humans has become popularized with zinc lozenges for colds and the flu.But to give you an idea of how new this scientific field is, which is known as “bioinorganic chemistry” (and we are marine bioinorganic chemists), a class of zinc proteins known as “zinc fingers” were only discovered in 1986.These proteins are aptly named because they cause a protein to have a tight fold that looks like a finger, and this structure allows a zinc finger protein to bind into the groove of DNA to control transcription processes.However, unlike many iron proteins, zinc fingers are presumed to be very low in copy number in any given cell, making their discovery and importance unknown until recently.With the sequencing of the human genome, it was realized how abundant these zinc proteins are, estimates have suggested that ~3% of the human genome encodes these types of proteins.For a trace chemical element that is used as a micronutrient for nutrition, that’s a surprisingly large number.

Our understanding of the importance of micronutrients beyond iron in phytoplankton is also similarly limited at this point.Zinc, cadmium and cobalt are what we’ve dubbed “the trace metal trio” because in diatoms all three can substitute for a common biochemical function, isoforms of the enzyme carbonic anhydrase.In fact, the discovery of a cadmium carbonic anhydrase was (and remains) the only biological function of cadmium known in all of life.It has been observed this cobalt-cadmium-zinc substitution capability also exists in a coccolithophore (E. huxleyii) and Phaeocystis antarctica (which is just starting to bloom here in the Ross Sea as I write this!), so we believe it is a widespread phenomenon in eukaryotic phytoplankton. However, this ability to substitute all three metals is not found in the marine cyanobacteria.For example, the cyanobacterium Prochlorococcus has an absolute requirement for cobalt that cannot be replaced by zinc or cadmium.This is evident visually in 3-D plots of growth rate relative to metal concentration (Figure 2 in L&O 2002 paper).As a result, we believe that the relative abundance of all three metals could influence the composition of phytoplankton species in the environment, as new niches are created by the “chemical signature” of the trace metal trio concentrations and bioavailabilities.We also believe that these very large differences in trace metal requirements between cyanobacteria and eukaryotic phytoplankton are remnants from their evolution in a changing chemical environment in the ocean throughout the history of the planet.In other words, the different physiologies of the trace metal trio appears to be evidence for the co-evolution of biogeochemical cycles and life itself.

Finally, do cobalt, cadmium and zinc limit productivity?It remains to be seen, but there is some evidence that phytoplankton communities could be co-limited by iron and one or more of these metals.Iron limited areas, like where we are in the Ross Sea, have abundant macronutrients (N, P, Si), and hence are prone to micronutrient limitation.We’re down here measuring the concentrations of cobalt, cadmium, and zinc and their potential biological availability in seawater to examine this possibility of limitation or co-limitation.But what we actually found last season was that the system was co-limited by iron and vitamin B12.B12 is a cobalt containing biomolecule, that only some organisms have the capability to synthesize.Interestingly, the system was not cobalt limited (elemental cobalt as opposed to vitamin B12 now), as we measured through bottle incubation experiments and analytical measurements of cobalt in seawater.This was an unexpected finding for us.I actually stopped by the lab at 5am on my way to the airport and picked up a small bottle of vitamin B12 as an afterthought of something in addition to our planned work to try out in the Ross Sea.We’re taught that science is supposed to be done with hypotheses, but it’s humbling to realize sometimes we don’t even know enough about nature to come up with the right hypothesis in the first place.

Thanks for your questions!

-Mak



Replies from Walker Smith

5) Are public outreach programs crucial to modern-day science, or are they a nuisance to researchers who have to update blogs and answer questions while trying to work?

I think that most scientists are committed to their work, and to explaining their work and its significance to the general public.  Like many tasks, researchers are often pressed to complete them all, often at once.  But I believe there is a general commitment to outreach in the form of education of the public, and to demonstrating why Antarctic research in general is a critical component of our understanding of the ocean.

15) Why is the polynia region important to study? Why not work in a more accessible regions? Relative to the rest of the ocean, where does the Ross Sea rank in terms of primary productivity.

