growth of Spirulina sp. is largely suppressed when the pH of the
medium is above 11.3 [11]. In the present study, the decreases in
the amperometric signals at the initial stage of the stationary phase
(after the 24 days of cultivation) resulted from the decline of the
metabolic and photosynthetic activities of the Spirulina cell based
on the low carbon consumption with pH variation (Fig. S4). When
the cultivation was prolonged over 54 days, the color of the S. maxima
changed from blue-green to brownish yellow (Fig. 2), which
could be attributed to the loss of chlorophyll and phycocyanin [16].
At this stage, amperometric monitoring of the cyanobacteria cultivation
was not possible with this photoelectrochemical cell due to
the increased noise with a very weak signal (results not shown).
In the present study, we aimed to establish the basic principles
of our method, which should be viewed as a “broad indicator” for
the measurement and growth monitoring of the cyanobacteria in a
cultivation process, with an efficient rapid test time of 10–30 min.
At present, an interpretation of electron transport mechanisms
between the light harvesting system of the cyanobacteria and the
vicinity of a poised working electrode isnot completelyunderstood.
Therefore, our current study focused on the optimization of sensor
systems for practical applications with various biological and
electrochemical experimental conditions.
4. Conclusions
In this article, we report on an application of photoelectrochemical
sensor for the growth monitoring of multicellular filamentous
cyanobacteria. During the cultivation period (over 54 days), the
strength of sensor signals was synchronized with the concentrations
of chlorophyll a and not largely related with the DCW. These
results conveyed that the amperometric signals from the system
are closely dependent on the metabolic activities of the cyanobacteria.
In the future, we will investigate the feasibility of the system,
especially for on-line monitoring of a cultivation process, as well
as carry out a modification of the system in order to enhance the
applicability of the system to practical microalgal industries.