Prokaryotes in the Ocean

Archaea are a group of single-celled microorganisms. Originally, Archaea were classed with bacteria as prokaryotes and called archaebacteria. However, after much research scientists discovered Archaea have an independent evolutionary history and show many differences in their biochemistry from other forms of life, so they are now classified separately in the three-domain system (or Phyolgenetic Tree of Life) as Archaea, Bacteria, and Eucaryota.

 Image of Archaea courtesy of NASA, US Government.

Bacteria and Archaea lack a nucleus, which is why they were originally lumped together under the name prokaryotes (before a nucleus).  Bacteria and Archaea have no organelles or membrane-bound compartments inside of them and are similar in that they can be difficult to distinguish from one another. Although often lumped together under the name Prokaryote, these two groups are only distantly related and each group performs very different functions.

In the ocean, Prokaryotes are abundant, with around 1 billion cells per liter of surface seawater. Prokaryotes can acquire energy to grow from a wide variety of chemical reactions or by photosynthesis. Photosynthetic bacteria (cyanobacteria) are the most abundant phytoplankton in the ocean. Prokaryotic cells range in size from 0.2 – 10.0 um.

Prokaryote Cell, image courtesy Mariana Ruiz Villarreal

Bacteria have four common cell morphologies: spherical, rod-shaped, spiral and comma-shaped. Most prokaryotes exist as individual cells, but some are filamentous or chain-forming.

 Bacteria captured with the MW4-HD1 digital microscope using phase contrast.

Prokaryotes have one or two cell membranes and may have flagella (thin whip-like filaments used to move about). Because Prokaryotes lack a nucleus or membrane-bound compartments inside of them, there is little internal structure visible by microscopy.