[L&O Featured Article]L&O Vol. 46, No. 6 (September 2001)

[lo-feature-admin@aslo.org]

Three related articles are featured in the September 2001 issue of 
L&O:  


Berman-Frank, Ilana, Jay T. Cullen, Yeala Shaked, Robert M. 
Sherrell, and Paul G. Falkowski. 2001. Iron availability, cellular iron 
quotas,  and nitrogen fixation in Trichodesmium. Limnol. Oceanogr. 
 46: 1249-1260.  


Hutchins, David A., Barbara J. Campbell, Matthew T. Cottrell, 
Shigenobu  Takeda, and S. Craig Cary. 2001. Response of marine 
bacterial  community composition to iron additions in three iron-
limited  regimes. Limnol. Oceanogr. 46: 1535-1545.  


Lenes, Jason M., Brian P. Darrow, Christopher Cattrall, Cynthia A. 
Heil,  Michael Callahan, Gabriel A. Vargo, Robert H. Byrne, Joseph 
 M. Prospero, David E. Bates, Kent A. Fanning, and John J.  
Walsh. 2001. Iron fertilization and the  Trichodesmium  response 
on the West Florida shelf. Limnol. Oceanogr. 46:  1261-1277. 


These articles are freely available on the Web at


          http://aslo.org/lo/toc/vol_46/issue_6/1249.pdf

          http://aslo.org/lo/toc/vol_46/issue_6/1261.pdf

          http://aslo.org/lo/toc/vol_46/issue_6/1535.pdf


Instructions for reading PDF files are located on the ASLO web 
page:


          http://aslo.org/help/loonline.html


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Introductory comments by Jon Zehr (the Associate Editor for all of 
these  papers)  


Until ten years ago research on nutrient limitation of productivity in 
the  sea largely focused on the macronutrients nitrogen and 
phosphorus  (Redfield 1958; Ryther and Dunstan 1971), although it 
was recognized  that micronutrients could be also used to model 
productivity (Dugdale  1967). Attention rapidly focused on the 
micronutrient Fe when =93clean  techniques=94 (Bruland et al.1979) 
demonstrated how easily trace elements  could contaminate 
primary productivity rate measurements. Experiments  showing 
that Fe additions could stimulate primary productivity in bottles  
(Martin et al. 1991), and ignited a wide range of studies over the 
globe,  with implications ranging from controls of the productivity in 
high- nutrient, low-chlorophyll (HNLC) regions, to controls on 
nitrogen  fixation in tropical oceans. More recent large-scale Fe 
fertilization  experiments clearly showed that Fe is a limiting factor 
at some times and  places (Coale et al. 1996; Boyd et al. 2000). It 
has also been suggested  that Fe availability is particularly 
important in controlling nitrogen  fixation, since many enzymes 
(particularly nitrogenase) require Fe.  Proposed applications of Fe 
to stimulate primary productivity as a  solution to increasing global 
atmospheric CO2 levels are extremely  controversial being of 
potentially dubious value and carrying large  uncertainties and risks 
(Chisholm 2000; Fuhrman and Capone, 1996).  Regardless, one 
thing is very clear: we need to know more about the  
biogeochemical and food web interactions with Fe to understand 
the  ecology and productivity of the oceans.  


In this issue of L&O, three papers provide important new 
perspectives  on aspects of the role of Fe in ocean ecosystems. 
Previous studies  showed that bacterial production responds to Fe 
additions along with  the phytoplankton (e.g., Cochlan 2001). 
However, the mechanisms  involved were not known=97bacteria can 
compete for Fe as a nutrient, but  they are also dependent on 
phytoplankton for fixed carbon. Thus, Fe- induced effects on 
bacterial communities could either be direct effects of  the nutrient 
enrichment or indirect, through the changes in  phytoplankton 
assemblages. In this issue of L&O, Hutchins et al. (2001)  address 
this uncertainty by following the dynamics of bacterial  phylotypes 
after Fe enrichment using the modern molecular DGGE  technique. 
They conclude that only a few phylotypes responded to the  Fe 
enrichment and that the phytoplankton and bacterioplankton  
communities become decoupled, at least over the time scale of a 
few  days. These results indicate that bacterial community 
composition is not  directly coupled to phytoplankton composition 
or Fe concentrations,  and raise intriguing questions regarding the 
factors controlling bacterial  communities.   


