Page 250 - (DK) Ocean - The Definitive Visual Guide

248 PLANT LIFE

Green Algae Anatomy

THESE MICROSCOPIC, Marine microscopic green algae mostly belong to a class of
DOMAIN Eucarya algae called the Prasinophyceae. Each consists of a single living
MOSTLY single-celled cell that is generally too small to be visible to the naked human
KINGDOM Plantae
plants live in the surface eye. Even the larger species, such as members of the genera
DIVISION Chlorophyta layers of the ocean in Halosphaera and Pterosperma, measure just 0.1–0.8 mm across, so
CLASS Prasinophyceae immense numbers, and appear as no more than a speck. Some green algae can swim, and
SPECIES 200 they form an important beat two or more hairlike structures, called flagellae, to move
through the water. Others lack flagellae and cannot propel
part of the phytoplankton
themselves. Several groups of these plants have a two-stage life
(see p.212). Sometimes referred to as the “grasses of history, including both swimming and non-swimming forms.
the sea,” like most plants, they produce their own food HALOSPHAERA
These microalgae All green algae possess
through photosynthesis. Large green algae, visible to the (shown greatly chloroplasts—structures GREEN BEACHES
naked eye, are called green seaweeds and are discussed enlarged) are green that contain the green
with chlorophyll
elsewhere in this book (see p.246). Microscopic algae and bear hairlike pigment chlorophyll A few green algae and worms
are often termed “microalgae.” Green microalgae are swimming appendages that plants use in form symbiotic partnerships, in
which both species gain. The
frequently classified as protists. Numerous other groups called flagellae. photosynthesis. beach-living worms ingest algae,
of protists (see p.236) are also termed algae, and also live giving them a green color. At low
as phytoplankton. tide, they move up through the
sand to pools on the surface,
Habitats where the algae photosynthesize.
In return, the worms absorb food
from the algae. Vast numbers of
With a few exceptions, marine microalgae swim and
the worms tinge beaches green.
float, in countless millions, in the sunlit layers of the
ocean so that photosynthesis can occur. They
are more numerous in nutrient-rich waters,
such as those benefiting from coastal
runoff. In temperate coastal waters, green
microalgae multiply rapidly each spring
in response to rising nutrient and light
levels, creating abundant food for
zooplankton. Such population
explosions, or blooms, can reduce the
water clarity for weeks. Some green ANIMAL–ALGA PARTNERSHIP
algae live inside the bodies of animals When young, these marine flatworms ingest
green algae, which may multiply until there are
(see panel, right) and inside protist 25,000 algal cells living in each worm. The adult
plankton—in the appendages (rhizopoda) worms obtain all their nutrition from the algae.
of radiolarians (see p.237) and within
compartments inside the dinoflagellate
Noctiluca (see p.236).

GREEN TIDE
Green algae grow quickly and are the first to respond
in spring when nutrients become available. While
grazer levels are low, the algae are free to
multiply until their density turns the ocean green.

CLASS PRASINOPHYCEAE CLASS PRASINOPHYCEAE
Halosphaera viridis Tetraselmis
convolutae
SIZE
20–30 micrometers
(motile phase) SIZE
10 micrometers

DISTRIBUTION Northeastern Atlantic, eastern Pacific
DISTRIBUTION Northeastern Atlantic, off the
Halosphaera viridis is a small, pear- western coasts of Britain and France
shaped cell with four swimming
flagellae at one end. It reproduces by Although it can survive free-living, the
splitting in two, allowing it to reach tiny cells of Tetraselmis convolutae often
OCEAN LIFE whose contents divide into small disks. in turn provides both algae and worm
live inside a worm host (see box, above)
high concentrations and from time
in a symbiotic relationship. The worm
to time some cells become small cysts
provides them with shelter and a
constant environment inside its
Each disk eventually becomes a
flagellated cell that will be released
body. The worm’s light-seeking
behavior gives the algae ideal
into the sea. There can be hundreds
conditions for photosynthesis, which
of cysts per square yard in the open
ocean, and they are probably a vital
with nutrients and energy.
food source for larger zooplankton.