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Algae: Ancestral Plants That Remained Aquatic

   What exactly are algae? We have already encountered cyanobacteria (blue-green algae) in the kingdom Eubacteria and golden-brown algae in the kingdom Protista. Are algae monerans, protists, plants, or all three? Actually, the term alga, from the Latin word for “seaweed,” has been used to describe simple chlorophyll-containing organisms that live in water and thus has come to include organisms in all three kingdoms. The algae of the plant kingdom are mostly multicellular aquatic organisms with simple reproductive structures. They are classified in three divisions (1)red algae,Rhodophyta;(2)brown algae, Phaeophyta;and(3)green algae, Chlorophyta.

    Multicellular algae probably arose about 600 million years ago from ancestral photosynthetic protists and were the only plants “on stage “ that early.They have since diversifide into about 12500 species, which vary from tiny threads to giant kelps and tangled seaweeds. Algae live in virtually all bodies of water , and sometimes on damp soils,rocks, trees, and evensnowbanks. Although modern algae are generally simpler than the complex plants now dominating land habitats, they continue to serve a critical ecological role:since aquatic habitats are so widespread and algae are so abundant, the algae probably capture 90percent of all the solar energy trapped by photosynthesis, notwithstanding the vast forests and prairies on land.

   The aquatic habitat is a relatively benign and unchanging place, and its properties helped shape the orgaisms that live there. Because water supports the algal plant body, most algae lack rigidity, and usually undulate gently with water currents and waves. Since water surrounds the plant on all sides individual algal cells absorb moisture and minerals directly frome the surounding water and have on need for specialized conduction tubes. Plant shape also reflects this direct contact with water: most algae are quite flattened, which maximizes the surface area for absorbing water, minerals, and sunlight. Finally, reproduction can be asexual, involving the fragmenting of cells or body parts , or it can be sexual, with the production of eggs and sperm. The main secret to the algae’s success is a range of photosynthetic pigments that can absorb the light of the different wavelengths that penetrate to varying water depths. Botanists use these same pigmentys to distinguish between red, brown, and green algae.

RED ALGAE: THE DEEPEST-DWELLING PLANTS

    Most red algae are small, delicate organisms that occur as thin filaments or flat sheets with an ornate, fanllike appearance; some, however, are single-celled or colonial. Red algae generally live in shallow, tropical ocean waters, but a few species survive at depths of about 270 m. These deep-dea denizens are 100times more efficient at capturing sunlight than are the red algae found in shallow waters. They rely on reddish accessory pigments that absorb light in the blue-green range –the only wavelengths that can penetrate to great depths. The  red pigments absorb light energy, and some of it os passed to chlorophyll, which traps the energy and makes it available for the synthesis of sugars.

    The cinspicuous body of a red alga is the haploid gametophyte, which produces a gel-lide protein that can be extracted commercially to make the laboratory growth medium agar; these algae also produce the starchy substance carrageenan, used as a stabilizing agent in ice crem, puddings , cosmetics,and paint.

 

BROWN ALGAE: GIANTS OF THE ALGAL WORLD

    Most species of brown algae inhabit cool, offshore waters and occur as small multicellular plants. However, there are notable exceptions: the largest members of the algal world are the kelps, brown algae that can grow 100 m long and float vertically like tall trees. Huge floating masses of the brown alga Sargassum thrive in the Sargasso Sea, a mass of still water that runs across the Atlantic north of the Caribbean, sometimes entangling hapless divers and ships alike.

   Brown algae range from golden brown to dark brown to black. While they also contain chlorophylls a and c, it is a golden-brown carotenoid pigment that colors the plants and enables them to collect the blue and violet wavelengths of light that penetrate medium-deep water. These pigments explain why kelps can exploit an environment that many other organisms cannot.

   Many brown algae possess complex sturctures analogous to parts of land plants : leaflike fronds collect sunlight and produce sugars; the stemlike stipe supports the plant vertically; and the rootlike holdfast anchors the plant to submerged rocks. Special tubelike conducting cells carry sugars produced in the fronds to the deeper plant parts. These tuves function like the more specialized internal transport systems of land plants, but are not related tissues; both types reflect similar evolutionary solutions to the need for internal transport on a large, multicellular organism. In kelps, the diploid sporophyte dominates the life cycle, but in most smaller brown algae, the gametophyte and sporophyte generations are both freeliving plants that resemble each other.

 

GREEN ALGAE: ANCESTORS OF LAND PLANTS

    Most species of green algae live in shallow freshwater environments or on moist rocks, trees, and soil, although a few inhabit shallow ocean water. Green algae usually occur as single cells or as multicellular, threadlike filaments , hollow balls, or wide, flat sheets.

   Green algae are most notable for producing orange carotenoids and chlorophylls a and b; together these pigments absorb the sunlight penetrating air or shallow water with maximum efficiency. Green algae share this pigment combination only with the land plants. This is one of the reasons botanists think that today’s green algae and land plants share a common ancestor.

   The unicellular green alga Chlamydomonas has been a favorite laboratory organism for investigating how genes control mating, the lashing movements of flagella, and the metabolic processes of photosynthesis and aerobic respiration. Chlamydomonas is an oval cell that typically lives in freshwater pools and moist soil, and is propelled by tow flagella. The most prominet organelles are an eyespot that orients the cell toward light and a large, cup-shaped chloroplast that nearly fills the cell. The haploid phase dominates the live cycle of Chlamydomonas. The single adult cell bears a striking resemblance to a flagellated spore from the multicellular marine green alga Ulva, or sea lettuce, suggesting close evolutionary ties.

   Ulva, an inhabitandt of tide pools, grows a delicate, leaflike body, or thallus, whixh resembles sheets of green cellophane and is just two cells thick. Ulva displays an alternation of conspicuous haploid and diploid phases, which are multicellular and virtually identical. The haploid thallus, the gametophyte, produces gametes, while the identical diploid thallus, the sporophyte, produces spores.

   The aquatic green algae probably gave rise to the simplest land plants, which still rely on standing water for reproduction. The next section recounts physical trends that emerged and allowed life to inhabit the land.