Fe has been shown to be important in the nitrogen-fixing  
cyanobacterium Trichodesmium, presumably because of the Fe  
requirement of nitrogenase (Reuter 1992).  In the second featured 
paper in this issue of L&O, Berman-Frank et al.  (2001) re-evaluate 
the link between Fe availability and nitrogen fixation in  
Trichodesmium, using field measurements and laboratory cultures. 
In  cultures, correlations between Fe limitation and ratios of the  
photosystem I and photosystem II reaction centers indicate that 
reduced  abundance of PSI reaction centers and electron flow 
through PSI may  affect the ability of Trichodesmium to generate 
ATP through cyclic  electron flow, and even affect oxygen 
dynamics by interactions with the  Mehler reaction generated in 
PSII. These findings provide a biochemical  mechanism for Fe 
limitation in Trichodesmium but also may provide an  index for Fe 
limitation in the field. Using the relationships between Fe  
concentrations and nitrogen fixation in cultures, the data were put 
in the  context of modeled surface Fe concentrations predicted 
from aeolian  dust fluxes. The authors conclude that 
Trichodesmium may be Fe limited  in approximately 75% of the 
world=92s oceans. Clearly, additional factors  are involved in 
controlling Trichodesmium distributions (Letelier and  Karl 1998; 
Sanudo-Wilhelmy et al. 2001), but these findings are  particularly 
relevant given the focus on nitrogen fixation in controlling  past, 
current and future global carbon fluxes (Gruber and Sarmiento 
1997;  Broecker and Henderson 1998).   


The third featured paper in this issue of L&O (Lenes et al. 2001) 
provides  complementary field evidence of stimulation of a 
Trichodesmium bloom  following a dust deposition event in the Gulf 
of Mexico. This paper  provides direct, but correlative, evidence of 
Fe limitation of  Trichodesmium nitrogen fixation and growth. 
During this deposition  event when dissolved iron increased 
Trichodesmium biomass increased  100-fold and dissolved 
phosphorus concentrations decreased. These  results highlight the 
interactions between Fe and other nutrients (Wu et  al. 2000), but 
also demonstrate another important aspect of  Trichodesmium 
bloom phenomena: stimulation of secondary blooms. In  this case, 
the authors believe that the nitrogen fixed during the  
Trichodesmium bloom was roughly equivalent to the nitrogen 
required  by the subsequent dinoflagellate Gymnodinium brevii 
bloom.   


References (the featured L&O papers in this issue are preceded by 
 asterisks)  


*Berman-Frank, Ilana, Jay T. Cullen, Yeala Shaked, Robert M. 
Sherrell,  and Paul G. Falkowski. 2001. Iron availability, cellular 
iron  quotas, and nitrogen fixation in Trichodesmium. Limnol.  
Oceanogr. 46: 1249-1260. 


Boyd, P. W., A. J. Watson, C. S. Law, E. R. Abraham, T. Trull, R.  
Murdoch, D. C. E. Bakker, A. R. Bowie, K. O. Buesseler, H.  
Chang, M. Charette, P. Croot, K. Downing, R. Frew, M. Gall,  M. 
Hadfield, J. Hall, M. Harvey, G. Jameson, J. LaRoche, M.  
Liddicoat, R. Ling, M. T. Maldonado, R. M. McKay, S.  Nodder, S. 
Pickmere, R. Pridmore, S. Rintoul, K. Safi, P.  Sutton, R. 
Strzepek, K. Tanneberger, S. Turner, A. Waite, and  J. Zeldis. 
2000. A mesoscale phytoplankton bloom in the  polar Southern 
Ocean stimulated by iron fertilization. Nature  407: 695-702.


Broecker, W. S., and G. M. Henderson. 1998. The sequence of 
events  surrounding Termination II and their implications for the  
cause of glacial-interglacial CO2 changes. Paleoceanography  13: 
352-364. 


