Methods
Habitats
Bibliography
Glossary
Syllabus
Students

PHYCOLOGICAL SOCIETY OF AMERICA

Pseudo-nitzschia

Distribution, Limiting Factors & Adaptations

Pseudo-nitzschia Distribution

Generally speaking, diatoms are found in bodies of water across the globe. In fact, diatoms are distributed throughout most waters, with the exception of the hottest and most saline. They are found in phytoplankton in both marine and fresh water in all latitudes. Pseudo-nitzschia maintains this widespread distributuion, however it is mostly marine and planktonic.

Limiting Factors

Though Pseudo-nitzschia has thrived in many parts of the world, there are still factors that limit its (as well as other diatoms's) growth and behavior. In a way, the main limiting factors facing these diatoms are centered around the diatom's use of silica. First, silica amounts limit the growth and behavior of diatoms. In addition, the frustules have a high density of silica which cases sinking. More specifically...

• Silica Concentration: Diatoms have the ability to uptake silica, reducing it to very low concentrations very rapidly and eventually leading to near depletion. In the open ocean, diatoms depend on transport of silicate-rich water from below the thermocline. That is, within the ocean there exists a temperature interface (thermocline) between the two layers of water. There is a top, nutrient poor, warm, and well lit layer as well as a bottom, nutrient rich, poorly lit, cold layer of water. This is an inherent problem because the nutrients (in this case silica) are found below the thermocline and away from the sunshine. Since diatoms, including those in Pseudo-nitzschia , are mostly photosynthetic, they usually exist above the thermocline. In order to access the nutrient rich waters, the diatoms must rely on storms for mixing of the layers as well as upwelling along the coast and equator.
• Sinking: Diatoms do not have propulsion systems, and, in addition, they are more dense than sea or fresh water due to their siliceous frustules. Both factors contribute to the nature of diatoms to sink out of the photic zone (where there is the light that they need) and into the deep waters, eventually being deposited into sediment. These diatoms thus must rely on turbulence from winds and currents to resuspend them.

Adaptations

Pseudo-nitzschia and other genera of diatoms have developed mechanisms in order to adapt to these limiting factors. Due to their small size and their subsequent low weight, they have a lesser tendency to sink. This tendency increases with the cube of the linear dimension. There is also a mucilage envelope around the cells that help lower then density and thus the sinking rate. The composition of ions in the vacuoles can be regulated in the marine species of Pseudo-nitzschia . In replacing heavy ions with lighter ones, the cells become lighter and thus more buoyant. This does not work for those species that inhabit freshwater habitats because there is a low ionic concentration in freshwater. Finally, the shape of diatoms can play a role in increasing surface area and thus viscous drag on a sinking organism.

Back to the Ecology & Natural History page...

copyright Jennifer Shin 1999.