7.30: Red Algae

Red algae, also known as rhodophytes, are primarily found in marine environments, though some species inhabit freshwater and terrestrial ecosystems. These organisms exist in both unicellular and multicellular forms, with some multicellular varieties reaching macroscopic sizes.

As phototrophic organisms, red algae contain chlorophyll a; however, their chloroplasts lack chlorophyll b. Instead, they possess phycobiliproteins, which serve as major light-harvesting pigments, similar to those found in cyanobacteria. The reddish hue exhibited by many red algae results from phycoerythrin, an accessory pigment that masks the green coloration of chlorophyll. This pigment, along with phycocyanin and allophycocyanin, is organized into structures called phycobilisomes, which function as light-harvesting antennae. Species dwelling in deeper aquatic environments tend to produce higher amounts of phycoerythrin, making them appear darker red, whereas those living in shallower waters have lower concentrations and may appear green.

Most red algae species are multicellular and lack flagella. Some varieties, commonly referred to as seaweeds, are valued for their commercial applications. They serve as sources of agar, a key solidifying agent in microbiological culture media, as well as carrageenans, which are widely used as thickening and stabilizing agents in the food industry. Certain red algae species, such as those from the genus Porphyra, are harvested, dried, and used as edible wraps in sushi. Morphologically, red algae can take on filamentous or leafy forms, while others, particularly those that deposit calcium carbonate, exhibit a coral-like appearance. These coralline red algae contribute to coral reef formation and fortification, helping to strengthen reefs against wave erosion.

The genus Polysiphonia represents a group of filamentous and branched red algae commonly found in marine environments worldwide. These organisms predominantly grow near shorelines, attaching themselves to rocks, other algae, and artificial surfaces such as jetties, retaining walls, docks, and concrete structures. With nearly 200 recognized species, Polysiphonia follows a complex reproductive cycle that includes an alternation of generations. During this cycle, haploid male and female gametes arise from a diploid multicellular organism, eventually maturing into haploid multicellular individuals of both sexes. The male algae release haploid sperm-like cells, which then fuse with specialized reproductive structures on female algae, forming diploid zygotes. These zygotes develop into multicellular organisms that, following meiosis, release male and female gametes, thereby completing the cycle.

In addition to multicellular species such as Polysiphonia, some red algae exist in unicellular forms. Members of the order Cyanidiales, including the genera Cyanidium, Cyanidioschyzon, and Galdieria, thrive in extreme environments such as hot, acidic, and metal-rich springs. These organisms endure temperatures ranging from 30°C to 60°C and pH levels as low as 0.5 to 4.0, conditions that are inhospitable to most other phototrophic microorganisms, including anoxygenic phototrophs.

Unicellular red algae exhibit several distinctive features. For instance, Cyanidioschyzon merolae possesses remarkably small cells, measuring only 1–2 μm in diameter, and has one of the smallest genomes known among phototrophic eukaryotes, approximately 16.5 megabase pairs. Molecular studies of Galdieria have revealed that this eukaryotic alga has acquired at least 75 genes through horizontal gene transfer from various prokaryotic sources. These transferred genes provide adaptations such as resistance to salt stress, protection against metal toxicity, and modifications that strengthen the cytoplasmic membrane, enabling survival in hot and acidic conditions. Additionally, Galdieria possesses an unusual ability to grow in complete darkness. This trait is linked to horizontally acquired genes that encode transport systems for utilizing organic compounds as energy sources.

Although horizontal gene transfer is typically observed among prokaryotic organisms, the case of Galdieria underscores its significant role in shaping microbial genomes, even across different phylogenetic domains.