7: Algae - an Introduction

Microalgae

Microalgae are single-cell organisms that can form filaments and colonies.  This group has one prokaryotic member, cyanobacteria, also known as blue-green algae. As a prokaryote, it does not have mitochondria or chloroplasts.   The rest of the microalgae are eukaryotic and include the phyla Chlorophyta, Rhodophyta, Glaucophyta, Cryptophyta, Euglenozoa, Cercozoa, Heterokontophyta, Haptophyta, and Miozoa (Myzozoa).  Another more straightforward organizational system divides microalgae into four categories:

• cyanophyta (blue-green algae/cyanobacteria)
• pyrrophyta (dinoflagellates and cryptomonads, and can be yellowish-green to golden-brown)
• chrysophyta (diatoms, heterokonts and golden brown algae)
• chlorophyta (microscopic green algae).  This term also applies to macroalgae (see below).

Green algae are diverse and are now broken up into two phyla, chlorophyta and charophyta, with a combined 17 classes.  AlgaeBase dynamic species counts show about 4,500 Chlorophyta species, including those living on land, in freshwater, and those considered macroalgae seaweeds.  There are about 2500 species of  Charophyta that are entirely freshwater.

Green, brown, and red algae are found within the microalgae. (There are also green, brown, and red macroscopic algae.) Microalgae are much more efficient at photosynthesis than land plants. The diatoms are an example of microalgae and are the precursor of the "fossil" fuel oil deposits. Spirogyra is a unicellular green alga that forms long filaments (colonies) up to 0.1 mm, so it looks like a multicellular organism. 

One class of microalgae, green algae, arose when a microbe acquired a cyanobacterium, which allowed photosynthesis. Green algae eventually evolved into higher plants, and a similar process led to red algae. Brown algae, diatoms, dinoflagellates, and euglenoids are other types of algae that arose from red or green algae uptake into other eukaryotic host cells.

Green microalgae, which absorb red wavelengths, are generally found on the surface. The red microalgae, which absorb green and blue wavelengths, are at lower levels. The brown microalgae are generally found in between these water layers. Figure \(\PageIndex{x}\) below shows the light penetration spectrum in water.

Microalgae exist as single cells or can form multicellular filaments and colonies. They proliferate in the presence of simple nutrients and can produce large amounts of polysaccharides for industrial bioethanol production or fatty acids/triacylglycerols for biodiesel production. They are, as mentioned above, the source of underground oil deposits. Both freshwater (like Chlorella and Haematococcus) and ocean microalgae (Dunaliella, Phaeodactylum, and Tetraselmis) can be used to produce biofuels. The cyanobacteria Spirulina sp. is commonly used for commercial purposes. It and Synechococcus have large amounts of glycogen that could be used for bioethanol production. Their triacylglycerols are low, so they can't be used for biodiesel (which we will discuss in a future section of the chapter). 

Most are familiar with algae blooms in freshwater lakes (and even in saltwater environments). Cyanobacteria (blue-green algae) are the main culprit. Microcystin, a potentially lethal toxin that targets serine/threonine protein phosphatases, is released from some algae blooms. Figure \(\PageIndex{5}\) shows a microalgae bloom in Lake Erie, a shallow freshwater lake polluted with agricultural runoff, making it an excellent site for cyanobacterial blooms.

Figure \(\PageIndex{5}\): Microalgae bloom in Lake Erie, October 2011.  https://commons.wikimedia.org/wiki/F..._Lake_Erie.jpg

Red tides are another type of toxic algae blooms that occur in coastal water.  Along the US coast, they are caused by dinoflagellates and diatoms, phytoplankton, both types of microalgae.  Around the Gulf of Mexico, the leading cause of red tides is the microalgae Karenia brevis.  They release large amounts of brevotoxin, a polycyclic ether that binds to and activates voltage-gated Na+ channels in nerve and muscle. Hence, it is a potentially deadly neurotoxin.  The frequency of red tides is increasing around the world.  Figure \(\PageIndex{6}\) shows two images of red tides.

Figure \(\PageIndex{6}\):  Red Tide.  License from Shutterstock

Microalgae are sources of glycan polymers that can be used for bioethanol production; some are excellent candidates for biodiesel fuel made from triacylglycerol reserves. Table \(\PageIndex{2}\) below shows a classification of microalgae and Cyanobacteria along with their characteristic pigments (that impart their distinctive colors) and their energy reserves that could be used for biofuel production.

Table \(\PageIndex{2}\):  Classification of microalgae and Cyanobacteria.  Hachicha, R.; Elleuch, F.; Ben Hlima, H.; Dubessay, P.; de Baynast, H.; Delattre, C.; Pierre, G.; Hachicha, R.; Abdelkafi, S.; Michaud, P.; Fendri, I. Biomolecules from Microalgae and Cyanobacteria: Applications and Market Survey. Appl. Sci. 2022, 12, 1924. https://doi.org/10.3390/app12041924. Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Cyanobacteria and red microalgae store glycogen and floridean starch (a hybrid between starch and glycogen), respectively, while green microalgae accumulate amylopectin-like polysaccharides.

