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Holy grails are difficult to catch- part 1

November 19, 2010
Isochrysis sp.

Isochrysis sp. due to its DHA content, a most valuable species (c) Bernd Kroon

The current state of economical development of algal production systems, after more than 70 years of both academic and commercial driven efforts, is in strong contrast with the vast array of potential applications of algal biomass and its commercial and societal relevance, despite recent biofuel-hype driven funding of many algal research or commercial projects.



In this post, part of a series of 3 posts (I think 🙂 ), I would like to focus a bit on the intrinsic problems that arise whenever one tries to  produce phytoplankton at reasonable costs – costs which would allow a successful entry into one or the other market segment. The text contains some mathematics, simply because it is the easiest way to explain processes, but even without understanding these simple equations, one can still get the point – just don’t give up reading. The kind of argumentation which I put forward here is not undisputed, but again, that is what makes blogs potentially useful: to offer a platform to voice complementary and even contrary opinions.

Marine phytoplankton are a diverse group of organisms that belong to the plant kingdom. Just like any other land plant, they use light as an energy source, and from their surroundings they capture carbon dioxide (CO2), which is used to synthesize the organic building blocks, that make up most of their biomass. Putting numbers to this process, 1 kg of phytoplankton requires the uptake of about 1.6 to 1.8 kg of carbon dioxide.

Land plants capture carbon dioxide from the air, phytoplankton pull the carbon dioxide out of the sea water. As the phytoplankton continues to grow, the surrounding sea water becomes depleted, or under-saturated, with respect to the amount of carbon dioxide that the water could contain, if no phytoplankton would be around. The balance is restored by a diffusive process, where the earth’s air at the surface of the ocean replenishes the carbon dioxide into the sea water. This process is part of, accidentally, what is called the biological carbon pump, without which life on earth would be impossible.

Under normal (natural) conditions there is a stable, dynamic equilibrium, where two processes (uptake and replenishment) keep each other in balance. If this balance, for any reason, is disturbed, then there will be a measurable change in the composition of the seawater. For example, if the CO2 in the earth’s atmosphere increases more than the natural fixation of carbon dioxide by the growth of phytoplankton, then the effect is one where the ocean acidifies (pH goes down) . In fact, recent studies have irrevocably shown that this is now happening on a global scale (look for recent publications by Ulf Riebesell et al.).

To understand the opposed situation, namely an alkalization (pH goes up) of the ocean, it is of help to understand the concepts of growth rate (Rx), productivity (P) and biomass density (X, or standing crop). In microbiology or biotechnology, the term Rx often expresses the rate of increase in biomass per unit volume per time and has the dimension of kg per m3 per day. Rx itself is defined by two different parameters:

Equation <1> Rx = μ · X

where μ is the specific growth rate : a kinetic parameter that quantifies the rate of increase of a given amount of biomass. It has the dimension of 1/(time unit), for example 1/day and can be derived from experimental data using equation <2>:

Equation <2> μ = 1/X · d(X)/d(t)

where d(X)/d(t) stands for rate of change in biomass as measured by a sufficiently small time interval, the so-called derivative of biomass with respect to time;

and X is the standing crop: a mass parameter with the dimensions of kg per volume, for example, kg/m3.

It must be noted, that the specific growth rate is not the same as the biomass doubling time (td), which can be derived from μ by taken the inverse value of μ and multiplying it with the natural logarithm of the number two.

The volumetric productivity (see equation <3>), expressed as produced kilograms of desired biomass per day, now follows by multiplying Rx with the volume in which the desired organism is growing .

Equation <3> P = Rx · V

where V represents the volume of the production system in m3.

The interested reader will see that the volumetric productivity transforms into the areal productivity by dividing the former with the depth of the water body, the depth being taken perpendicular to the direction of the footprint of the system (in which the phytoplankton grows) on the earth’s surface.

Equations <1>,  <2> and <3> show that a high biomass density, if combined with a high specific growth rate, leads to a high growth rate and also to a high productivity (without increasing the production volume).

Logically, any commercial production system should aim for high productivities in order to produce phytoplankton at minimal costs. Take it for a fact that harvesting the cells from water is an economically costly process, it becomes easy to grasp that  high biomass density is a knife with two sharp edges. In other words, one aims to achieve high concentrations of biomass, while creating conditions where the biomass retains the ability to express high specific growth rates. With phytoplankton, higher growth rates are only possible when certain light levels are available (only light energy fuels the growth process) ; unfortunately, since the cells grow in a water environment, high cell concentrations also lead to strong diminution of light as the light travels through the culture suspension.

It is therefore normally not possible to have high growth rates, in large volumes, at high cell densities. Which explains, and this is the this most important point of this post, why it is so difficult (and most large scale facilities fail at this most primary task) to produce phytoplankton (as governed by equation <3>) in large amounts at economic feasible ways.

Technology offers the only way out of this dilemma. I’ll try to continue with another post tomorrow to look at technological solutions.

3 Comments leave one →
  1. January 2, 2011 11:49 AM


    • Bernd Kroon permalink
      January 2, 2011 11:51 AM

      In a way it is.


  1. The Blog ‘Marine Agriculture’ 2010 in review « Marine Agriculture

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