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The dynamics of phytoplankton growth and replication in controlled conditions

There is another way to address the potential rates of cell replication of phytoplankton and that is through the dynamics of isolates under and carefully controlled conditions in the laboratory. We might begin by assessing how well the cells of a given species perform under the most idealised conditions it is possible to devise. Once an optimum performance is established, further experiments may be devised to quantify the influences of each of the suspected controlling factors. Finally, the various impositions of sub-ideal growth conditions can be evaluated. The following sections apply this approach to a selection of freshwater species of phytoplankton.

5.3.1 Maximum replication rates as a function of algal morphology

The collective experience of culturing isolates of natural phytoplankton in the laboratory has been summarised by Fogg and Thake (1987). The fastest rates of species-specific increase are attained in prepared standard media, designed to saturate resource requirements, when exposed to constant, continuous light of an intensity sufficient to saturate photosynthesis, and at a steady, optimal temperature. Even then, maximal growth rates are not established instantaneously. There is usually a significant 'lag phase' during which the inoculated cells acclimatise to the ideal world into which they have been introduced. Within a day or two, however, the isolated population will be increasing rapidly and, generally, will be doubling its mass at approximately regular intervals. It is early in this exponential phase that the maximal rate of replication is achieved, when r' is supposed to be equal to the observed net rate of increase, rn, solved by Eq. (5.2). Later, as the resources in the medium become depleted or the density of cells in suspension begins to start self-shading, the rate of increase will slow down considerably (eventually, the stationary phase). Care is taken to discount the biomass increase in this phase from the computation of the exponent of the maximum specific growth rate, r'.

The fastest published rate of replication for any planktic photoautroph is still that claimed for a species of Synechococcus (at that time named Anacystis nidulans) by Kratz and Myers (1955). At a temperature of 41 °C, the Cyanobacterium increased its mass 2896-fold in a single day, through the equivalent of 11.5 doublings (tG = 2.09 h), and sustaining a specific rate of exponential increase (r') of 7.97 d-1 (or 92.3 x 10-6 s-1). Numerous other algal growth rates are recorded in the literature cover wide ranges of species, culture conditions and temperatures. Several notable attempts to rationalise and compile data for interspecific comparison include those by Hoogenhout and Amesz (1965), Reynolds (1984a, 1988a) and Padisak(2003). The selection of entries in Table 5.1 is hardly intended to be comprehensive but it does refer to standardised measurements, made at or extrapolated to 20 °C, on a diversity of organisms of contrasting sizes, shapes and habits. The temperature is critical only insofar as it is uniform and that it has been a popular standard among culturists of microorganisms. It is probably lower than that at which most individual species (though not all) achieve their best performances (see Section 5.3.2 below).

The entries in Table 5.1 show a significant range of variation (from ~0.2 to nearly 2.0 d-1). There is no immediately obvious pattern to the distribution of the quantities - certainly not one pertaining to the respective phylogenetic affinities of the organisms, nor to whether they are colonial or unicellular. The variations are not random either, for in instances where more than one authority has offered growth-rate determinations for the same species of alga, the mutual agreements between the studies has been

Table 5.1 Maximum specific growth rates (r20 d reported for some freshwater species of phytoplankton in laboratory cultures, under continuous energy and resource saturation at 20°C

Phylum

Species

r2o d 1

References

Cyanobacteria

Synechococcus sp.

l.72a

Kratz and Myers (l955)

0 0

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