[L&O Featured Article] L&O Featured Articles, Vol. 50 (5) September 2006

[lo-feature at aslo.org]

There are three Featured Articles in the September 2006 issue of L&O.  They
are:

Tortell, Philippe D., Cheryl L. Martin, and Miranda E. Corkum. 2006.
Inorganic carbon uptake and intracellular assimilation by subarctic Pacific
phytoplankton assemblages. Limnol. Oceanogr. 51: 2102-2110.

http://aslo.org/lo/toc/vol_51/issue_5/2102.pdf  

Martin, Cheryl L., and Philippe D. Tortell. 2006. Bicarbonate transport and
extracellular carbonic anhydrase activity in Bering Sea phytoplankton
assemblages: Results from isotope disequilibrium experiments. Limnol.
Oceanogr. 51: 2111-2121.

http://aslo.org/lo/toc/vol_51/issue_5/2111.pdf 

Downing, J. A., Y. T. Prairie, J. J. Cole, C. M. Duarte, L. J. Tranvik, R.
G. Striegl, W. H. McDowell, P. Kortelainen, N. F. Caraco, J. M. Melack, and
J. J. Middelburg. 2006. The global abundance and size distribution of lakes,
ponds, and impoundments. Limnol. Oceanogr. 51: 2388-2397. 

http://aslo.org/lo/toc/vol_51/issue_5/2388.pdf

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Tortell et al., 2006 and Martin and Tortell, 2006

Introductory Comments by John Raven (L&O Associate Editor)

The availability of inorganic carbon is not as great a determinant of the
rate of primary production by marine phytoplankton as are nitrogen,
phosphorus, iron, and (possibly) zinc in different parts of the ocean. This
general lack of inorganic carbon limitation is a result of the widespread
occurrence of inorganic carbon concentration mechanisms (CCMs). CCMs work by
concentrating CO2 at the site of the core carboxylation enzyme ribulose
bisphosphate carboxylase-oxygenase (Rubisco). This overcomes the enzyme’s
relatively low CO2 affinity and inhibition by O2, and the characteristics of
the inorganic carbon system in seawater, which would restrict primary
productivity if CO2 moved from bulk seawater to Rubisco by diffusion alone.
CCMs involve the entry of CO2 and/or HCO3-, the active transport of one or
both of these species, the activity of carbonic anhydrases (CAs) in one or
more compartments within the cell, or outside the cell, and possibly C4
metabolism.

Laboratory studies on algal cultures were crucial in reaching our current
understanding of the mechanism, regulation, and phylogenetic occurrence of
CCMs.  While studies on natural phytoplankton assemblages have shown that
CCMs occur in the real world as well as in the laboratory, more work is
needed on the variety of CCMs expressed as a function of location, season,
and the diversity of organisms. Such studies are especially urgent in view
of the effects of anthropogenically induced changes (increase in
concentration, and changes in speciation), of inorganic carbon in surface
seawater.

The featured articles in this issue of L&O by Tortell, Martin and Corkum,
and by Martin and Tortell, provide important new information on the nature
of the CCMs in two colder areas of the ocean. This work on the Bering Sea
and the NE Subarctic Pacific complements and extends earlier studies on the
equatorial Pacific and the Southern Ocean. Tortell’s group, on separate
cruises to the two areas, used a variety of techniques to study CCMs in
natural assemblages as a function of time and location. The methods used
included carbon isotope disequilibrium studies of the roles of CO2 and HCO3-
in the overall uptake of inorganic carbon, with an excellent description of
the methodology, including a novel analysis of the uncertainty in the
estimates of parameters. Other measurements made were the determinations of
extracellular, and total cell, CA activity, and the activity of Rubisco and
phosphoenolpyruvate carboxylase (PEPc). PEPc is an essential catalyst in the
majority of phytoplankton organisms of the production of carbon skeletons
required for nitrogen assimilation and growth, and also for the C4
photosynthetic pathway, which may constitute (or contribute to) the CCM in
diatoms.

