PANEL 1

In this sort of "continuous culture" apparatus, fresh medium is pumped in at a steady rate (such as 100 ml/day) into a fixed culture volume (such as 1 liter). As medium comes in culture (media + algae) flows out the overflow at the same rate. The dilution rate is inflow/V. In this example the dilution rate is 0.1 per day.

Algae growing in this apparatus have to eventually grow (after adaptation) at the dilution rate. If they grow too slowly, i.e. if diltution rate is greater than the cell growth rate, the cells will be diluted out. For example if the dilution rate is 0.25 day^-1 and the cells are only growing at 0.2 day^-1, then the cells will be diluted out at a reate of 0.05 d^-1.

 

PANEL B

The medium coming in is all nutrients (with no algae), however the culture is a combination of the dissolved nutrient and the amount of that nutrient in the cells. The total has to add up to the input. For example if there is 100 umol NO3- per liter in the fresh media, then the sum of the soluble NO3- plus nitrogen in the cells has to equal 100 umol L^-1.

The distribution of external soluble nutrient vs. nutrient in the cells is adjusted by cell growth and dilution as described in the next two panels.

 

PANEL C

The growth rate of the aglae depends on the amount of external nutrient with a saturating function (as shown). This can be parameterized as the maximum growth rate and the 1/2 saturation constant and using the saturating growth equation: u = umax * [nut]/([nut] + K1/2)

It takes higher external concentrations of nutrients to grow faster.

 

PANEL D

The continuous culture apparatus, with the combination of growth rates, dilution rates and external nutrient concentration relationships will reach a steady state nutrient concentration and growth rate.

The growth rate will converge on the dilution rate as described above.

The nutrient concentration will adjust to support the growth rate needed to match the dilution rate in the following way.

if the growth rate of the algae is too fast, they take up extra nutrient from the culture, thereby depleting the external nutrient concentration and slowing the growth down

if the growth rate of the algae is too slow, the fresh medium that is diluting the culture comes in faster than the algae are growing and taking up the nutrients, thus the concentration in the vessel will rise

By setting different dilution rates, the culture will reach steady states at different combinations of algal growth rates and external nutrient concentrations (see the small figure at the bottom right of this panel).

 

PANEL E

This culture apparatus is very good at setting a steady state (or letting the algae grow into a steady state) in which the growth rate = dilution rate. There is no net growth at steady state, and thus each point is a Zero Neg Growth point.

In the next lecture we are going to look at how algae can be at ZNG for many combinations of several nutrients, and this forms what we will call a Zero Net Growth Isocline, a ZNGI (pronounced zingy).