This month we take a look at another impressively diverse, often not so welcome, group of Caribbean residents, the macroalgae or seaweeds. On a coral reef the balance between these notorious competitors and coral is a delicate one, easily disrupted if there are not enough herbivores to keep their growth under control with their continuous grazing. However, while these organisms might not be as captivating as the colorful coral animals, they are equally important and interesting once you delve deeper into their lives.
Macroalgae can be either calcified and/or fleshy in structure. They exhibit a vast array of growth forms. Highly adaptable, they colonize coral reefs, seagrass beds, mangrove forests, and the intertidal zone. Three broad groups of macroalgae can be recognized: red macroalgae (Rhodophyta), brown macroalgae (Phaeophyceae), and green macroalgae (Chlorophyta). This two-part article will focus on the ancestral relatives of vascular plants, the Chlorophyta or green seaweeds.
A true treasure among the green macroalgae is the very unique and aptly named sea pearl (Valonia ventricosa). These squishy, mostly solitary, bubbles have fascinated me from the first time I stumbled across them nestled in depressions in the reef. The silvery sheen of the glitzy baubles catches your eye and draws you in for a closer look.
Their tight, spherical cell wall makes them highly reflective under water, giving them that unique sheen that led to them to be also called sailor’s eyeballs. That beautiful sheen, however, can sometimes be obscured by a thin layer of lavender crust alga (Hydrolithon farinosum), a pesky encrusting red alga.
Sea pearls are not picky where they set down roots, and can be found among coral rubble, mangrove roots or seagrass, and on sandy bottoms. They are attached to the substrate by minute, hair-like threads called rhizoids that provide a surprising strong hold. Their coloring, which depends on the amount of chloroplasts present, ranges from grass green to a dark, olive green. Algae bubbles can reach the respectable size of 2 inches or 5 cm, a fact that makes them one of the largest single celled organisms in the world. This is actually a big deal since unicellular organisms are normally microscopic, limited in growth by surface-area-to-volume-ratio restrictions. Sea pearls have overcome these limitations by having multiple nuclei and multiple chloroplasts. Squishing and bursting a sailor’s eyeball will result in more of them popping up as they only need to have one nucleus to grow into an entirely new ball. Their large cell size and unique function have made them the subject of many a scientific study, especially regarding the transfer of water and other fluids across biological membranes.
A green algae that truly illustrates the structural beauty and diversity of macroalgae for me is the beautifully named green mermaid’s wineglass (Acetabularia caliculus). Hard to believe that these dainty, uniquely designed algae also arose from a single plant cell. Since its discovery in the 1800s the genus Acetabularia has been a source of fascination for many a scientist. In fact, Acetabularia was actually the genus used to first demonstrate that genetic information is contained in the nucleus of organisms whose cells have a nucleus, which essentially means all animals and plants. These delicate algae grow singly or in clusters, and each alga body, or thallus, represents a single cell with a single nucleus. The unusually large nucleus is situated at the base of the stalk or stipe, allowing it to regenerate if damaged. Calcium absorbed from the water provides it with the light calcium carbonate skeleton that gives it that fluid rigidity, allowing its stunning umbrella shaped cup to face the life giving rays of the sun.
These hardy beauties can be found on a host of substrates, from pebbles, shells, pieces of rock to seagrass beds, mudflats, and mangrove roots. The cup of the green mermaid’s winegglass has rounded ridges that radiate from the center to form bulbous edges.
Reproduction mirrors fungi growing from spores. During sexual reproduction tiny cysts form in the cap that produce gametes. The cysts have tiny caps that open when the gametes are ready for release. Each individual produces only one type of gametes which unites with a different type to form a zygote. It takes approximately six months of the zygote to develop into the striking thallus that is fed upon by many fish and sea urchins. These tiny organisms are also unique in their ability to absorb mercury from the water and concentrate it in polypeptides in their tissues.
Another uniquely structured green alga that relies on a supportive calcium carbonate skeleton is the bristle ball brush (Penicillus dumetosus). This common Caribbean resident of sand flats and seagrass beds can be recognized by its rounded ball shape of tightly packed filaments that arise from a stout green stalk. It is also referred to as Neptune’s shaving brush because of its rounded shape. Their calcium carbonate framework allows bristle ball brushes to reach heights of 6 inches (15 cm). The carbonate component becomes part of the seafloor sediments when the alga dies and the soft parts decay away.
Another Penicillus family member to look out for is the flat-top bristle brush (Penicillus pyriformis), whose tightly packed bristles, as the name indicates, end in a flat top, forming an inverted cone.
Calcifying green algae, such as Penicillus, are major contributors of the carbonate that forms key reef habitats. In many tropical locales the beautiful white, sandy beaches we enjoy are mostly composed of the bleached and eroded calcium carbonate skeletons of one of the most common green algae on the reef, Halimeda. Studies have shown that, when considered globally, carbonate from Halimeda spp. accounts for an estimated eight percent of the total global production.
A typical Halimeda alga consists of flattened, leaf-like segments reinforced with calcium carbonate and covered with algal protoplasm. The tiny, calcified segments are interconnected by a thin strand, giving the plant its flexibility. It has been shown that 60 to 80 percent of these segments are composed of aragonite, a crystal form of calcium carbonate — no wonder they can contribute so greatly to marine sediments when they die and decay. Their crunchy nature also makes them unpalatable to most herbivores, contributing to their success on a reef.
Once you can recognize these segmented algae, you will spot them all over a reef nestled among coral, hanging down in a crevice or forming dense carpets across sandy bottoms. It is not the easiest task to identify individual species — for example, the watercress alga (Halimeda opuntia) grows crammed closely together in dense, spreading mats where it is often difficult to discern individual algal bodies or thalli. The large leaf hanging vine (Halimeda copiosa), as the name indicates, grow in bunches as long chains of beautiful green, rectangular segments joined down the middle by a single stalk, a species more easily recognized because of its distinctive growth form. Members of the genus can reproduce asexually and sexually, with sexual reproduction being completed within an amazing 36 hours. Members of this diverse genus are also known for the noxious metabolites they synthesize to further protect them from those pesky herbivores. It is these secondary metabolites that have become the focus of many scientific studies and their potential use as phytopharmaceuticals, especially for their hydrophilic antioxidant properties. Methanol and dimethylformamide extracts of the watercress alga (Halimeda opuntia) have shown positive antibacterial action against microorganisms such as Staphylococcus aureus, Escherichia coli and Bacillus subtilis. Ethanol extracts of the same species exhibited activity against hepatitis C virus, while extracts from other species have shown cytotoxicity towards cancer cells. Again it shows how much potential is locked up in nature in the most unlikely places. Imagine being saved from the hazards of cancer by what would normally be considered a lowly alga.
Next month we will look at more of these unique organisms with beautiful names such as mermaid’s fan and mermaid’s teacup.
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