Aeroplankton
Aeroplankton are tiny lifeforms that float and drift in the air, carried by wind. Most of the living things that make up aeroplankton are very small to microscopic in size, and many can be difficult to identify because of their tiny size. Scientists collect them for study in traps and sweep nets from aircraft, kites or balloons. The study of the dispersion of these particles is called aerobiology.
Aeroplankton is made up mostly of microorganisms, including viruses, about 1,000 different species of bacteria, around 40,000 varieties of fungi, and hundreds of species of protists, algae, mosses, and liverworts that live some part of their life cycle as aeroplankton, often as spores, pollen, and wind-scattered seeds. Additionally, microorganisms are swept into the air from terrestrial dust storms, and an even larger amount of airborne marine microorganisms are propelled high into the atmosphere in sea spray. Aeroplankton deposits hundreds of millions of airborne viruses and tens of millions of bacteria every day on every square meter around the planet.
Small, drifting aeroplankton are found everywhere in the atmosphere, reaching concentration up to 106 microbial cells per cubic metre. Processes such as aerosolization and wind transport determine how the microorganisms are distributed in the atmosphere. Air mass circulation globally disperses vast numbers of the floating aerial organisms, which travel across and between continents, creating biogeographic patterns by surviving and settling in remote environments. As well as the colonization of pristine environments, the globetrotting behaviour of these organisms has human health consequences. Airborne microorganisms are also involved in cloud formation and precipitation, and play important roles in the formation of the phyllosphere, a vast terrestrial habitat involved in nutrient cycling.
Overview
The atmosphere is the least understood biome on Earth despite its critical role as a microbial transport medium. Recent studies have shown microorganisms are ubiquitous in the atmosphere and reach concentration up to 106 microbial cells per cubic metre and that they might be metabolically active. Different processes, such as aerosolisation, might be important in selecting which microorganisms exist in the atmosphere. Another process, microbial transport in the atmosphere, is critical for understanding the role microorganisms play in meteorology, atmospheric chemistry and public health.Changes in species geographic distributions can have strong ecological and socioeconomic consequences. In the case of microorganisms, air mass circulation disperses vast amounts of individuals and interconnects remote environments. Airborne microorganisms can travel between continents, survive and settle on remote environments, which creates biogeographic patterns. The circulation of atmospheric microorganisms results in global health concerns and ecological processes such as widespread dispersal of both pathogens and antibiotic resistances, cloud formation and precipitation, and colonization of pristine environments. Airborne microorganisms also play a role in the formation of the phyllosphere, which is one of the vastest habitats on the Earth's surface involved in nutrient cycling.
The field of bioaerosol research studies the taxonomy and community composition of airborne microbial organisms, also referred to as the air microbiome. A recent series of technological and analytical advancements include high-volumetric air samplers, an ultra-low biomass processing pipeline, low-input DNA sequencing libraries, as well as high-throughput sequencing technologies. Applied in unison, these methods have enabled comprehensive and meaningful characterization of the airborne microbial organismal dynamics found in the near-surface atmosphere. Airborne microbial organisms also impact agricultural productivity, as bacterial and fungal species distributed by air movement act as plant blights. Furthermore, atmospheric processes, such as cloud condensation and ice nucleation events were shown to depend on airborne microbial particles. Therefore, understanding the dynamics of microbial organisms in air is crucial for insights into the atmosphere as an ecosystem, but also will inform on human wellbeing and respiratory health.
In recent years, next generation DNA sequencing technologies, such as metabarcoding as well as coordinated metagenomics and metatranscriptomics studies, have been providing new insights into microbial ecosystem functioning, and the relationships that microorganisms maintain with their environment. There have been studies in soils, the ocean, the human gut, and elsewhere.
