Page 261 - Algae Anatomy, Biochemistry, and Biotechnology
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244 Algae: Anatomy, Biochemistry, and Biotechnology
CULTURE OF SESSILE MICROALGAE
Farmers of abalone (Haliotis sp.) have developed special techniques to provide food for the juvenile
stages, which feed in nature by scraping coralline algae and slime off the surface of rocks using their
radulae. In culture operations, sessile microalgae are grown on plates of corrugated roofing plastic,
which serve as substrate for settlement of abalone larvae. After metamorphosis, the spat graze on
the microalgae until they become large enough to feed on macroalgae. The most common species of
microalgae used on the feeder plates are pennate diatoms (e.g., Nitzchia, Navicula). The plates are
inoculated by placing them in a current of sand filtered seawater. Depending on local conditions, the
microalgae cultures on the plates take between 1 and 3 weeks to grow to a density suitable for
settling of the larvae. As the spat grow, their consumption rate increases and becomes greater
than the natural production of the microalgae. At this stage, the animals are too fragile to be trans-
ferred to another plate and algal growth may be enhanced by increasing illumination intensity or the
addition of fertilizer.
QUANTITATIVE DETERMINATIONS OF ALGAL DENSITY AND GROWTH
Although tedious, time-consuming, and requiring an excellence in taxonomic identifications, the
traditional counting of phytoplankton is still unsurpassed for quantifying plankton, especially at
low limits of detection. The examination and counting of preserved material also allows for
direct observation and assessment of cell condition. The fundamental issue that must be addressed
prior to choosing a counting technique is whether samples must be concentrated. The three routine
methods used to concentrate phytoplankton samples are centrifugation and filtration for live
samples and sedimentation (gravitational settling) for preserved samples. Centrifugation and sedi-
mentation are the most commonly used, although filtration onto membrane filters is an effective
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procedure for fluorescence microscopic enumerations. If the cell density is less than 10 cells l ,
the sample needs to be concentrated; if cell density is too high to allow direct counting,
the sample will be diluted. Dilution or concentration factors need to be taken into account for
calculating the final cell concentration.
Species containing gas vacuoles (cyanobacteria) are unlikely to fully sediment despite the
Lugol’s fixation. Two methods are available for collapsing vacuoles to assist sedimentation.
Samples can either be exposed to brief ultrasonification (,1 min, but may vary for different
species) or alternatively, pressure can be applied to the sample by forcing it through a syringe
with a fine needle; this will collapse the vacuoles and the cells will then settle through gravity.
If the water sample contains sufficient number of algae and concentration is not required,
a direct count can be undertaken. The sample is thoroughly mixed, treated with a few drops of
Lugol’s solution, mixed again and allowed to stand for 30–60 min. The sample is mixed again
and the subsamples are used for direct counting and taxonomic identification.
When concentration is required, a simple procedure for gravitational settling is to pour a well-
mixed volume of the sample of water containing the microalgae in a measuring cylinder (100 ml),
add Lugol’s solution to it (1% by volume), and either allow the sample to stand overnight or use a
centrifuge so that the cells sink. The iodine in Lugol’s solution not only preserves and stains the
cells but also increases their density. When the column of water appears clear, the top 90 ml
will be gently siphoned off without disturbing the sediment at the base of the cylinder. This
leaves the cells concentrated in the bottom 10 ml. Subsamples of this sedimented fraction can be
used for counting procedure and examined under the microscope for identification, keeping in
mind that the cells of the original sample have been concentrated 10 times.
At present, there are many kinds of counting procedures available for enumerating phytoplank-
ton, depending on whether counting algae in mixtures, as from field sampling, or unialgal samples,
such as in growth or bioassay experiments in a laboratory. Some techniques are relatively “low-