Photo synthesizers capture solar energy. The chloroplasts in their cells use light energy to synthesize glucose (a kind of sugar) from water and carbon dioxide. Therefore your algae need access to light.

Important:  seaweeds need access to light.

     All living things need water, and some of the chemicals that dissolve in water. Algae that live under water do not have to worry about drying out, so their survival strategies are about light capture and reproduction.

The Open Ocean

     The water is deep in the open ocean, too deep for light to penetrate to the ocean floor. Organisms dependent on photosynthesis cannot hope to live on the bottom, which may be miles below. To survive in the open ocean, seaweeds must float on the surface of the water, where the sun shines brightly, and the only the shade that falls on the water is that cast by clouds.

    The photo synthesizers here are mostly different kinds of single celled organisms that we call phytoplankton.  We cannot see the tiny individual cells, but they are abundant, and sometimes these tiny cells even change the color of the water.  They are the base of the food chain in the ocean, and are eaten by tiny animals and the young of larger animals that we refer to as zooplankton.  Then larger animals eat the zooplankton.  The members of the phytoplankton, many of whom are algae, produce a good part of our atmospheric oxygen.

      In addition to sunlight, the phytoplankton also need minerals and nutrients. These are carried down to the coasts by rivers and runoff water, so the organisms near the land can use these resources. On the surface of the deep ocean the number of living creatures varies with the availability of nutrients. At certain times of year there is an upwelling of waters from deeper levels. These waters bring up chemicals from the deep levels, where dead animals and seaweeds have drifted down and decomposed. When the nutrients are carried up, the surface of the ocean becomes thick with phytoplankton, and tiny photosynthesizes and animals form a vast floating community. When nutrients are scarce, only a few organisms live in these sunny waters.  These organisms reproduce very quickly when nutrients are abundant,.

     In addition to the tiny algae, some multicellular algae, which we would call seaweeds, may form floating mats.  These maps form in areas where sea currents circle, like those in the Sargasso Sea. The Sargasso Sea has large tangled masses of a variety of kelp and a whole ecosystem of smaller algae and small and microscopic animals. Larger floating seaweeds may have special structures that they fill with gas, like tiny, very tough balloons, which help the seaweed to float. These palgae get lots of light, but they are at the mercy of storms and winds. They may get blown into shallow water, or even onto shore, where they dry out and die.

     Many of these large multicellular algae reproduce vegetatively, that is, they just grow, and when pieces get broken off by wind or big waves the pieces are also complete seaweeds. These floating surface algae are most abundant in warmer waters, because chemical reactions (what life does) take place more quickly in warmer temperatures. Those in cold waters grow more slowly than algae in warmer seas.

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Seaweeds  in Shallow Water

     Shallow water may lie above rocky, sandy, or muddy bottoms. Water is usually shallow close to land, so floating algae in shallow water risk being thrown up onto the beach by the tides and by storms. Light can penetrate for quite some distance into clean water: our textbook, The Earth Atlas, says that the "good light" zone goes down 75 feet. Many shallow water algae are close to the surface of the water, perhaps only inches from the surface. They find some way to attach themselves to the bottom of the ocean so that they can grow up toward the light and not be swept away.

The strategies that these algae use depend on what the bottom is like. If the bottom is rocky, seaweeds develop some way of holding onto it. On earth, seaweeds develop structures called hold holdfasts. These are not roots, because they do not absorb water or nutrients: they just hold onto a rock. Kelp anchor themselves with holdfasts. Kelp  may be up to 90 feet in length, and they form undersea forests alive with fishes and other life forms.

Small seaweeds also cling to the rocks. Some of them are only a few inches long and cling to rocks near the shore. Some even appear in the challenging intertidal zone, where the rocks they cling to may be out of water for short periods of time.

     Where the sea bottom is made of sand or mud, there is nothing for algae to hold onto. Some sea grasses  in these areas develop runners that grow through the mud and help to hold them down, however, seagrasses are not algae: they are land plants that moved back into the underwater environment about 100 million years ago on earth.  Sea grasses are real plants and so have roots, which allow them to live in an area where the algae that are native to the sea cannot survive..

     Sometimes there are places where rivers flow into the sea, carrying a lot of silt (fine particles of clay) with them. These areas can be difficult for photo synthesizers to live in: if they get buried by the silt, they will die from lack of light.

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Temperatures Affect Algal Growth

    Algae can grow more quickly in warm water than in cold water, because chemical reactions occur more quickly where there is more heat. You would expect to find many water seaweeds near the equator where the water is warm. However, nutrients must also be present if the seaweeds are to grow.

     Where the water is very cold, as in arctic seas, you also find phytoplankton, and even algae that live under the ice. Seaweeds grow there because deep sea currents bring nutrients up from the depths. Seaweeds are dependent on both light and the availability of the chemicals needed for growth.

Special Problems for the Earliest Photo synthesizers

     The earliest organisms were able to develop only because they were under water. The ultraviolet light contained in unshielded solar radiation kills living organisms. Nothing could live on the surface of the earth until the planet had enough oxygen in the atmosphere to allow the protective ozone layer to form. Water provided shielding from this lethal radiation, providing an environment for the earliest forms of life.  Later some of the oxygen that the cyanobacteria and the algae produced became the ozone layer that protects all earth dwellers.