Macroalgae

As mentioned above, macroalgae, known as seaweeds, do not have roots, stems, leaves, or flowers. Kelp in underwater forests appear to have roots, stems, and leaves, but they are not plants. They have analogous structures such as holdfast, stipes, and blades, which serve the functions of roots, stems, and leaves found in plants. As do the microalgae, variants of macroalgae include red  (Rhodophyta), green (Chlorophyta, also a type of microalgae), and brown  (Phaeophyta) algae. It grows up to 30 times as quickly as land-based groups. In addition, it does not have lignin. They grow much faster than terrestrial plants and can form lots of biomass for commercial processing in much less area than land groups. They can be grown cheaply in sea farms without adding nutrients or pesticides. Hence, they are ideal for both food and biofuel production. Brown and red algae have many present commercial uses. They have lots of carbohydrates for potential bioethanol production and triacylglycerols for biodiesel. Their carbohydrate composition includes mannitols and cell wall constituents, which could also be used for fermentation.   In contrast to present petroleum sources, the biodiesel from macroalgae does not contain sulfur.   

Red macroalgae

(some material below for red and green macroalgae from https://bio.libretexts.org/Bookshelv...nd_Green_Algae)

The red algae are almost exclusively marine; some are unicellular, but most are multicellular. They have true chloroplasts with two membranes (no remnant peptidoglycan) containing chlorophyll. Like the cyanobacteria, they use phycobilins as antenna pigments, phycoerythrin (which makes them red), and phycocyanin. Red pigment allows the red algae to photosynthesize at deeper depths than the green or brown algae, harnessing more of the blue light waves that penetrate deeper into the water column.   Unlike green algae and plants, red algae store carbohydrates as Floridean starch,  which has glucose in α(1,4) linkages and occasional α(1,6) linkages, similar to amylopectin. Agar, the base for culturing bacteria and other microorganisms, is extracted from a red alga.   Multicellular forms can be filamentous, leafy, sheet-like, coralloid, or crust-like. Some examples are shown in Figure \(\PageIndex{7}\).

Figure \(\PageIndex{7}\): These images show multicellular red algae, which can range from filamentous  (first image) to "leafy" (second image, left) to sheet-like (second image, right). The red color is due to an abundance of the red pigment phycoerythrin, which gives this group reddish chloroplasts. First image by Melissa Ha CC-BY-NC. Second image by Maria Morrow CC-BY-NC. Right image:  https://commons.wikimedia.org/wiki/F...ed_algae_3.jpg

Green macroalgae

These algae exhibit great diversity of form and function. Similar to red algae, green algae can be unicellular or multicellular. Many unicellular species form colonies, and some green algae exist as large, multinucleate, single cells. Green algae primarily inhabit freshwater and damp soil and are a common component of plankton. They have chloroplasts and the photosynthetic pigments chlorophyll a and b, carotene, and xanthophylls. Examples include Chlamydomonas, Chlorella, Pediastrum, Netrium, Hydrodictyon, Acetabularia, Ulva, and Spirogyra. Lichens are a symbiotic combination of fungi and green algae. 

The nature of the evolutionary relationships between green algae is still debatable. As of 2019, genetic data supports splitting the green algae into two major lineages: chlorophytes and streptophytes. The green algae exhibit similar features to the land plants, particularly in chloroplast structure. They have chlorophyll a and b, have lost phycobilins but gained carotenoids, and store carbohydrates as starch inside plastids. Green algae are an important source of food for many aquatic animals.  Figure \(\PageIndex{8}\) shows two types of green macroalgae.

Figure \(\PageIndex{8}\):Two types of green macroalgae

left:  Figure 5.3.3.125.3.3.12: Trentepohlia is a genus of green algae in terrestrial environments. It forms fluffy orange colonies on trees and is a photobiont in many lichens. One might not know they were looking at a green algae due to the orange pigmentation. However, green algae have carotenoids. These terrestrial green algae produce an abundance of carotenoids, perhaps for protection from sun damage. Photo by Scott Loarie, CC0.

Right: freshwater green algae. https://upload.wikimedia.org/wikiped...reen_Algae.jpg

Green macroalgal blooms (called green tides) can also occur (just as blooms from the microalgae cyanobacteria). Green tides in the Yellow Sea (between China and Korea) are the largest known. A particularly large one is shown in Figure \(\PageIndex{9}\):

Figure \(\PageIndex{9}\): Green Seaweed in the Yellow Sea , June 2021.  https://earthobservatory.nasa.gov/im...the-yellow-sea

This particular green color was from the nontoxic green macroalgae Ulva prolifera. Because it is edible, it is often called sea lettuce. Blooms can affect the local ecosystem and, as they decay, lead to hypoxic zones. 

