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.