I am attempting to isolate picocyanobacteria from seawater (pre-filtered with a GF/D membrane) using the pour plate technique and filter plating method (Kearney et al., 2022) on Pro99 and SNAX agar media (0.3% agar) supplemented with cycloheximide to suppress the growth of eukaryotic plankton. After a few days of incubation under a 12/12 h light-dark cycle, I saw many bacterial colonies, which were likely to be heterotrophic bacteria (gram-negative, rod-shaped). For the filter plating method, small mucoid colonies were seen on the filter paper. For the pour plate technique, white turbid colonies were found throughout the agar. Despite extending the incubation period to a month, only a few cyanobacteria colonies were obtained on some agar plates.
I also used Pro99 and SNAX broth in hopes of enriching cyanobacteria before isolation, but the media turned white and turbid instead of green or any other color typically associated with cyanobacteria.
Is it common for heterotrophic bacteria to grow on these media?
According to the recipe to make these media, how is this possible since no organic carbon source is included?
This seems a common issue. Also, in the paper you mention (Kearney et al., 2022) it was stated that cyanobacteria need helper bacteria so bacterial growth on this medium is somewhat desirable. As for the organic carbon content - natural seawater does contain some dissolved organic carbon, especially if it was procured from a coastal region. This can be reduced by "aging" the water (https://www.researchgate.net/post/Why-must-seawater-be-aged-for-use-in-media-prep). Using a high purity agar (Noble agar, electrophoresis agar) can also help minimize the carbon content.
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