Carbon dioxide: algal growth & ammonium oxidation

“We’re running the most dangerous experiment in history right now, which is to see how much carbon dioxide the atmosphere… can handle before there is an environmental catastrophe.” – Elon Musk

In 1896, Arrhenius posited that the carbon dioxide (CO₂) produced from fossil fuel combustion would be directly proportional to the increase in global temperature. We can see from today’s events that his hypothesis was very much proven. One of my previous posts, about Earth day, highlighted the importance of photosynthesis in the consumption of CO₂ from the atmosphere. Algae, much like trees, photosynthesise consuming inorganic carbon (dissolved CO₂ or bicarbonate) from their aquatic environment. Another process countering the rising CO₂ concentrations is ammonium oxidation which is carried out by ammonium oxidising bacteria (AOB). As the name states, these bacteria oxidise ammonium into nitrite (NO₂). The energy produced from this oxidation is used to fix CO₂ into their biomass through the Calvin Cycle.

The Calvin Cycle

Algal growth

For this research project, the 2 green alga species (Chlorella and Scenedesmus) were documented in literature to be excellent at nutrient fixation while the other 2 species (Anabaena and Spirulina) were relatively unknown. In terms of CO₂ consumption, the 2 different types of algae had 2 different ways of consuming dissolved CO_{2 .}They differed in efficiency too because by the end of the study, only Anabaena remained. The other species died out from the reactor as a result of low dissolved CO₂ concentrations and Anabaena survived due to the species utilising a carbon concentrating mechanism (CCM). This CCM is characterised by the conversion of bicarbonate (from the environment) to CO₂ by the enzyme carbonic anhydrase. The CO₂ is then consumed through photosynthesis therefore, maintaining function even in a below optimum environment.

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Ammonium oxidation

Ammonium oxidation, or nitritation, is part of a very important process (nitrification) in biological nitrogen removal from wastewater. When I first learnt of ammonium oxidation I thought AOB had the same metabolic pathway as most bacteria which was to release CO₂. However, much like a photosynthetic organism, AOB consumed CO₂ to incorporate into biomass. In order to ensure ammonium oxidation was occurring in the bioreactor, the dissolved oxygen (DO) concentration had to be 3 mg/L and above. Photosynthesis made it easy to maintain this concentration however, it still caused one of the bigger challenges for me which was to create an environment where AOB thrived above all the other oxidising microorganisms. Since that DO concentration was optimal for all “oxidisers”, I was constantly manipulating the lights which controlled photosynthesis and subsequently the DO concentration.

An observation…

After reviewing the dissolved CO₂ profiles in the bioreactor, we noticed that there was an increase in the concentration which meant that CO₂ was getting into the reactor after feeding. Now, when dealing with algae and bacteria biomass, the reactors are usually completely sealed to reduce the introduction of new microorganisms that could present as competition to the ones inside. Thus CO₂ was being produced inside the reactor by the ordinary heterotrophic organisms (OHOs) that break down organic matter resulting in CO₂ as one of the byproducts. This should have been a a positive for the algal strains that died out but I believe Anabaena was already taking over by this time.

My two cents…

In the mitigation of climate change, these processes would be beneficial but on a very large scale. This research also showed me that perfecting (CO₂) consumption and wastewater treatment is quite challenging but quite necessary. Imagine reducing (CO₂) concentrations in the atmosphere and simultaneously cleaning water that could be reused instead of continuously depleting the already low fresh water reserves. What a way to change the tide.

–the Awkward Chemist

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