During this year a number of experiments were carried out to determine productivity of the cultures as a function of several variables, this time studied independently (Laws 1984b, c; see also Laws et al. 1985). One task was to screen candidate species in the laboratory for possible growth outdoors. However, as the author concludes, “the fact that a given species grows well in the laboratory is no guarantee that it will perform well in an outdoor culture system.” One reason the project switched to different algal species was that the P. tricornutum strain used in the experiments described above was quite sensitive to even moderate (above 25°C) temperatures, and required temperature control (cooling) of the reactors. A Platymonas sp. was thus tested without temperature control in the outdoor flumes, at several dilution rates and maximal pH levels of 7 to 8. This strain showed a maximum productivity of about 26 g/m2/d, about the same as observed with P. tricornutum with temperature control.

Another question raised was the reproducibility of the data obtained with the flumes. During this year all four flumes were operated under identical conditions for two periods, resulting in very similar productivities (5% and 10% standard deviations in the two separate experiments). The Platymonas sp. strain was then inoculated into the larger (48 m2) flume which was operated at various depths. A depth of about 12-cm gave the best results, producing about 26 g/m2/d during February 1984.

One important experiment carried out, again with the Platymonas sp., was to determine the effects of N and P limitation on algal lipid content. Lipids increased from less than 20% to almost 40% of dry weight upon combined N and P limitation. However, no actual productivity data were reported for this 2-week experiment.

The major experiment carried out this year (Laws 1984b, c), was the cultivation of Platymonas in the 48-m2 flume, over most of the year. One major variable was dilution, with the culture diluted every 2, 3, or 4 days. As seen in Figure III. B.4., maximal productivity (50 g/m2/d) was achieved by diluting the culture every 3 days. Further investigation of this phenomenon suggested that this was not an artifact, but a reproducible effect. Indeed, such a culture diluted every third day had twice the productivity on the third as on the first 2 days. This was a most unexpected, and controversial, result, and a major focus for the following years of this project.

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Figure III. B.4. Productivity as a function of dilution. (Source: Laws 1984.)