MICROBIOLOGY FOR THE UNITINTIATED – PART 8: ALGAE

Algal growth coating cooling tower backfill.
Source: https://petapixel.com/2016/10/13/photos-reveal-guts-massive-cooling-towers/

Introduction

Today’s post continues my survey of microorganisms that commonly infect industrial systems. Algae are photosynthetic members of the Domain Eukarya, Kingdom Protista. Taxonomically and genetically diverse, all algae are photosynthetic (although some can obtain their energy through chemoheterotrophic metabolism – see What’s New, May 2023). The one common feature of all algae is the presence of chloroplasts (see below). Morphologically, algae range from single cells (unicellular) microalgae (e.g., Chlorella species, dia < 100 μm, Figure 1a) to multicellular, giant kelp (Macrocystis pyrifera, which can grow to 200 m long, Figure 1b). Given that all but one genus of algae are photosynthetic, algal contamination is only problematic in systems exposed to light. Most commonly these are cooling towers (title photo).

Fig 1. Algal diversity – a) Chlorella species; b) Macrocystis pyrifera.

Agal Taxonomy

There’s still considerable debate about taxonomic classification which still uses the system Carl Linnaeus developed during the 16th century. One classification (Figure 2), proposed in 2008, assigns algae to seven Divisions, based on:

• Gross morphology (algal body – unicellular, multicellular, branched or unbranched thalli, etc.)
• Primary cell wall chemical composition
• Types of chlorophyll and carotenoids
• Phycobiliproteins
• Storage products.

Fig 2. Algal Divisions, after Lee (Phycology, 4th ed. Cambridge University Press, London, ISBN 10-9780521682770).

Notes:

1. Chlorophyta can be unicellular or multicellular. The latter can be mononucleated, coenocytic (>1 nucleus per cell), and form branching or non-branching filaments.
2. Chl a and Chl b – Chlorophyll a and Chlorophyll ; the chlorophyll types involved in photosynthesis.
3. Neg. – Phycobiliproteins are only found in Rhodophyta.
4. CHO – carbohydrates.

I have not seen a more recent, genomics-based taxonomy. It will be interesting to see how alga taxonomy changes when relationships among taxa are assessed based on genetics rather than traditional tools.

Except for diatoms, which are members of the Division, Bacillariophyta, the algae most commonly recovered from cooling towers are all members of the Division, Chlorophyta. Figure 3 illustrates examples of algae most commonly recovered from cooling tower systems.

Fig 3. Algae commonly recovered from cooling systems – a) Chlorella sp.; b) Diatoms; c) Oocysts; d) Pediastrum sp.; E) Scenedesmus sp.

Agal Cell Morphology

Figure 4 shows a simplified sketch of an algal cell. Except for algae in the Divisions Chlorophyta and Euglenophyta, all algal cells have cell walls. Cell wall chemistries differ among the algae Divisions. The cell wall functions primarily to maintain cell form and structural integrity. It also maintains the cell membrane’s integrity. The cell membrane regulates the movement of nutrients, salts, and other substances between the cell and its environment. The cytoplasm is a complex, gel-like substance that contains the cell’s organelles. The primary organelles are the nucleus, mitochondria, ribosomes, and chloroplasts. As with all other eukaryotic organisms, the nucleus contains the cell’s chromosomal deoxyribonucleic acid (DNA). It also contains the nucleolus – the site of ribosomal ribonucleic acid (rRNA) production.

Ribosomes are the sites of protein production. Messenger RNA (mRNA) takes (transcribes) coding from DNA in the nucleus and transports it to the ribosomes. Within the ribosomes mRNA transfers the code to rRNA which then aligns transfer RNA (tRNA) to which amino acids are attached. Thus, the DNA’s gene sequence dictates the order in which amino acids are assembled into proteins. Mitochondria are the organelles in energy metabolism is centered. The citric acid cycle (also known as the tricarboxylic acid cycle) catabolizes macronutrients (alcohols, carbohydrates, fats, and proteins) to produce Acetyl-CoA. Acetyl-CoA oxidation generates adenosine triphosphate (ATP) – which, as I have discussed elsewhere, is the cell’s “coin of the realm” for energy metabolism. Mitochondria also function to regulate cell metabolism, control cell signally, heat generation (thermogenesis), and death (apoptosis). Chloroplasts are the organelles in which chlorophyll and carotenoids convert solar energy and carbon dioxide into chemical energy (ATP) and organic molecules (sugars, amino acids, and lipids. Chloroplasts and mitochondria evolved from independent prokaryotes to intracellular symbionts (i.e., microbes living within other organisms’ cells to the mutual benefit of both) to organelles. The storage products listed in the bottom row of Figure 2 are stored in the cell’s vacuoles.

