Lesson 4 - Page 1



 Fresh water ponds, lakes, and rivers throughout the world have many different microscopic and macroscopic organisms.  This microworld is full of small plants, protozoa, bacteria, fungi, and animals.  These critters provide the food source for larger organisms in complex food webs.  They can also tell us whether the environment is healthy.  The microorganisms have different roles.  Some are autotrophic or can make their own food from the Sun.  Some are heterotrophic or ingest food including plants and other heterotrophic organisms. Some live their entire live in the water (holoplankton) and others only have their larval stages in the water (meroplankton) and then change into flying or land-born organisms.

Surirella  sp., a diatom

Holoplankton spends their entire lives as part of the plankton.  It includes any organisms whether autotrophic (phytoplankton) or heterotrophic (zooplankton), that is controlled by the water movement in which they reside.  However, some components are capable of slight movement, but cannot counter stronger water currents.  The marine environment also uses this classification of living styles of their littlest creatures.

Pennate Diatom Gyrosigma sp.

Diatom parts


The autotrophic representatives include diatoms and green algae.   Autotrophic

organisms have two major strategies to produce food by photosynthesis or through chemical means.  Photosynthesis is a chemical process that requires light and chlorophyll-A which in found in chloroplasts.  Green plankton produce food in the form.  Chemotrophs are organisms that obtain their food or energy through the exchange of molecules from their environment.  Inorganic or organic material can be used by these organisms.  So these are organisms  that use chemistry to eat!

Autotrophic unicellular organisms are the primary source of food for zooplankton.  Common autotrophic organisms called diatoms are plankton that lack flagella (except the male gametes).  Frustules  (shells or valves) are overlapping like a “pill” box and are made of opaline silica.  They are identified by the frustule by its appearance..  They possess raphe, which is a slit on the top valve.  They are fresh or marine and contain chlorophyll a and c.  They are autotrophic with mainly asexual reproduction.  There are over 37,000 species.


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