There are several methods for measuring bacteria in seawater.
The most common method, heterotrophic plate count (HPC), involves spreading the
water sample on a culture plate, waiting for bacterial colonies to grow, and
then counting the number of colonies. Unfortunately, this growth period can
take up to 21 days. By the time you get results, biofilm may have already
clogged the membranes. Another issue with HPC is that you can only count the bacterial
colonies capable of growing on a culture plate (1–10% of all bacteria species),
which means a majority of bacteria are missed from this method.
A faster method
called flow cytometry involves running samples through a specialized instrument
that automatically counts the number of bacteria in the sample. However, the
number of bacteria does not necessarily reflect the biological activity of
bacteria. “We can have one million bacteria in seawater, but it means nothing
if these bacteria are not active,” says Motasem. Another issue with flow
cytometry is that it often underestimates the number of bacteria if they are
clumped together—it might count a clump of a few thousand bacteria as one.
The ideal method for Motasem’s application would be fast,
reliable and measures all active biomass. This is possible with an assay that
measures the concentration of a tiny molecule called adenosine triphosphate
(ATP)—the energy currency of all living cells. “Through ATP testing, we can
know the activity of the bacteria. Maybe there’s only 1,000, but they could be
very active,” says Motasem. He found that there were ATP testing kits available
for testing freshwater, such as drinking water. However, they did not work with
seawater because salt interfered with the reaction. “We were looking for a way to measure ATP in seawater because it's clearly a very
good method; very fast, very robust. It gives a lot of information within
Motasem collects seawater samples from a sampling point of a full-scale seawater desalination plant in Oman.