Page 237 - (DK) Ocean - The Definitive Visual Guide

CHROMISTS 235

Habitats

Chromists live in every ocean in the world. Others need sunlight
to photosynthesize, so the microscopic forms, most of which are
phytoplankton, are only found in the well-lit surface layers. Some, such as
foraminiferans can live on the seabed, along with brown seaweeds, which
often dominate rocky seashores in cooler climates. Great underwater forests
of brown kelps grow in colder waters but they and most other brown
seaweeds do not usually grow below about 66 ft (20 m). Some brown
seaweeds grow unattached in sheltered lagoons and sea lochs, and a few
grow in salt marshes anchored in mud. The brown sargassum or gulfweed,
Sargassum natans, is
unusual in that it floats
at the surface of the
open ocean, forming
the basis for a unique
ecosystem (see p.238).

PLANKTON BLOOM
The milky bloom in this satellite
image off Cornwall, UK, is of
the coccolithophore Emiliania
huxleyi, which has multiplied
rapidly in favorable conditions.

Life Strategies

Most single-celled chromists drift in the ocean surface layers as
phytoplankton. They reproduce by splitting into two and some can do
so very rapidly (see above). They can also reproduce sexually, forming the
equivalent of egg and sperm cells. Their number increases dramatically
with the warmth and longer days of
spring and summer. Many seashore
brown seaweeds produce slippery
mucus, both to protect from drying
out and to deter grazers. While
some brown seaweeds are annuals,
living for only a year, large species
are usually perennial.
NEW KELP GROWS FROM OLD
A new, yellow frond is growing from the top
of this kelp stipe. The old frond, which will
drop off, is covered in white animals called
bryozoans, which block vital photosynthesis.
upper valve of HUMAN IMPACT
pillbox-shaped
diatom
FARMING
THE OCEANS
DIATOM DIVISION
When this diatom Coscinodiscus
granii divides, the two halves will Seaweeds are harvested wild, but
separate such that each daughter more are now grown in ocean
cell inherits one valve from its nurseries and farms, especially
parent and creates another itself. in Asia. They are used for food,
and seaweed extracts are used in
a wide range of products—for
example, in gels as a stabilizer, in
cosmetics and pharmaceuticals,
in beer-making, and as a fertilizer.
SEAWEED HARVEST IN ZANZIBAR
Many seaweeds grow readily on floating rafts,
as seen here. They flourish in strong light,
away from grazing invertebrates, and provide
vital income for coastal communities. OCEAN LIFE