Excellent questions!  Polynyas are the most productive of any region in the Antarctic.  Most primary productivity and biomass maxima in the Southern Ocean occur on the shelf, and polynyas (which also largely occur on the shelf) are "maxima within maxima" - the single most productive locations in the Southern Ocean.  Daily productivity in the Ross Sea polynya can be up to 10 g c/m2/d, and chlorophyll concentrations can exceed 20 µg/l in extreme conditions.  These are high rates and standing stocks, no matter what situation you are considering.  However, when integrated over the annual cycle, the productivity of the Ross Sea (the Antarctic's most productive location) is very small - approximately equal to that off Bermuda in the Sargasso Sea.  The extreme seasonality drives unique ecological and evolutionary adaptations throughout the food web, and is a critical issue to understanding the region's food webs and biogeochemical cycles.

17) The katabatic winds will tend to be stronger on this CORSACS trip in November compared to last year’s trip. Will this cause a significant change in the depth of the mixed layer?

Winds during austral spring are stronger than summer; in fact, it is the periods of transition within the seasonal cycle where winds are strongest.  Katabatics can be strong near their source; however, the critical issue is how far they extend once the pressure gradient is diminished.  In my experience the answer is "not far", and physical oceanographers have actually sat down and computed the distances for me.  My understanding is that they do not extend more than 60 km from the source; hence, any influence on vertical mixing is relatively restricted to coastal regions.

We also know in the Antarctic that mixed layers are largely driven by salinity changes i.e., inputs of brine from freezing ice.  Relative to this physical forcing, winds are a very inefficient means to deepen the mixed layer.

While I used to think that the Ross Sea mixed completely everywhere on the shelf during winter, I now am less sure.  We have data to show that near the ice shelf (which always is freezing seawater, and hence is a region of substantial ice formation) mixing is to the bottom, but in other areas removed from the ice shelf, we don't have winter data.  My guess now is that the mixed layers in the central Ross Sea are greater than 200 m, but not all the way to the bottom.



Jack DiTullio's replies

2) Since diatoms need light to grow and photosynthesize, how do they grow when there is no daylight for months, and when there are thick ice sheets that cover the Ross Sea for much of the rest of the time?

Obviously, diatoms and all other phytoplankton in the Ross Sea cannot photosynthesize or grow during the winter months (May through September) when there is nearly 24 hours of darkness in the Ross Sea. During this time of darkness and very cold temperatures most diatom cells will form spores and dinoflagellates will form cysts to reduce their metabolic costs. The onset of light in the spring and melting of the pack ice are the environmental cues that will trigger the excystment. Thick ice sheets alone will not prevent ice algae from growing. Ice algae typically grow on the bottom of the ice at the ice/seawater interface. Depending on the ice thickness and snow cover light is attenuated to very low levels (typically on the order of < 10 µE/m2/s). It is actually the snow thickness that is most effective in reducing light through the ice. Antarctic sea ice diatoms are extremely low light adapted and have very low light saturation values of approximately 10-25 µE/m2/s.

4) Could differential viral lysis of diatoms and Phaeocystis be a factor contributing to their relative dominance?

Very little virus work has been done in the Ross Sea. One interesting phenomena that happens almost every year is the rapid demise of the Phaeocystis bloom. Typically, Chl concentrations within the Phaeocystis bloom in the southern Ross Sea will peak near 20 µg Chl per liter in early December. Then sometime between mid December and early January the bloom collapses. Macronutrient concentrations are still high (e.g. nitrate > 10-15 µM) but iron levels are low enough (ca. 0.1 nM) to limit the growth of Phaeocystis at this time. At present, it is unclear if low iron levels, per se, are enough to trigger the rapid demise of the bloom, which sometimes can occur virtually overnight. Viral lysis could certainly be important in contributing to the demise of these blooms. But at this point there are no data to back up that hypothesis. We collected samples last year to determine viral abundance by flow cytometry. However, those samples have not been analyzed yet.

11) How will this second cruise build upon the information gained from last year's results?

The main rationale for having two CORSACS field seasons was to allow us to investigate how changes in environmental parameters (e.g. light, carbon dioxide and iron concentrations) during the spring and summer seasons would affect algal community structure and physiology. We will conduct similar manipulative incubation experiments this spring as we did last summer but we will be starting with a physiologically different algal community. In addition, we will conduct areal transects and profiles to investigate the natural algal community structure. Statistical analyses will be conducted to correlate various biological and chemical parameters. Our objective is to compare our manipulative and observational approaches during the two seasons and hopefully some consistent trends will become evident.



 

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Last updated November 30, 2006
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