Bruland, K. W., R. P. Franks, G. Knauer, and J. Martin. 1979. 
Sampling  and analytical methods for the determination of copper,  
cadmium, zinc, and nickel in seawater. Anal. Chim. Acta. 105:  
233-245. 


Chisholm, S. W. 2000. Oceanography - Stirring times in the 
Southern  Ocean. Nature 407: 685-687. 


Coale, K. H., K. S. Johnson, S. E. Fitzwater, R. M. Gordon, S. 
Tanner, F. P.  Chavez, L. Ferioli, C. Sakamoto, P. Rogers, F. 
Millero, P.  Steinberg, P. Nightingale, D. Cooper, W. P. Cochlan, 
M. R.  Landry, J. Constantinou, G. Rollwagen, A. Trasvina, and R.  
Kudela. 1996. A Massive Phytoplankton Bloom Induced by  an 
Ecosystem-Scale Iron Fertilization Experiment in the  Equatorial 
Pacific Ocean. Nature. 383: 495-501. 


Cochlan, W. P. 2001. The heterotrophic bacterial response during 
a  mesoscale iron enrichment experiment (IronEx II) in the  eastern 
equatorial Pacific Ocean. Limnol. Oceanogr. 46: 428- 435. 


Dugdale, R. C. 1967. Nutrient limitation in the sea: Dynamics,  
identification and significance. Limnol. Oceanogr. 12: 685-695. 


Fuhrman, J. A., and D. G. Capone. 1991. Possible 
Biogeochemical  Consequences of Ocean Fertilization. Limnol. 
Oceanogr. 36:  1951-1959. 


Gruber, N., and J. L. Sarmiento. 1997. Global patterns of marine 
nitrogen  fixation and denitrification. Global Biogeochem. Cycles 
11:  235-266. 


*Hutchins, David A., Barbara J. Campbell, Matthew T. Cottrell,  
Shigenobu Takeda, and S. Craig Cary. 2001. Response of  marine 
bacterial community composition to iron additions in  three iron-
limited regimes. Limnol. Oceanogr. 46: 1535-1545. 


*Lenes, Jason M., Brian P. Darrow, Christopher Cattrall, Cynthia 
A. Heil,  Michael Callahan, Gabriel A. Vargo, Robert H. Byrne, 
Joseph  M. Prospero, David E. Bates, Kent A. Fanning, and John 
J.  Walsh. 2001. Iron fertilization and the  Trichodesmium  
response on the West Florida shelf. Limnol. Oceanogr. 46:  1261-
1277. 


Letelier, R., and D. Karl. 1998. Trichodesmium spp. physiology 
and  nutrient fluxes in the North Pacific subtropical gyre. Aquat.  
Microb. Ecol. 15: 265-276. 


Martin, J. H., M. Gordon, and S. E. Fitzwater. 1991. The case for 
iron.  Limnol. Oceanogr. 36: 1793-1802. 


Redfield, A. C. 1958. The biological control of chemical factors in 
the  environment. American Scientist. 46: 205-222. 


Rueter, J., D. A. Hutchins, R. W. Smith, and N. L. Unsworth. 
1992. Iron  nutrition of Trichodesmium: establishment of culture 
and  characteristics of N2-fixation, p. 289-306. In E. J. Carpenter, 
D.  G. Capone, and J. G. Rueter (eds), Marine Pelagic  
Cyanobacteria: Trichodesmium and Other Diazotrophs.  Kluwer 
Academic Publishers, The Netherlands. 


Ryther, J. H., and W. M. Dunstan. 1971. Nitrogen, phosphorus, 
and  eutrophication in the coastal marine environment. Science  
171: 1008-1013. 


Sanudo-Wilhelmy, S. A., A. B. Kustka, C. J. Gobler, D. A. 
Hutchins, M.  Yang, K. Lwiza, J. Burns, D. G. Capone, J. A. 
Raven, and E. J.  Carpenter. 2001. Phosphorus limitation of 
nitrogen fixation by  Trichodesmium in the central Atlantic Ocean. 
Nature 411: 66- 69. 


Wu, J., W. Sunda, E. A. Boyle, and D. M. Karl. 2000. Phosphate  
depletion in the western North Atlantic Ocean. Science  289:759-
762.