5.4 Algae

Introduction to Algae

Algae are photosynthetic eukaryotes that range from single-celled organisms to massive kelp forests. They serve as primary producers in aquatic ecosystems, meaning they convert light energy into organic compounds that feed entire food webs. In microbiology, algae matter because of their ecological impact, their ability to produce dangerous toxins, and their growing use in biotechnology.

Relevance of Algae in Microbiology

As primary producers, algae are responsible for a significant portion of global oxygen production and carbon fixation. They also form key symbiotic relationships with other organisms, such as the partnership between algae and fungi in lichens, or between algae and coral polyps in coral reefs.

Beyond ecology, algae have practical importance:

• Harmful algal blooms (HABs): Certain species produce toxins that accumulate in shellfish and water supplies, causing poisoning syndromes and respiratory problems in humans and animals.
• Biotechnology: Algae are used to produce biofuels (biodiesel), pharmaceuticals (antioxidants, antibiotics), and food additives (omega-3 fatty acids, thickening agents like agar and carrageenan).
• Environmental applications: Algae are used in wastewater treatment and studied for their potential in carbon sequestration.

Key Characteristics of Algae

Algae are eukaryotic, so they have membrane-bound organelles including a nucleus, mitochondria, and chloroplasts. The chloroplasts are where photosynthesis happens.

• Photosynthetic pigments: All algae contain chlorophyll for capturing light energy. Different groups also carry accessory pigments like carotenoids and phycobilins, which let them absorb different wavelengths of light.
• Size and organization: Algae range from microscopic unicellular forms (like Chlamydomonas) to large multicellular seaweeds (like kelp). Between those extremes, you'll find colonial forms (like Volvox) and filamentous forms.
• Cell walls: Most algae have cell walls made of cellulose and other polysaccharides. Diatoms are distinctive because their walls contain silica. Some dinoflagellates lack cell walls entirely.
• Body structure: The algal body is called a thallus, which lacks true roots, stems, and leaves. This is a key distinction from true plants.
• Reproduction: Algae reproduce both asexually (cell division, fragmentation, spore formation) and sexually (fusion of gametes). Some groups display alternation of generations, cycling between haploid and diploid life stages.

Harmful Algal Blooms and Toxins

When nutrient levels in water spike (often from agricultural runoff), toxin-producing algae can multiply rapidly, creating harmful algal blooms (HABs). These blooms disrupt ecosystems and pose serious health risks to humans and animals. The toxins typically accumulate in shellfish that filter-feed on the algae, then cause illness when humans eat the contaminated shellfish.

Toxin-Producing Algae and Their Effects

Dinoflagellates are the most notorious toxin producers:

• Alexandrium produces saxitoxins, causing paralytic shellfish poisoning (PSP). Symptoms include numbness, paralysis, and respiratory failure.
• Karenia produces brevetoxins, causing neurotoxic shellfish poisoning (NSP). These toxins can also become airborne in sea spray, triggering respiratory distress in coastal areas.
• Dinophysis produces okadaic acid, causing diarrhetic shellfish poisoning (DSP) with severe gastrointestinal symptoms.

Cyanobacteria (technically prokaryotic, but historically studied alongside algae) also produce dangerous toxins:

• Microcystis produces microcystins, which cause liver damage.
• Anabaena produces anatoxins, which are neurotoxic.
• Cylindrospermopsis produces cylindrospermopsin, which damages the liver and gastrointestinal tract.

Diatoms can be toxic too. Pseudo-nitzschia produces domoic acid, causing amnesic shellfish poisoning (ASP). This toxin targets the nervous system and can cause memory loss, seizures, and disorientation.

Major Algal Groups and Taxonomy

Comparison of Major Algal Groups

Each major algal group is distinguished by its pigments, storage molecules, cell structure, and habitat. Here's a breakdown:

• Chlorophyta (green algae): Contain chlorophyll a and b, store energy as starch. These are the algae most closely related to land plants. Examples: Chlamydomonas (unicellular), Volvox (colonial), Ulva (multicellular sea lettuce).
• Rhodophyta (red algae): Contain accessory pigments called phycobilins (phycoerythrin and phycocyanin), which give them their red color and allow them to photosynthesize at greater depths. They store floridean starch. Examples: Porphyra (nori, used in sushi), Gracilaria (source of agar), Chondrus (source of carrageenan).
• Phaeophyta (brown algae): Contain chlorophyll a and c plus fucoxanthin (the pigment responsible for their brown color). They store energy as laminarin. These are the largest algae. Examples: Laminaria (kelp), Fucus (rockweed), Sargassum (floating seaweed that forms massive mats in the Atlantic).
• Bacillariophyta (diatoms): Unicellular algae with distinctive silica-based cell walls called frustules, which fit together like a petri dish lid and base. They are major components of phytoplankton. Examples: Pseudo-nitzschia (toxic), Thalassiosira (planktonic), Chaetoceros (chain-forming).
• Dinoflagellata (dinoflagellates): Unicellular algae with two flagella that create a characteristic spinning movement. Some species are bioluminescent, and many are responsible for "red tides." Examples: Alexandrium (toxic), Karenia (red tides), Dinophysis (produces okadaic acid).

Classification of Algal Species

Most algal groups are classified within the Kingdom Protista, including Chlorophyta, Rhodophyta, Phaeophyta, Bacillariophyta, and Dinoflagellata. Some multicellular green algae (like Ulva and Caulerpa) are sometimes placed in the Kingdom Plantae because of their close evolutionary relationship to land plants.

Within each group, species are classified using standard taxonomic ranks: Phylum, Class, Order, Family, Genus, Species. Classification is based on morphological features, biochemical traits (pigments, storage molecules), and genetic analysis.

For example:

1. Chlamydomonas reinhardtii: Chlorophyta → Chlorophyceae → Chlamydomonadales → Chlamydomonadaceae → Chlamydomonas → reinhardtii
2. Porphyra umbilicalis: Rhodophyta → Bangiophyceae → Bangiales → Bangiaceae → Porphyra → umbilicalis

Algal Ecology and Evolution

Ecological Importance

Algae form the base of aquatic food webs. As primary producers, they convert $CO_2$ and light energy into organic molecules through photosynthesis, feeding everything from zooplankton to fish.

Phytoplankton (mostly diatoms and dinoflagellates) are responsible for roughly 50% of global oxygen production. That means about every other breath you take comes from algal photosynthesis, not trees.

Algae also drive biogeochemical cycles. They fix carbon from the atmosphere into organic matter, and when they die and sink, they transport that carbon to the ocean floor (the "biological pump"). Certain cyanobacteria and algal symbionts also contribute to nitrogen cycling by fixing atmospheric nitrogen.

Evolutionary Significance

The origin of algae is tied to endosymbiosis. According to endosymbiotic theory, an ancient eukaryotic cell engulfed a photosynthetic cyanobacterium. Instead of being digested, the cyanobacterium became an endosymbiont and eventually evolved into the chloroplast. This is called primary endosymbiosis, and it gave rise to green algae, red algae, and land plants.

Some algal groups (like brown algae and diatoms) acquired their chloroplasts through secondary endosymbiosis, where a eukaryote engulfed another eukaryote that already had a chloroplast. This is why these groups have chloroplasts surrounded by additional membranes.

Algal life cycles reflect this diverse evolutionary history. Some groups reproduce through simple cell division, while others have complex alternation of generations that cycles between a haploid gametophyte stage and a diploid sporophyte stage.