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15.3: Life in the Ocean

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    The traditional system of categorizing life defines five Kingdoms: animal, plant, bacteria, fungi and protist. And just as all five can be found across the continents, they exist and flourish in the sea as well. Here, life is adapted to various hazards including temperature, pressure, darkness, and salinity. The greatest depths are where the plant kingdom begins to show itself less while the other kingdoms exist in a quiet dark stillness. Their network of interactions changes with each type of ocean environment and results in a colorful diversity in life throughout the oceans.



    Bacteria are single-celled organisms lacking membrane-bound nuclei. They are found all throughout the water column, some being aerobic and some anaerobic. They exist in the water, in sediment, and of course, in many organisms throughout the ocean. Marine bacteria play different roles when compared with bacteria in other habitats. Some marine organisms harbor light-emitting bacteria, which forms a symbiotic relationship working to create an effect called bioluminescence, and the light produced could be used to hunt prey, defend against predators, and also find mates1. Other bacteria, like cyanobacteria, photosynthesize and are said to be the original source of oxygen on the early planet. They build stromalites, which also provide scientists with a key piece of evidence as to the Earth's past geological history. In addition to that, deep-sea bacteria that thrive on minerals from hydrothermal vents also develop important relationships with the surrounding communities, such as providing essential food sources.

    Courtesy of UCSB

    Marine Bacteria: Courtesy of UCSB


    Marine fungi mainly have continental origins, many being transported out to sea by drifting organic matter from land. They exist largely as parasites on the other kingdoms of life. They can be found living within sand grains, inside coral, and in/outside plants and animals. Many marine fungi have been found to originate from mangrove swamps in which floating pieces of driftwood harbor fungi that may disperse down the water column. Examples include:

    • Rhyzophydium littoreum
    • Arthopyrenia halodytes
    • Fusarium
    • Candida albicans


    Salmon with fungal disease: Courtesy of Wikipedia


    Plants are autotrophic organisms. That means that they can build their own organic matter, rather than consuming it. Plants use sunlight to build this organic matter through the process of photosynthesis. Therefore, they can be called photoautotrophs. All plants are multicellular, which means their bodies are composed of multiple cells. Underwater plants are adapted to higher levels of salinity and are found all throughout the upper ocean. Here, plants are adapted in similar ways to their terrestrial counterparts. Both have developed ways to anchor themselves and deal with wind or water currents. 


    Mangroves: Courtesy of Wikipedia


    In comparison to plants, animals cannot create their own organic matter. Therefore, they must eat. Organisms that must consume organic matter are referred to as heterotrophs. The most visible kingdom of life in the oceans is the animal kingdom. Marine animals are adapted to a variety of hardships at all ranges across the ocean. The main food web lies between members of the animal kingdom, where the other kingdoms function mainly as food sources or recyclers. Marine animals are both predators and herbivores, creating a complex diversity in the way ecosystems must interact to sustain themselves. Most animals are invertebrates, or organisms that lack a vertebral column. Vertebrates include fish, seabirds, sea turtles, and marine mammals. Some important Phyla of animals are described in the next sections. 


    Blacktip Reef Shark, Courtesy of Tracking Sharks



    Protists cannot be classified as plants, animals, or fungi, and many scientists no longer include Protista as a Kingdom. Nevertheless, it is an easy way to categorize some organisms that don't fit into the other Kingdoms. This group includes both unicellular and multicellular organisms, and organisms that are autotrophic and those that are heterotrophic. They are found anywhere containing liquid water and exist in the oceans as plankton and algae. Many marine protists provide a food source for the upper levels of the oceanic food web. Familiar marine protists include:

    • macroalgae
    • dinoflagellates
    • diatoms
    • coccolithophores


    Protists: Courtesy of UMD

    15.3: Life in the Ocean is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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