Fig 4. Algal cell simplified diagram.

Biodeteriogenic Activity

Heat exchange inhibition

Algae growing on heat exchanger surfaces form insulating masses (biofilms) that can reduce heat exchange dramatically. Figure 5 shows four examples of algal masses coating heat exchanger surfaces. Figure 6 illustrates the relationship between biofilm thickness and heat exchange efficiency through three metals (carbon steel, stainless steel, and copper). A 10 μm thick biofilm can decrease heat exchange though carbon steel, stainless steel and copper by 22 %, 31 %, and 80 %, respectively.

Fig 5. Algal biofilms of four heat exchanger surfaces.

Fig 6. Relationship between biofilm thickness and heat transfer for three metals.
(Source: https://aquariustech.com.au/wp-content/uploads/bsk-pdf-manager/Biofilm_and_Energy_Loss_3.pdf)

Building deterioration

Except for aquatic structures such as pier foundations (Figure 7a), algae typically grow as aerophytes (organisms that obtain moisture and nutrients from the air, rather than from the substrates on which they are growing) on buildings and other structures (Figure 7b through 7e). Algae can grow on structure surfaces (epiphytic growth), in the pores of those surfaces (endophytic growth), or both. Algal growth on structure surfaces affects their aesthetics. When the effect is deemed negative it is called aesthetical biodeterioration. In addition to the aesthetic impact, aerophytic algae can increase the water absorptive capacity of the structure – thereby increasing its susceptibility to expansion and contraction due to freezing and thawing of the trapped water. Algal biofilms create habitats for biodeteriogenic bacteria and fungi. Organic acids produced by algal biofilm communities also decrease the pH of interstitial water absorbed by the structures on which they grow. The end result in loss of structural integrity.

Fig 7. Structure biodeterioration caused by algae – a) concrete pier, India; b) stone wall, Palas du Roi, Spain; c) brick wall; d) house siding; e) statue, Courances castle, France.

Harmful algal blooms

Harmful algal blooms (HAB) occur when aquatic environments are enriched with nitrogen and phosphorus nutrients – stimulating the rapid proliferation of one or more algal species. Most commonly, the algal biomass becomes sufficiently thick to block sunlight from reaching plants below the surface (Figure 8). As the algae in the bloom die off, they provide nutrients for heterotrophic bacteria. Although algae produce oxygen, the rate of oxygen consumption through respiration can exceed the rate of oxygen generation. When this occurs, the body of water eutrophication occurs (Figure 8). The oxygen concentration becomes depleted, causing marine life to suffocate (Figure 9d).

Fig 8. Eutrophication

Fig 9. Harmful algal blooms – a) freshwater pond; b) lakeshore; c) pond, showing CO₂ gas bubbles forming on water surface; d) fish asphyxiated by algal bloom.

Red tides are a form of HAB. The algae most commonly found in red tides are dinoflagellates and diatoms. Table 1 lists several dinoflagellates associated with red tides. The Table also lists the toxins that these algae produce. Most of the toxins listed are neurotoxins that accumulate in shellfish tissue and cause paralytic shellfish poisoning. Figure 9 shows two examples of red tides.

Table 1. Red tide dinoflagellates and their toxins.

Fig 10. Red tides – a) Karenia brevis off the Florida coast; b) Noctiluca scintillans off the Washington coast.

Summary

Algae are phylogenetically diverse photosynthetic organisms that range in size from <100 μm unicellular diatoms to multicellular giant kelp (50 m to 200 m long). In industry, algae adversely affect heat exchange and fluid flow in cooling water systems. Algal biomass can also clog tricking bed wastewater treaters. As primary oxygen producers, algal growth is generally beneficial. However, harmful algal blooms can cause eutrophication – creation of oxygen depleted zones on lakes and ponds – and produce toxins that can be lethal to fish and marine mammals. Humans who eat shellfish in which algal toxins have become concentrated, can experience fatal paralytic shellfish poisoning.

As always, please share your comments and questions with me at fredp@biodeterioration-control.com.