3.3 Bacterial contamination of algal cultivations
Waste streams rich in organic carbon may also contain abundant heterotrophic
microorganisms. In our preliminary cultivation of algae in these wastewaters we
found that sterilization of the waste streams was necessary to avoid adverse contamination
originating from the waste stream. At industrial scale, sterilization may
not be feasible and thus we assessed how reduction of contaminants prior to
cultivation affected subsequent algal growth. Quantitative PCR (qPCR) with appropriate
primers was used to estimate the number of bacterial and algal cells
during the cultivation of Selenastrum sp. in small scale laboratory experiments
designed to test whether the growth of contaminating bacteria during the cultivation
was a reason for poor growth of algae in some conditions. To validate the
qPCR results, which is a relatively new method for determination of algal growth,
microscopy was also used to determine algal and bacterial cell numbers.
Process water, press water and pre-composting water were used to assess
bacterial growth. All waste waters were sterilized and used at two dilutions. Algae
were grown in non-sterile environments. Initially, the number of bacterial cells was
below the detection limit, but bacterial populations developed during the cultivation.
In only one out of six experiments did strong bacterial growth correlate with
poor algal growth. In the other cultivations, algal and bacterial growth curves were
similar, implying that the populations could co-exist and did not out-compete each
other. Thus bacterial contamination would probably not inhibit algal growth if bacterial
numbers can be reduced, e.g. by sterilization of the waste stream, at the
beginning of the process. Since complete sterilization is unlikely to be feasible at
an industrial scale, other approaches to limiting bacterial growth will be needed.