The largest cells on Earth
Well, perhaps not as big as the one Kirk and Spock are facing off, but pretty big nonetheless. Researchers at the Scripps Institute of Oceanography have sent remote submersibles into the Mariana Trench, one of the deepest spots in the South Pacific Ocean at 10,600 meters (6.6 miles) below the surface. The abyssal plains of the oceans have been only poorly explored, due to the tremendous pressures experienced by anything we send to those depths. However, there are a lot of living organisms which are well adapted to the high pressures, the cold, and the lack of light. Many of these organisms will depend upon organic materials which drop down from the surface as sources of energy and carbon for their metabolisms. Some organisms are primary producers, however, and utilize chemolithotrophy by growing at undersea geothermal vents.
A remote operated submersible device in July 2011 identified the presence of xenophyophores, a unique class of protozoans. The xenophyophores are 10 cm wide unicellular organisms, similar in cellular makeup to terrestrial amoebas, that feed by adsorbing nutrients via endocytosis and pinocytosis. Although xenophyophores are unicellular in that they appear to have one continuous plasma membrane surrounding them, they do have multiple nuclei throughout the body of the organism, a phenomenon not dissimilar to that found in aseptate fungi or even many terrestrial plants where the cytoplasm is contiguous throughout the organism.
These organisms trap particles that fall from the surface for their nutrition, and consequently contain high levels of many heavy metals. So far, the xenophyophores are a group of organisms only found in the deep sea environment, with no know related organisms a bit closer to home. Their restriction to the high pressure environment at this depth means that we cannot currently study them in the laboratory until a suitable high pressure aquarium can be constructed to keep them alive.
In BIO230, we have discussed some of the reasons that cellular life is size-constrained, and a previous blog posting further explained how eukaryotic cells are able to circumvent some of these constraints. Still, the 10 cm diameter of the xenophyophores far exceeds what had been previously seen for the upper size limit of a cell. By looking at the picture of one of these organisms above, can you conjecture what aspect of the organism is allowing it to achieve such a large size?