Environmental Engineering Reference
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diversity within the same standing population, at
different depths in the water column and within
a time sequence during seasonal progression.
Studies by Dean
et al
. (2006), for example, on
epilimnion and hypolimnion populations of
Apha-
nizomenon
and
Anabaena
within a water column
demonstrated distinctive subpopulations in the for-
mer case - with surface algae (depth 0-5 m) having
significantly higher levels of carbohydrate com-
pared to those sampled at 10-15 m.
and anionic (P, S, Cl) elements, with other elements -
such as Si, also detectable in some preparations. As
with FTIR microspectroscopy, XRMA can be used
to characterise single-species micropopulations (20-
30 cells) within the mixed phytoplankton sample,
and to make comparisons both within and between
species. Micropopulations can be characterised sim-
ply in terms of elemental frequency distributions
and mean elemental compositions, or by multivari-
ant analysis - carrying out correlation plots, prin-
cipal component analyses and hierarchical cluster
analysis.
The potential use of XRMA to resolve intra-
specific phytoplankton populations is illustrated in
Fig. 2.21, where cells within a colony of
Micro-
cystis
have either a high-Si or low-Si content. The
distinct sub-populations are demonstrated by a clear
bimodal frequency distribution of Si concentration,
which occurred in all colonies analysed and at all
depths sampled within the water column (Sigee
and Levado, 2000). In high-Si cells, Si is associ-
ated with Al and appears to be mainly located at
the cell surface, as indicated by the elemental cor-
relation pattern and by a greater mean diameter
compared to low-Si cells. Both high-Si and low-Si
populations include dividing and non-dividing cells,
and the biological significance of this bimodality
is not known. It is interesting to note that similar
data have also been obtained for another blue-green
alga (
Anabaena flos-aquae)
) in the same water col-
umn (Sigee
et al
., 1999), but analysis of other algae
including
Ceratium
and
Staurastrum
did not show
Si-bimodality.
X-raymi analysis
X-ray microanalysis
(XRMA) provides information on the elemental com-
position of microsamples (such as algal preparations)
and has considerable potential for studying phyto-
plankton populations. The technique combines the
high spatial resolution of electron microscopy (ability
to locate and analyse single algal cells) with the use of
a fine-electron probe and an X-ray detector to collect
generated X-rays (Sigee
et al
., 1993). These can then
be analysed (multi-channel analyser) to produce an
X-ray emission spectrum. Analysis of phytoplankton
samples is more readily carried out using a scanning
electron microscope (SEM) rather than in transmis-
sionmode.SEMpreparationiscarriedoutbydeposit-
ing freshly isolated, washed phytoplankton samples
onto a cellulose membrane, freeze-drying the sample,
then coating with a fine layer of carbon. Chemical
fixation must be avoided (to retain the natural ele-
mental composition) and gold coating (normally used
for conventional SEM) should not be carried out.
A typical X-ray emission spectrum (Fig. 2.21)
shows clear peaks of major cationic (Mg, K, Ca)
B. NON-PLANKTONIC ALGAE
Non-planktonic algae comprise a diverse range of
organisms, differing from phytoplankton in terms
of their substrate-association - which may involve
attachment (fixed location) or free motility (move-
ment over the substrate surface). The distinction
between planktonic and non-planktonic algae is not
absolute, and benthic forms may simply occur as a
seasonal phase or be in dynamic equilibrium with
planktonic populations (Fig. 2.1).
Although many of the non-planktonic algae occur
in the low-light benthic zones of lakes, rivers and
estuaries, they can also be found in environments of
high light exposure. These include the edge of lakes
(littoral zone), shallow streams (Fig. 2.23), exposed
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