Focus on Algae - Part I: Bioremediation

After spending the last few blog posts on different aspects of dissimilatory bacteria, I want to switch the focus to a different class of organisms I have been interested in for a long time now. These are the algae. Algae comprise a large diversity of "sea weeds" and an even larger variety of single-celled organisms that mostly are capable of doing photosynthesis. They include the ordinary sea-weed, and make up a portion of the green slime found around the edges and the bottom of a pond. More exotic types of algae can live symbiotically - that is together with another organism in a mutually beneficial way. Lichens are an example of symbiotic relationship between algae and fungi. More information about the evolution and lineage of algae can be found in this wiki article.

Image via Wikipedia

Typically, these organisms are either not mentioned at all or only in conjunction with toxic algal blooms. But lately, algae, of course, have been in the news recently because of the promise of these organisms have for lessening the effects of global-climate disruption and energy dependency on other countries. In the next few blogs, we shall explore the different ways how algae can be used to combat the things mentioned above. We will start with waste water treatment.

Bioremediation in General

Waste water treatment is a wide field because waste water can come from a big variety of sources ranging from agricultural waste, to residential waste to specialty industrial waste. Within the scope of this overview it would be too much to go into details for all of them.

Typical residential waste water treatment consists of three stages. In the first two stages, large debris and physical matter are removed through a variety of filtration and sedimentation techniques. Generally, though contaminants in waste water such as excessive organic compounds (think of human or animal excretion), including excessive nitrogen or phosphate sources and heavy metals are not removed.

The third stage in waste water treatment deals with the above problems. Here, microbial organisms are stimulated to degrade these contaminants. This is either done by a process called "activated sludge" in which the waste water is aerated and mixed to stimulate the microorganisms to degrade the waste. Although very effective in waste removal, this process also takes up a lot of energy because it takes a lot of energy to continually mix large amounts of waste water. An alternative to activated sludge, are trickling filters. In this process, the waste water is drizzled over large pools containing many rocks. The rocks are colonized by a variety of bacteria, fungi and some algae. As the waste water slowly dribbles along the many layers of moist rocks, again microbial organisms degrade the waste.

Third stage water treatment has several disadvantages. As previously mentioned, third stage water treatment can be extremely energy intensive and therefore financially expensive. In addition, irregularities in the water conditions can lead to fouling. Communities surrounding the area may therefore experience bad odor on occasion. Lastly, waste water treatment itself produces waste which is known as "sludge". This sludge consists of water insoluble waste products the bacteria themselves excrete and a lot of dead cells from the dividing bacteria. The sludge is usually considered toxic as well and has often been discarded by dumping into land fill sites - an unideal solution.

Algal Use in Bioremediation

Instead of using mainly bacteria, it is also possible to use mainly algae to clean waste water because many of the pollutant sources in waste water are also food sources for algae. Nitrates, and phosphates are common components of plant fertilizers for plants. Like plants, algae need large quantities of nitrates and phosphates to support their fast cell cycles. Certain heavy metals are also important for the normal functioning of algae. These include iron (for photosynthesis), and chromium (for metabolism). Because marine environments are normally scarce in these metals, some marine algae especially have developed efficient mechanisms to gather these heavy metals from the environment and take them up. These natural processes can also be used to remove certain heavy metals from the environment.

The use of algae has several advantages over normal bacteria-based bioremediation processes. One major advantage in the removal of pollutants is that this is a process that under light conditions does not need oxygen. Instead, as pollutants are taken up and digested, oxygen is added while carbon dioxide is removed. Hence, phytoremediation could potentially be coupled with carbon sequestration. Additionally,Because phytoremediation does not rely on fouling processes, odors are much less a problem.

Of course, phytoremediation is not without problems. The main problem in phytoremediation is contamination and competition with bacteria. Eukaryotic algae have much larger genomes, and more complex cell machinery. Therefore, takes much longer for an algae to divide (a few hours) compared to bacteria in ideal conditions (less than an hour). If conditions are not carefully monitored, bacterial populations could out compete the algae and push them out of the water ecosystem at the treatment plant. Furthermore, algae although sturdy are more sensitive compared to bacteria. So water conditions have to be monitored carefully to not overexpose the algal populations to the pollution sources and thereby killing them.

Nevertheless, algae have several very exciting alternative uses that could theoretically be coupled to the remediation processes. We will go over some of these applications in the next section.