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Holy Grails are difficult to catch – part 2

November 20, 2010

Isochrysys sp (c) Bernd kroon

The previous post concluded that light availability basically limits the productivity of algal systems if they are left on their own. One has to use technology to grow phytoplankton economically sound, designed to achieve high productivities (= high densities multiplied with high specific growth rates).

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While there are many different technologies thinkable that can overcome the nature-given energy (=light) constraint, they all have one aspect in common: the total culture volume is distributed in many smaller units, which each have a geometry that allow the penetration of available sunlight in as much of the overall desired production volume as possible.

The most promising production sites are located at a geographical position where high light intensity is combined with high ambient temperature (e.g. deserts). Where the former is always good as production parameter, the latter is not always good. Besides the existence of a lethal temperature beyond which life is not possible (and the ambient temperature in most deserts is close to the biological limit for almost all phytoplankton species), temperature also exerts strong effects on chemical processes (the Arrhenius law, first formulated by the Dutch scientist van ‘t Hoff,  says that the speed of a chemical process doubles with every 10 degrees Celsius increase).

Sea water is a very complex mixture of many salts dissolved in water. There are many sats that can react with each other, and thereby, form all kinds of other salts that will not be present at lower salt concentrations. Some salts are highly soluble, others less so. The reactivity of a mixture of chemicals, as explained by the Arrhenius law, quickly increases as temperature increases. Another factor plays a dominant role in the reaction rate, and the solubility, of chemicals, namely the acidity, or pH (pH ranges from 0 to 14; a medium with a pH of 7 is neutral, below 7 it is acid, above 7 it is alkaline). A high pH implies the presence of a high concentration of hydroxide ions in water. Hydroxide ions strongly react with many salt ions leading to salt hydroxides. Many so called hydroxides, which are created at high pH, are insoluble; their rate of formation is stimulated by a high temperature.

At this point of the discussion, two phenomena will converge, namely the need for a high density algal culture which grows at a high specific growth rate, and the preceding paragraph which explained why the chemistry of  salts at high salt concentrations, high temperatures, and high alkalinity lead to insoluble complexes.

When phytoplankton takes up CO2 faster than it can be delivered by exchanging this gas at the interface of air/water, the water compositions is driven into non-equilibrium conditions. In this particular case, the fast removal of carbon dioxide from sea water leads to the creation of free hydroxide ions. This can be seen as an increase in pH up to values as high as 9 or 10 (normal sea water is moderate alkaline at a stable value of about 8.3). We now have all elements in place to understand the following sequence of events:

  • a high phytoplankton productivity is desired. This requires:
  • short optical pathways, which allow for high biomass densities, leading to:
  • above equilibrium uptake rates of the carbonate system in sea water, inducing:
  • high pH values (caused by high concentration of hydroxide ions);
  • leading to the formation of significant insoluble hydroxide complexes;
  • these gel-like complexes trap algal cells, which normally should be in suspension. This leads to:
  • biomass in the system which cannot be harvested, but does consume away carbon dioxide and other minerals, which leads to
  • low standing crops of biomass in the system, which means:
  • the creation of a condition which is characterized by low productivity.

So, conditions which promote the specific growth rate of phytoplankton at high cell densities, in combination with the cascade of chemical phenomena and biological events, while they were in essence created to stimulate high phytoplankton productivity, may result in the opposite effect, namely a significant lowering of phytoplankton production of what should, by all means, be normally possible.

The solution, again, is technology. By going over all arguments, it is easy to understand, that adding CO2 at the place where the gas exchange between water and air in the system takes place will prevent the carbon disequilibrium….. and hence, ultimately, will lead to higher productivities. There will be other positive side-effects, such as minimizing or even completely mitigating the formation of calcium scales which, in absence of extra CO2, pollute the system and, hence, cause all kinds of microbial and hydrodynamical maintenance complications.

Concluding, one solution to increase phytoplankton productivity is to add a small flow of pure carbon dioxide into the air stream which is supplied to the system.

Since carbon dioxide is known to be a greenhouse gas, contributing to the anthropogenic increase in global temperature, one might wonder if it makes sense to create algal systems to minimize climate change. Tomorrow’s post will look a bit further into this aspect of phytoplankton technologies.

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