Picoplankton
Picoplankton is the fraction of plankton composed by cells between 0.2 and 2 μm that can be either prokaryotic and eukaryotic phototrophs and heterotrophs:
They are prevalent amongst microbial plankton communities of both freshwater and marine ecosystems. They have an important role in making up a significant portion of the total biomass of phytoplankton communities.
Classification
In general, plankton can be categorized on the basis of physiological, taxonomic, or dimensional characteristics. Subsequently, a generic classification of a plankton includes:However, there is a simpler scheme that categorizes plankton based on a logarithmic size scale:
- Macroplankton
- Microplankton
- Nanoplankton
In order to differentiate between autotrophic picoplankton and heterotrophic picoplankton, the autotrophs could have photosynthetic pigments and the ability to show autofluorescence, which would allow for their enumeration under epifluorescence microscopy. This is how minute eukaryotes first became known.
Overall, picoplankton play an essential role in oligotrophic dimicitc lakes because they are able to produce and then accordingly recycle dissolved organic matter in a very efficient manner under circumstance when competition of other phytoplankters is disturbed by factors such as limiting nutrients and predators. Picoplankton are responsible for the most primary productivity in oligotrophic gyres, and are distinguished from nanoplankton and microplankton. Because they are small, they have a greater surface to volume ratio, enabling them to obtain the scarce nutrients in these ecosystems.
Furthermore, some species can also be mixotrophic. The smallest of cells are on the order of nanometers, not picometers. The SI prefix pico- is used quite loosely here, as nanoplankton and microplankton are only 10 and 100 times larger, respectively, although it is somewhat more accurate when considering the volume rather than the length.
Role in ecosystems
Picoplankton contribute greatly to the biomass and primary production in both marine and freshwater lake ecosystems. In the ocean, the concentration of picoplankton is 105–107 cells per millilitre of ocean water. Algal picoplankton is responsible for up to 90 percent of the total carbon production daily and annually in oligotrophic marine ecosystems. The amount of total carbon production by picoplankton in oligotrophic freshwater systems is also high, making up 70 percent of total annual carbon production. Marine picoplankton make up a higher percentage of biomass and carbon production in zones that are oligotrophic, like the open ocean, versus regions near the shore that are more nutrient rich. Their biomass and carbon production percentage also increases as the depth into the euphotic zone increases. This is due to their use of photopigments and efficiency at using blue-green light at these depths. Picoplankton population densities do not fluctuate throughout the year except in a few cases of smaller lakes where their biomass increases as the temperature of the lake water increases.Picoplankton also play an important role in the microbial loop of these systems by aiding in providing energy to higher trophic levels. They are grazed by a various number of organisms such as flagellates, ciliates, rotifers and copepods. Flagellates are their main predator due to their ability to swim towards picoplankton in order to consume them.
Oceanic picoplankton
Picoplankton are important in nutrient cycling in all major oceans, where they exist in their highest abundances. They have many features that allow them to survive in these oligotrophic and low-light regions, such as the use several nitrogen sources, including nitrate, ammonium, and urea. Their small size and large surface area allows for efficient nutrient acquisition, incident light absorption, and organism growth. A small size also allows for minimal metabolic maintenance.Picoplankton, specifically phototrophic picoplankton, play a significant role in the carbon production of open oceanic environments, which largely contributes to the global carbon production. Their carbon production contributes to at least 10% of global aquatic net primary productivity. High primary productivity contributions are made in both oligotrophic and deep zones in oceans. Picoplankton are dominant in biomass in open ocean regions.
Picoplankton also form the base of aquatic microbial food webs and are an energy source in the microbial loop. All trophic levels in a marine food web are affected by picoplankton carbon production and the gain or loss of picoplankton in the environment, especially in oligotrophic conditions. Marine predators of picoplankton include heterotrophic flagellates and ciliates. Protozoa are a dominant predator of picoplankton. Picoplankton are often lost through processes such as grazing, parasitism, and viral lysis.