These papers show that, as in the equatorial Pacific and the Southern Ocean,
HCO3- rather than CO2 is the predominant inorganic carbon species used by
phytoplankton. Furthermore, the low extracellular CA activity shows that
HCO3- uptake, rather than the extracellular conversion of HCO3- to CO2 with
subsequent uptake of CO2, accounts for the majority of HCO3- use. This is
true for all stations regardless of the ambient dissolved CO2 concentration,
is also the case for short term manipulations in which natural assemblages
are acclimating to a range of CO2 concentrations. The work also extended,
and applied to natural populations, the finding that the inhibitor used to
inhibit extracellular CA also partially inhibited HCO3- uptake, so that
there is the likelihood that the fraction of inorganic carbon uptake
attributed to HCO3- uptake is underestimated. 

The work on CA showed activity in almost all samples, and that CA expression
was highest when CO2 was lowest, in agreement with work on laboratory
cultures. The PEPc:Rubisco activity ratio showed a positive correlation with
sea-surface CO2 concentration, in contrast with the data on laboratory
cultures of a marine diatom with C4-like photosynthetic metabolism. However,
the authors are careful to point out that the field data on the PEPc
activity almost certainly includes non-photosynthetic micro-organisms with
the enzyme, so the results should not be over-interpreted in relation to
phytoplankton.

While the results and interpretations in these two featured papers raise
more questions than they answer, the work provides an excellent practical
and theoretical background for further work.

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Downing et al, 2006

Introductory comments by Joe Ackerman (L&O Associate Editor) 

As we know, the earth looks very much like a blue marble from space due to
the fact that ~ 71 % of the surface is covered by oceans, which contain ~
97% of the volume of water on our planet.  Not surprisingly, we tend to
minimize the importance of, or even neglect, the aerial coverage of
continental waters in global considerations and models. This is due in part
to the difficulty in partitioning the remaining ~ 3% of freshwater among and
within the reservoirs that include glaciers and other land ice, groundwater,
rivers, and lakes.  In the case of the latter – lentic systems – the
question of spatial scale and type emerge as the most problematic issues.
For example, it is quite easy to enumerate large lakes (> 1 km2) and even
the largest lakes (> 105 km2), but how does one account for the smaller
basins of less than a 10th of a hectare or 0.001 km2 in size (i.e., a basin
< 36 m in diameter) be it a lake, a prairie pothole, a dam or an
agricultural impoundment?  This is not a trivial task as many of these water
basins are smaller than the mapping resolution – hence the difficulty in
developing a sense of the global estimate of lentic systems.

The featured L&O article by John Downing and his 10 co-authors addresses the
issue of the global estimate and size distribution of lakes, ponds and
impoundments by incorporating new data and techniques in their analysis.
Their research builds on the well recognized power relationship between the
lake frequency (dL) and lake area (A), which the late Robert Wetzel (1990)
suggested had an exponent of ~ 1 for large lakes. When other size classes of
water basins were included from newer sources and GIS informed regional
observations, the authors recognized that there was good agreement for lake
size class and a more general power law presented by Lehner and Döll (2004).
These and earlier estimates suggest that large lakes contribute most to the
total of 2 – 2.8 x 106 km2 of continental land area, which represent 1.3 –
1.8% of land surface.

Downing et al. recognized that their data followed a truncated lognormal
distribution (probability density function; PDF) that can be modelled by the
Pareto (or Bradford) Distribution.  The Pareto PDF was used originally to
model the distribution of wealth in which 20 % of the people owned 80 % of
the wealth (Pareto’s Principle).  In this case, the authors demonstrated
that this PDF can be used to model the frequency of lakes that contribute to
lake area in each size category of basin under consideration.  Using this
observation and the previous data sets, they suggest that the smallest two
categories, ponds and small lakes > 0.001 km2 (i.e., 1 Ha), contribute the
most to the lacustrine area, which totals 4.2 x 106 km2 of continental land
area or 2.8 % of land surface.  They extend this estimate further by
examining the ever increasing number of dams and farm ponds, which
contribute another 0.077 x 106 km2 and 0.26 x 106 km2 of area, respectively,
which increases the estimate of aerial coverage of freshwater to > 3 % of
land surface.

This research is significant for a number of reasons least of which is the
knowledge of the natural system and how it is/might be changing.  From an
ecological and biogeochemical perspective, there is a potential for a great
deal of “edge effect” in these smaller basins that may contribute greatly to
global estimates of productivity, nutrient budgets, and elemental cycles.
Moreover, this observation needs to be accounted for in regional and global
estimates as well in modeling efforts directed at the aforementioned
processes.  The first step in understanding a system is characterizing it,
and Downing et al. have made an important step in that direction.