Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 53

TABLE 5.1
Key Requirements for Algal Growth in Relation to PBR Design

Consequences if
Too Low

Consequences if
Too High Key Requirement Function of

Light Insufficient for

photosynthesis,
slow growth

Photo-inhibition,
photo- and
oxidative damage

Reactor surface:volume ratio
Geometry, orientation, and
inclination of reactor
Material and thickness of
reactor walls
Culture depth and density
Mixing

Temperature Slow growth,

dormancy

Cell death Heat input (ambient

temperature, solar radiation,
angle to sun, shading, heat
generation by algal
metabolism)
Heat dissipation (evaporation,
airflow, heating/cooling
mechanisms)

Nutrient provision Growth inhibition Toxicity Media composition

CO 2 provision and O 2 removal
(mass transfer, sparging and
degassing mechanisms, gas
concentration and flow rate,
headspace, gas holdup
volume)
Mixing

Mixing Poor mass transfer

Biomass settling
Anaerobic zones

Shear stress
High energy use

Reactor geometry, mixing
technique (e.g., mechanical,
air flow, gravity flow)

a certain point, cells become photo-inhibited due to damage to the photosynthetic
apparatus, and the photosynthetic rate declines with further increases in irradiance
(Chisti, 2007; Grobbelaar, 2009). In most algae, photosynthesis is saturated at about
1,700 to 2,000 µmol m −2 s −1 , while some plankton are photo-inhibited at much lower
levels (130 µmol m −2 s −1 ). Photo-inhibition occurs rapidly; irreversible destruction can
occur in a few minutes, exceeding 50% damage after 10 to 20 minutes (Pulz, 2001).
In dense algal cultures under high irradiance (e.g., mid-day sunlight), it is likely
that the illumination at the culture surface will be sufficient to induce photo-
inhibition, while that a few centimeters below the surface will be insufficient for
growth. The light conditions experienced by an individual cell within a reactor are
constantly changing as a function of

• Culture depth or optical cross-section (the deeper or wider the culture
vessel, the longer cells spend in low light conditions)