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-
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