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Prokaryotes play important roles in ecosystems as decomposers recycling nutrients. Some prokaryotes are pathogenic, causing disease and even death in plants and animals. [5] Marine prokaryotes are responsible for significant levels of the photosynthesis that occurs in the ocean, as well as significant cycling of carbon and other nutrients. [6]
Prokaryotes evolved before eukaryotes, and lack nuclei, mitochondria, and most of the other distinct organelles that characterize the eukaryotic cell. Some unicellular prokaryotes, such as cyanobacteria, form colonies held together by biofilms, and large colonies can create multilayered microbial mats.
Cyanobacteria, the only prokaryotes performing oxygenic photosynthesis and the only prokaryotes that contain two types of photosystems (type I (RCI), also known as Fe-S type, and type II (RCII), also known as quinone type). The seven remaining prokaryotes have anoxygenic photosynthesis and use versions of either type I or type II.
Cyanobacteria is the only prokaryotic group that performs oxygenic photosynthesis. Anoxygenic photosynthetic bacteria use PSI- and PSII-like photosystems, which are pigment protein complexes for capturing light. [5] Both of these photosystems use bacteriochlorophyll. There are multiple hypotheses for how oxygenic photosynthesis evolved.
Synechococcus is one of the most important components of the prokaryotic autotrophic picoplankton in the temperate to tropical oceans. The genus was first described in 1979, [5] [6] and was originally defined to include "small unicellular cyanobacteria with ovoid to cylindrical cells that reproduce by binary traverse fission in a single plane and lack sheaths". [7]
The heliobacteria are phototrophic: they convert light energy into chemical energy using a type I reaction center. [6] [7] The primary pigment involved is bacteriochlorophyll g, which is unique to the group and has a unique absorption spectrum; this gives the heliobacteria their own environmental niche. [5]
Oscillatoria uses photosynthesis to survive and reproduce. Each filament of Oscillatoria consists of a row of cells called a trichome. The tip of the trichome oscillates like a pendulum. Reproduction takes place asexually by fragmentation. Usually the filament breaks into a number of fragments called hormogonia.
Many protist species can switch between asexual reproduction and sexual reproduction involving meiosis and fertilization. [6] In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic.