[L&O Featured Article]Vol. 49, Issue 1, January 2004

[lo-feature@aslo.org]

Current Featured Article

The Featured Article in the January 2004 issue of L&O is:

De La Rocha, Christina, L., and Uta Passow. 2004. Recovery of 
Thalassiosira weissflogii from nitrogen- and silicon-starvation. 
Limnology and Oceanography 49: 245-255.

This paper is freely available at this Web address:

          http://aslo.org/lo/toc/vol_49/issue_1/0245.pdf

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

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

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Introductory comments by John Raven:

The featured article in this issue adds to the list of possible 
functions of silicon in diatoms. The suggested function is novel in 
that it concerns the ecological and evolutionary consequences of 
silicon deprivation in diatoms, rather the more obvious consequences 
of the presence of silicon in the environment and of silicified 
structures in the diatoms.

The suggestion made by De La Rocha and Passow concerns the different 
effects on a diatom of limitation by silicon and limitation by 
nitrogen on the performance of the cells when nutrients are re-
supplied. Flynn and Martin-Jezequel (2000) suggested, on the basis of 
modelling, that silicon-deprived diatoms will recover more rapidly 
than nitrogen-deprived diatoms upon re-supply of nutrients. De La 
Rocha and Passow have experimentally verified this suggestion for 
Thalassiosira weissflogii, and have carried out further modeling 
exercises. The experimental findings are that, starting from closely 
similar lengths of nutrient deprivation and of population sizes of 
nutrient deprived cells, the nitrogen-starved cells do not catch up 
with the silicon-starved cells within the two days of the experiments 
after nutrient re-supply. Modeling indicates that this difference 
should be perpetuated over at least nine days.

Like all good research, more questions are raised than are answered. 
One requirement for future work is to extend the analysis 
experimental analysis to phytoplankton cells other than diatoms. De 
La Rocha and Passow point out that the changes seen in nitrogen-
deficient diatoms also occur in non-diatom species, so that their 
hypothesis could relate ecologically to a situation in which diatoms 
run out of silicon and other phytoplankton species running out of 
nitrogen. In a situation with a low silicon:nitrogen  ratio in the 
surface ocean where diatoms run out of silicon well before other 
phytoplankton cells run out of nitrogen, what, all else being equal, 
is the outcome as the smaller populations of silicon-starved diatoms 
compete with the larger populations of nitrogen-starved cells of 
other groups of algae when nutrients are re-supplied? Does a more 
rapid recovery of the diatoms from a smaller population base still 
give an advantage in terms of population size at, say, 10 days after 
re-supply relative to the population sizes of non-diatoms? De La 
Rocha and Passow suggest that the diatoms would be at an advantage, 
citing the generally high specific growth rate of diatoms as a 
contributing factor in speeding the growth of diatom populations.

A possibility for future cogitation, if not experimentation, is the 
evolutionary significance of the hypothesis. Could the selective 
advantage in running out of a resource, i.e. silicon, which few non-
diatom competitors need, as a means of maximizing the long-term 
survival and growth potential of the silicon-requiring cells in an 
environment with fluctuating resource supply, be an evolutionary 
rationale for the silicon requirement in the first place? At first 
sight this seems unlikely, since the phenomenon elaborated by De La 
Rocha and Passow involves a silicon sequestration by diatoms 
sufficient to cause local depletion of the resource. Such a 
quantitatively significant sequestration would have to involve 
production of a solid mineral phase with implications for cell 
density. Further, if the mineral phase was externalized as in fossil 
and extant diatoms, there would also be implications mentioned by De 
La Rocha and Passow for grazing, ultraviolet absorption and the 
functioning of extracellular carbonic anhydrase. These lines of 
reasoning suggest that the role of silicification suggested by De La 
Rocha and Passow is an important emergent property of silicification 
in diatoms, but is probably not the original selective advantage of 
the silicon requirement.

Flynn, K. J., and V. Martin-Jézéquel. 2000. Modelling Si-N-limited 
growth of diatoms. J. Plankt. Res 22: 447-472

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