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Fig. S1. Reproducibility of diatom-biofilm growth as followed by turbidity measurement (A) and linear correlation of turbidity and chlorophyll a (chl-a) content of the biofilms (B). Growth systems were inoculated in parallel and incubated under continuous flow of culture fluid, in this example (A) axenicmarine diatom P. tricornutum (n = 3). For the example shown here (A), the turbidity readings were recorded every 10 min, and such ‘oversampling’ of data appeared an oscillation of the baseline that represented the periodical exchange of culture fluid in the flow lanes (i.e. the filling/overflow intervals, see text); a short period of tenfold-increased medium flow (30 min, 15 ml h−1) is indicated by an arrow. (B) A sufficiently linear correlation (R2 = 0.97) between biofilm turbidity reading and chlorophyll a content throughout growth of the biofilms was confirmed when individual biofilms were sacrificed at intervals for chlorophyll a determination (see Experimental procedures). In the example shown (B), axenic freshwater diatom Planothidium sp. was used (n = 6). We calculated with a Planothidium-specific conversion factor of 1.5 for changing arbitrary biofilm turbidity-units (B) into chl-a content per surface area (μg chl-a cm−2) for the calculation of Planothidium sp. biofilm growth rates in later experiments (see main text).

Fig. S2. Effect of treatment of pre-established Planothidium sp. biofilms with an algicidal agent, as followed by biofilm-turbidity measurement (left) and macroscopical appearance of the biofilm (right). The photographic illustrations refer to the time points indicated in the biofilm-turbidity measurement. The treatment (3 days) resulted in bleaching of diatom biofilm from naturally brown to colourless during the following incubation (4 days) with growth medium (Fig. 5). The eradication of phototrophic biofilm was resembled in the biofilm-turbidity measurement (Fig. 5) as a peak observed directly after the start of the treatment (1 day), followed by stagnation (2 days) and gradual decrease (4 days) of the biofilm turbidity to approximately 75% of the maximal value before treatment. The remainder turbidity observed (≅ OD600) was attributed to the opacity of debris of bleached diatom biofilm in the flow lane, as indicated by microscopy (not shown).

Fig. S3. Supplemental information on the characteristics of the LEDs used. Relevant excerpts are shown of the Technical Datasheet for LED ‘LM560A’ provided by the manufacturer Seoul Semiconductor, Korea.

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