Brown macroalgae (also known as Kelp)

(some material below for brown macroalgae from https://bio.libretexts.org/Bookshelv...3A_Brown_Algae)

Macroscopic brown algae arose when a heterotrophic eukaryote merged with a unicellular photosynthetic eukaryotic red algae chloroplasts. This time, the red alga became a chloroplast with four membranes -- the engulfing membrane from the oomycete, the red alga's plasma membrane, and the two membranes of the original chloroplast within the red alga. The chloroplast has lost one of these membranes in many groups derived from secondary endosymbiosis. Figure \(\PageIndex{10}\) shows this secondary endosymbiosis event.

Figure \(\PageIndex{10}\):  In the diagram above, we see a unicellular photosynthetic eukaryote with a 2-membrane chloroplast. In step one, this organism is engulfed by a heterotrophic eukaryote. In step two, we see the photosynthetic organism inside the heterotrophic organism. In step three, the original photosynthetic organism has been reduced to a chloroplast with four membranes. Artwork by Nikki Harris CC-BY-NC with added labels by Maria Morrow.

Brown algae are brown due to the large amounts of carotenoids they produce, primarily fucoxanthin. These organisms are exclusively multicellular and can grow so large that they require special conductive cells to transport photosynthesized products from their blades to the rest of their tissues. These conductive cells are called trumpet hyphae, have sieve plates, and resemble sieve tubes found in flowering plants.

Much like Saprolegnia, the body of an alga is called a thallus because it is not differentiated into specialized tissues. The general morphology of a brown alga includes a holdfast, stipe, gas bladder(s), and blade(s). Figure \(\PageIndex{111}\) shows a diagram of the kelp structure.

 

Figure \(\PageIndex{11}\):  A diagram of kelp structure.  In the diagram above, there are two kelp thalli. The one on the left side is labeled. At the bottom of the thallus is a network of root-like projections that make up the holdfast. The stem-like structure that travels up from the holdfast is the stipe, which terminates in an inflated gas bladder. There are several leaf-like structures attached to the gas bladder. These are blades. The thallus on the right has all these components but in a slightly different arrangement. Can you find them? Artwork by Nikki Harris CC-BY-NC with added labels by Maria Morrow.

 

Figure \(\PageIndex{12}\) shows a beautiful image of a kelp forest.

Figure \(\PageIndex{12}\):  Kelp forest.  https://commons.wikimedia.org/wiki/F...1115735%29.jpg

Figure \(\PageIndex{13}\) shows another type of a brown macroalga.

Figure \(\PageIndex{12}\): Underwater "roses" of a brown algae Padina pavonica, commonly known as the peacock's tail (Israel).  https://commons.wikimedia.org/wiki/F...l_(Israel).jpg

Now we can discuss algae as a source of biofuels and nutritive foods in the next chapter section.

Summary

This chapter introduces third-generation bioethanol production derived from algae, presenting it as a promising alternative to first-generation (corn starch) and second-generation (lignocellulosic biomass) biofuels. It highlights how algal feedstocks—ranging from unicellular microalgae to multicellular macroalgae (seaweeds)—offer significant advantages in terms of rapid growth, high photosynthetic efficiency, and the potential to minimize environmental impacts by using non-arable lands or marine environments.

Key points discussed in the chapter include:

• Overview of Bioethanol Generations:
  A comparative flowchart illustrates the pre-fermentation processing of feedstocks from corn, lignocellulosic biomass, and algae. The chapter sets the stage by reviewing previous generations and emphasizing the environmental and ethical issues associated with land use for food versus fuel.

• Diversity and Classification of Algae:
  The chapter reviews the vast diversity of algae, covering both microalgae and macroalgae. It explains the complexities of algal taxonomy—highlighting groups such as cyanobacteria (blue-green algae), chlorophytes (green algae), rhodophytes (red algae), diatoms, and dinoflagellates—and discusses their evolutionary relationships within the broader tree of life.

• Algal Physiology and Light Utilization:
  The physical basis of light absorption in water is explored, explaining how variations in the light spectrum affect the distribution and productivity of different algal groups. This discussion links the efficiency of algal photosynthesis to their potential as a robust feedstock for biofuel production.

• Biofuel Production Potential:
  The chapter details the biofuel applications of algae, emphasizing that their rapid growth and high biomass yield can lead to efficient production of bioethanol and biodiesel. It contrasts the energy conversion and processing steps required for algae with those of terrestrial crops, noting that algae can be cultivated with fewer environmental impacts.

• Future Directions:
  The chapter concludes by setting the stage for a deeper discussion on the practical applications of algal biofuels, including potential value-added processes, the integration of biofuel production with other biotechnological applications, and the broader implications for sustainable energy systems.

Overall, this chapter equips junior and senior biochemistry majors with an understanding of the unique biological properties of algae, the challenges and opportunities in harnessing them for biofuel production, and the broader environmental and technological context in which third-generation bioethanol is emerging as a viable renewable energy source.