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Marine microorganisms are defined by their habitat as microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism (or microbe ) is any microscopic living organism or virus , which is invisibly small to the unaided human eye without magnification .
In the ocean, animal–microbial relationships were historically explored in single host–symbiont systems. However, new explorations into the diversity of marine microorganisms associating with diverse marine animal hosts is moving the field into studies that address interactions between the animal host and a more multi-member microbiome .
The ocean represents the largest continuous planetary ecosystem, hosting an enormous variety of organisms, which include microscopic biota such as unicellular eukaryotes (protists). Despite their small size, protists play key roles in marine biogeochemical cycles and harbour tremendous evolutionary diversity.
Pelagibacter ubique and its relatives may be the most abundant microorganisms in the ocean, and it has been claimed that they are possibly the most abundant bacteria in the world. They make up about 25% of all microbial plankton cells, and in the summer they may account for approximately half the cells present in temperate ocean surface water.
The Pelagibacterales are an order in the Alphaproteobacteria composed of free-living marine bacteria that make up roughly one in three cells at the ocean's surface. [2] [3] [4] Overall, members of the Pelagibacterales are estimated to make up between a quarter and a half of all prokaryotic cells in the ocean.
A new strain of aerobic anoxygenic phototrophic bacteria, JF-1, was recently isolated from deep-ocean hydrothermal vent waters. These bacteria were found to be pleomorphic which shapes varying from coccoid and ovoid rods, to bean-shaped. The coccoid bacteria were from 0.4 to 0.5 μm in size. The ovoid rods were 0.4-0.5 by 1.0-1.2 μm in size.
Here they are exposed to water-borne micro-organisms including fungi during their long period of development. The lobster has a symbiotic relationship with a gram-negative bacterium that has anti-fungal properties. This bacterium grows over the eggs and protects them from infection by the pathogenic fungus-like oomycete Lagenidium callinectes.
The biofilm-like habitat at the surface of the ocean harbours surface-dwelling microorganisms, commonly referred to as neuston. [20] The sea surface microlayer (SML) constitutes the uppermost layer of the ocean, only 1–1000 μm thick, with unique chemical and biological properties that distinguish it from the underlying water (ULW).