Environmental Engineering Reference
In-Depth Information
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Eyepiece micrometer scale
Algal filament
(a) Measurement of filament length in eyepiece scale units
Figure 2.14
Measurement of algal
filament length: use of eyepiece and
stage micrometers. In this illustra-
tion, the algal filament (composed of
7 cells) has a length of 46 eyepiece
units (a); 50 eyepiece units are equiv-
alent to 33μm (b), so the length of the
filament in absolute units is 30.4μm.
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30
40
50
Eyepiece micrometer scale
0
10
20
30
Stage micrometer scale
(
μ
m − micrometers)
(b) Calibration of the eyepiece scale
size. Simple colony counts are difficult to interpret,
and more useful approaches for enumeration are
either to measure colony sizes and determine biovol-
umes (see Section 2.5.4) or to estimate the number
of cells contained within the colonies. Estimating cell
numbersisnotnormallycarriedoutonaroutinebasis,
but can be carried out either by breaking up colonies
into their constituent cells or by estimating cell num-
bers from colony size (Reynolds and Jaworski, 1978).
number of cells (
y
) in a healthy quasi-spherical
colony of
Microcystis
can be approximated from
its diameter (
x
) by the regression equation:
Log
10
y
= 2
.
99log
10
x
− 2
.
80
(2.4)
Although all of the above methods give compa-
rable estimates of single cell counts of
Microcystis
,
the authors consider that the regression method is
least useful because of variation both in colony shape
(often not approximating to a sphere) and also of
cell packing (cell number per unit volume) within the
mucilaginous matrix.
Colony fragmentation.
Colonies of algae such
as
Microcystis
can be broken up by chemical
(alkaline hydrolysis) or physical (sonication) pro-
cedures. Reynolds and Jaworski (1978) carried
out alkaline hydrolysis by adding a single pellet
of sodium hydroxide to 100 ml of phytoplank-
ton suspension, then heating the alkaline suspen-
sion at constant temperature (90
◦
) for 30 min.
This normally causes complete disruption of the
mucilaginous colonies, allowing individual cells of
Microcystis
to be counted. The use of sonication
can also be very effective in disrupting colonies.
Reynolds and Jaworski (1978) found that 1 min
ultrasound treatment (∼12 μm, 20 kHz) of phyto-
plankton suspension resulted in homogeneous cell
suspensions, but the required duration will clearly
varywiththespeciicdetailsofphytoplanktonsam-
ple and sonication apparatus.
The Lund nanoplankton counting chamber
Although the Sedgwick-Rafter slide is ideal for
medium- to large-sized algal species, it is difficult to
visualise and count smaller species - which may thus
be entirely missed. This problem arises because high-
power lenses, with their small depth of focus, cannot
be used due to the depth of the counting chamber and
the thickness of the coverslip (non-inverted micro-
scope) or the thickness of the slide itself (inverted
microscope).
The optical problems encountered with the
Sedgwick-Rafter slide have been largely overcome
with the introduction of the Lund nanoplankton
counting chamber, which is ideal for counting small
phytoplankton cells (Fig. 2.15). This has reduced the
Cell estimates from colony size.
According to the
observations of Reynolds and Jaworski (1978) the
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