In the atmosphere, though, microbial gene expression and metabolic functioning remain largely unexplored, in part due to low biomass and sampling difficulties. So far, metagenomics has confirmed high fungal, bacterial, and viral biodiversity, and targeted genomics and transcriptomics towards ribosomal genes has supported earlier findings about the maintenance of metabolic activity in aerosols and in clouds. In atmospheric chambers airborne bacteria have been consistently demonstrated to react to the presence of a carbon substrate by regulating ribosomal gene expressions.
Types
Pollen grains
Effective pollen dispersal is vital for maintenance of genetic diversity and fundamental for connectivity between spatially separated populations. An efficient transfer of the pollen guarantees successful reproduction in flowering plants. No matter how pollen is dispersed, the male-female recognition is possible by mutual contact of stigma and pollen surfaces. Cytochemical reactions are responsible for pollen binding to a specific stigma.Allergic diseases are considered to be one of the most important contemporary public health problems affecting up to 15–35% of humans worldwide. There is a body of evidence suggesting that allergic reactions induced by pollen are on the increase, particularly in highly industrial countries.
File:Misc pollen colorized.jpg|thumb|upright=1.5|left| Colourised SEM image of pollen grains from common plants
Fungal spores
, a major element of atmospheric bioaerosols, are capable of existing and surviving in the air for extended periods of time. Both the spores and the mycelium may be dangerous for people suffering from allergies, causing various health issues including asthma. Apart from their negative impact on human health, atmospheric fungi may be dangerous for plants as sources of infection. Moreover, fungal organisms may be capable of creating additional toxins that are harmful to humans and animals, such as endotoxins or mycotoxins.Considering this aspect, aeromycological research is considered capable of predicting future symptoms of plant diseases in both crops and wild plants. Fungi capable of travelling extensive distances with wind despite natural barriers, such as tall mountains, may be particularly relevant to understanding the role of fungi in plant disease. Notably, the presence of numerous fungal organisms pathogenic to plants has been determined in mountainous regions.
A wealth of correlative evidence suggests asthma is associated with fungi and triggered by elevated numbers of fungal spores in the environment. Intriguing are reports of thunderstorm asthma. In a now classic study from the United Kingdom, an outbreak of acute asthma was linked to increases in Didymella exitialis ascospores and Sporobolomyces basidiospores associated with a severe weather event. Thunderstorms are associated with spore plumes: when spore concentrations increase dramatically over a short period of time, for example from 20,000 spores/m3 to over 170,000 spores/m3 in 2 hours. However, other sources consider pollen or pollution as causes of thunderstorm asthma. Transoceanic and transcontinental dust events move large numbers of spores across vast distances and have the potential to impact public health, and similar correlative evidence links dust blown off the Sahara with pediatric emergency room admissions on the island of Trinidad.
Pteridophyte spores
s are vascular plants that disperse spores, such as fern spores. Pteridophyte spores are similar to pollen grains and fungal spores, and are also components of aeroplankton. Fungal spores usually rank first among bioaerosol constituents due to their count numbers which can reach to between, while pollen grains and fern spores can each reach to between.Arthropods
Many small animals, mainly arthropods, are also carried upwards into the atmosphere by air currents and may be found floating several thousand feet up. Aphids, for example, are frequently found at high altitudes.Ballooning, sometimes called kiting, is a process by which spiders, and some other small invertebrates, move through the air by releasing one or more gossamer threads to catch the wind, causing them to become airborne at the mercy of air currents. A spider, or spiderling after hatching, will climb as high as it can, stand on raised legs with its abdomen pointed upwards, and then release several silk threads from its spinnerets into the air. These automatically form a triangular shaped parachute which carries the spider away on updrafts of winds where even the slightest of breezes will disperse the arachnid. The flexibility of their silk draglines can aid the aerodynamics of their flight, causing the spiders to drift an unpredictable and sometimes long distance. Even atmospheric samples collected from balloons at altitude and ships mid-ocean have reported spider landings. Mortality is high.
Enough lift for ballooning may occur, even in windless conditions, if an electrostatic charge gradient is present in the atmosphere.