A dominant mode of life on Earth includes parasitism, and both protozoan and parasitic metazoan eukaryotes are known to cause some of the most intractable and damaging diseases to domestic animals and humans. The lives of millions of children living in Sub-Saharan Africa alone are taken by malaria, a leading mortality cause while other infections including parasitic worms cause chronic disorders to nearly 33 percent of the global population yet are considered neglected diseases (Ajioka, Budjuso).
These pathogens are able to survive and reproduce without significant difficulty due to the diverse transmission modes in endemic regions which often involve complex interactions between environmental, vector, and host factors. Consequently, the study of parasitic infections has remained throughout history a scientific undertaking spanning multiple disciplines yet are uniquely oriented towards developing nations that often suffer from widespread poverty. However, the primary area of study with respect to pathogens includes pathology and the primary substrate for harboring this organisms includes soil contamination (Ajioka, Budjuso).
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Fungi represent any member of eukaryotic organisms that produce microorganism offspring including molds and yeasts as well as the most familiar known as mushrooms. One particular characteristic that differentiates this pathogen from bacteria, plants, and a handful of protists includes the chitin found within the boundaries of their cell walls (Alexopoulos et al). Like animals, fungi operate as heterotrophs; harvesting their food and sustenance through the absorption of dissolved molecules which is often accomplished by the secretion of their environment’s digestive enzymes.
However, it is important to keep in mind that unlike plants, fungi do not engage in photosynthesis and their primary method of transportation is through growth or spores which are able to travel through the water or the air. Two examples of pathogenic fungi include Ustilago maydis and Cryptococcus neoformans (Alexopoulos et al). The former is responsible for smut disease in both teosinte and maize. Although plants have adapted efficient defense systems against invading microbes through oxidative bursts, the fungus often introduces a stress response for protection in addition to their natural DNA repair system that acts as a recombinant during meiosis and mitosis. The latter exists as a yeast which encapsulates itself in both animals and plants. Typically infecting the lungs, the fungus often undergoes phagocytosis via alveolar macrophages.
In a symbiotic relationship, lichens are composite organisms that arise from either cyanobacteria or algae that live across multiple fungi filaments. Combined together, the lichen contains properties that differ from the component organisms and grow in a variety of forms, colors, and sizes (Ajioka, Budjuso). They may contain fruticose (small, leafless branches), foliose (flat leaves), or crustose (flakes that appear on the surface). Although they do not possess the roots necessary to absorb nutrients and water in the same way that plants do, they are able to produce their own sustenance through photosynthesis. On plants, they do not operate as parasites and simply utilize the host as a substrate. Nearly 6 percent of the entirety of Earth’s land surface is populated with lichen due to the fact that they can grow on rock, gravestones, exposed soil surfaces, biological crust, walls, or roofs.
Several deviations have developed the adaptations necessary to survive in more extreme environments including arctic tundra, toxic slag heaps, hot dry deserts, and rocky coasts. They are able to reproduce asexually through vegetation or the dispersal of diaspores or simply by breaking off into individual fragments. However, only the lichen’s fungal partner can reproduce sexually (Ajioka, Budjuso). Although they officially considered substrates, there is sufficient evidence to suggest that depending on the species, they could be considered pathogenic. For example, throughout the course of nutrient exchange between a fungal partner, photobiont cells are destroyed as part of a routine.
Protozoa is a term used to describe a diverse group of eukaryotic organisms which are unicellular in nature. Given that they exist as a critical source of food for microinvertebrates, the ecological role of protozoa is to transfer algal and bacterial production to trophic levels that are successive (Lumen Candela). They naturally prey upon filamentous or unicellular algae, microfungi, and bacteria as a way of controlling biomass and populations. In the food chain’s decomposer link, species can span across both consumers and herbivores which can stimulate organic matter decomposition, digest cellulose, and mobilize nutrients. The most infamous protozoan pathogen suffered through parasitic attacks in humans includes that of malaria; a disease that often results in headaches, fatigue, vomiting, and in more severe cases seizures or even death (Lumen Candela). Typically transmitted via a bite from an infected mosquito, the saliva enters a human’s bloodstream and the parasites travel to the region of the liver to cultivate and reproduce.
Algae is defined by a significant group of photosynthetic eukaryotic organisms that are considered polyphyletic given that most species are not closely related with each other nor do they share a common ancestor. Furthermore, there is no official definition to describe this species as each exhibit a diverse range of reproductive strategies through cell division or complex forms of partner to partner reproduction. They are prominent in terrestrial water bodies and in more unusual locales such as ice and snow. In aquatic ecology specifically, they play essential roles. For example, phytoplankton which live suspended in water columns are the food base for the majority of marine wildlife (Lumen Candela). However, they are considered pathogenic when existing in higher densities as they often discolor the water and poison, outcompete, and even asphyxiate other forms of life. Perhaps the most well known example of pathogenic algae in mammalians includes the disease known as Protothecosis, which is known to cause significant infection in cattle, dogs, and cats given the lack of chlorophyll (Lumen Candela).
