No abstract

The use of microalgae suspensions in PAM-fluorometers such as the Water-PAM (Walz GmbH, Germany) presents the problem of maintaining a homogeneous sample. The Water-PAM is marketed with an optional accessory for stirring the sample within the cuvette while in the emitter-detector (ED) unit. This stirring device can help to prevent cells from settling out of suspension over the time-course of chlorophyll-a fluorescence measurements. The ED unit was found to provide a vertically heterogeneous light environment and, therefore, cells within a single sample can exist in different quenched states. Enhancing cell movement by stirring was found to substantially influence measured fluorescence yield while performing induction curve and rapid light curve analyses. This is likely to result from relatively unquenched cells outside the main light-path moving into a higher light region and thus emitting disproportionately more fluorescence than quenched cells. Samples containing cells with high sinking rates or motile species may encounter similar (but reduced) problems. This effect can be mitigated by: (a) reducing analysis time to minimise the distance cells can sink/swim during the measurement procedure and avoiding the necessity of stirring; (b) limiting the proportion of sample outside the light path by minimising sample volume or; (c) by activating the stirrer only for short periods between saturation pulses and allowing enough time after stirring for quenching to stabilise before activation of the saturation pulse. Alternatively, modifications to the instrument providing a vertical dimension to the LED-array could resolve the issue by providing a more homogeneous light environment for the sample.

No abstract

Microalgae growth in closed photobioreactors is greatly inhibited by elevated temperatures caused mainly by the infra-red portion of light. Current passive evaporative cooling systems for temperature control in outdoor photobioreactors are neither economical nor sustainable. Here we built a novel flat plate photobioreactor with its illumination surface customized with insulated glazing units (IGP). The IGP design enabled transmission of more than 50% of visible light while blocking 90% of ultraviolet and infrared radiations. The growth and productivity of Nannochloropsis sp. (MUR 267) in the IGP was compared against conventional flat plate photobioreactors subjected to the full spectrum (HLP) and also externally modified spectrum (CLP) of halogen lights. High temperature (up to 42 o C) resulted in no growth in the HLP. Biomass productivities of Nannochloropsis sp. grown in the CLP was significantly higher than the IGP due to higher light transmission and lower temperature profiles recorded in the CLP. Lipid content of Nannochloropsis was highest in the CLP (60.23%) while protein was highest in the IGP (42.43%). All photosynthesis parameters were negatively affected in the HLP. The IGP's ability to remove infrared (heat) makes this newly developed photobioreactor a promising and sustainable cultivation system for mass algal production especially for high value products.

The mixing of microalgae bioreactors is important as it circulates cells in and out of the light zone and reduces boundary layers, benefiting photosynthesis and nutrient uptake. However, mixing is also a major economical factor, impacting on power consumption and operating costs, and it remains a barrier for those locations where no electrical supply is available. Combining solar panels with algae ponds can help to alleviate the power cost burden, but if solar panels are the sole source of generated electricity for a regional raceway pond system, the paddle wheels will only operate during day time. This study investigated the growth of halophilic green alga Tetraselmis suecica, in outdoor paddle wheel-driven open ponds under three different mixing regimes. The first experimental condition was a continuously mixed regime, while in the second and third conditions, the paddle wheels were stopped overnight and restarted 1 h after sunrise or 1 h before sunrise, respectively. Growth rate, biomass productivity and cell weight showed no statistically significant difference during the period when the culture was growing under optimal environmental conditions. Chlorophyll fluorescence measurements of photosystem II maximum quantum yield in the light (F v ′/F m ′) over a 24-h period found no significant differences between cultures, further suggesting that under favourable conditions ceasing mixing overnight may have no undesirable impact on Tetraselmis health or productivity. These findings imply a potential 37 % power saving and a 33 % reduction in energy-associated costs. During a heat stress event, Tetraselmis was negatively impacted in all ponds to the same degree; however, the continuously mixed pond recovered faster than ponds that were not mixed overnight.