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23.4A: Protists as Primary Producers, Food Sources, and Symbionts - Biology

23.4A: Protists as Primary Producers, Food Sources, and Symbionts - Biology


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Protists function as sources of food for organisms on land and sea.

Learning Objectives

  • Give examples of how protists act as primary producers

Key Points

  • Photosynthetic protists serve as producers of nutrition for other organisms.
  • Protists like zooxanthellae have a symbiotic relationship with coral reefs; the protists act as a food source for coral and the coral provides shelter and compounds for photosynthesis for the protists.
  • Protists feed a large portion of the world’s aquatic species and conduct a quarter of the world’s photosynthesis.
  • Protists help land-dwelling animals such as cockroaches and termites digest cellulose.

Key Terms

  • zooxanthella: an animal of the genus Symbiodinium, a yellow dinoflagellate, notably found in coral reefs
  • primary producer: an autotroph organism that produces complex organic matter using photosynthesis or chemosynthesis

Primary Producers/Food Sources

Protists function in various ecological niches. Some protist species are essential components of the food chain and are generators of biomass.

Protists are essential sources of nutrition for many other organisms. In some cases, as in plankton, protists are consumed directly. Alternatively, photosynthetic protists serve as producers of nutrition for other organisms. For instance, photosynthetic dinoflagellates called zooxanthellae use sunlight to fix inorganic carbon. In this symbiotic relationship, these protists provide nutrients for the coral polyps that house them, giving corals a boost of energy to secrete a calcium carbonate skeleton. In turn, the corals provide the protists with a protected environment and the compounds needed for photosynthesis. This type of symbiotic relationship is important in nutrient-poor environments. Without dinoflagellate symbionts, corals lose algal pigments in a process called coral bleaching and they eventually die. This explains why reef-building corals do not reside in waters deeper than 20 meters: insufficient light reaches those depths for dinoflagellates to photosynthesize.

The protists themselves and their products of photosynthesis are essential, directly or indirectly, to the survival of organisms ranging from bacteria to mammals. As primary producers, protists feed a large proportion of the world’s aquatic species. (On land, terrestrial plants serve as primary producers. ) In fact, approximately one-quarter of the world’s photosynthesis is conducted by protists, particularly dinoflagellates, diatoms, and multicellular algae.

Protists do not only create food sources for sea-dwelling organisms. Certain anaerobic parabasalid species exist in the digestive tracts of termites and wood-eating cockroaches where they contribute an essential step in the digestion of cellulose ingested by these insects as they bore through wood.


23.4A: Protists as Primary Producers, Food Sources, and Symbionts - Biology

Multitrophic microbial loop interactions drive sustainable agriculture and ecosystem services.

These interactions are relatively less investigated with respect to the development of sustainable eco-and agro-biotechnologies.

Climate and land use changes are adversely affecting the earth ecosystem – thus causing microbial trophic downgrading such as microbial diversity loss, which may impair ecosystem services.

Theory-driven research may not only enhance the implications of these microbial interactions in microbial based eco-biotechnologies, but it may also help in the preservation of aquatic and agro-ecosystem health and productivity.

Future research should combine ecological, environmental, and molecular genetic approaches to dissect the functional patterns of microbial multitrophic interactions to elucidate whether these are driven by species or functional trait diversity to develop successful strategies for sustainable agriculture and ecosystem services.

Multitrophic level microbial loop interactions mediated by protist predators, bacteria, and viruses drive eco- and agro-biotechnological processes such as bioremediation, wastewater treatment, plant growth promotion, and ecosystem functioning. To what extent these microbial interactions are context-dependent in performing biotechnological and ecosystem processes remains largely unstudied. Theory-driven research may advance the understanding of eco-evolutionary processes underlying the patterns and functioning of microbial interactions for successful development of microbe-based biotechnologies for real world applications. This could also be a great avenue to test the validity or limitations of ecology theory for managing diverse microbial resources in an era of altering microbial niches, multitrophic interactions, and microbial diversity loss caused by climate and land use changes.


History of Classification

Protists include a remarkable number and variety of living organisms that far outnumber bacteria and viruses in their species diversity. It is estimated that there are nearly three times as many undiscovered protists as there are ones that have been described. Their functional diversity and the cosmopolitan nature of the niches they inhabit make them crucial for conservation and the maintenance of biodiversity.

Protists were first classified as a group of organisms by Ernst Haeckel in the 1860s, using the term derived from the Greek word protistos meaning ‘the very first’. It was initially used to indicate that these organisms were probably primitive forms of plants and animals. This term appeared in the backdrop of the invention of the microscope and the discovery of a wide variety of microorganisms.

DNA sequencing and molecular genetics have made it easier to establish evolutionary lineages and the relationships between different groups of organisms. This has further contributed to the redistribution of protists among the other five eukaryotic kingdoms. Some scientists however classify them based on their ultrastructure and biochemistry. Classification of protists continues to be an area of active research, even as new tools are emerging for the study of phylogenetics.


Section Summary

Prokaryotes existed for billions of years before plants and animals appeared. Microbial mats are thought to represent the earliest forms of life on Earth, and there is fossil evidence, called stromatolites, of their presence about 3.5 billion years ago. During the first 2 billion years, the atmosphere was anoxic and only anaerobic organisms were able to live. Cyanobacteria began the oxygenation of the atmosphere. The increase in oxygen concentration allowed the evolution of other life forms.

Prokaryotes (domains Archaea and Bacteria) are single-celled organisms lacking a nucleus. They have a single piece of circular DNA in the nucleoid area of the cell. Most prokaryotes have cell wall outside the plasma membrane. Bacteria and Archaea differ in the compositions of their cell membranes and the characteristics of their cell walls.

Bacterial cell walls contain peptidoglycan. Archaean cell walls do not have peptidoglycan. Bacteria can be divided into two major groups: Gram-positive and Gram-negative. Gram-positive organisms have a thick cell wall. Gram-negative organisms have a thin cell wall and an outer membrane. Prokaryotes use diverse sources of energy to assemble macromolecules from smaller molecules. Phototrophs obtain their energy from sunlight, whereas chemotrophs obtain it from chemical compounds.

Infectious diseases caused by bacteria remain among the leading causes of death worldwide. The excessive use of antibiotics to control bacterial infections has resulted in resistant forms of bacteria being selected. Foodborne diseases result from the consumption of contaminated food, pathogenic bacteria, viruses, or parasites that contaminate food. Prokaryotes are used in human food products. Microbial bioremediation is the use of microbial metabolism to remove pollutants. The human body contains a huge community of prokaryotes, many of which provide beneficial services such as the development and maintenance of the immune system, nutrition, and protection from pathogens.



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