In the animal kingdom, it is extremely difficult to find an organism that lives in isolation. Many depend on other living things for food, protection and other resources they require for survival. For example, microbes and photosynthetic plants provide oxygen for human beings. Trees also provide shelter to some animals and other smaller plants. While such relationships are basic, there are many instances that associations are more involved. For instance, shrimps dig burrows at the seabed and share them with goby fish, which cannot dig on their own. In return, goby fish will warn the near blind in the event of danger. Such deep relationships, in which animals need each other for survival, are common. This phenomenon is referred to as symbiosis, a term that loosely translates to ‘living together’. Bacteria have an extensive history of forming beneficial symbiotic relationships with other organisms. This paper will discuss these relationships.
Bacteria and other Organisms
Nitrogen and carbon are essential to the lives of all organisms on Earth. Even though the former is abundant on the planet, it would possibly become useless if bacteria had not existed. Rhizobia, bacteria found in the soil, have established a symbiotic relationship with legumes that benefit both organisms and play a crucial in nitrogen fixation. Legumes have nodules on their roots and stems to enable them fix nitrogen into a form that can be used by plants directly and animals. However, the nodules cannot do so on their own. Therefore, they depend on rhizobia to convert atmospheric nitrogen into inorganic chemicals compounds such as NH4+ (ammonium) that is necessary in producing amino acids and proteins for plant usage. Rhizobia have an enzyme called nitrogenase, which is capable of converting atmospheric nitrogen into ammonium. The bacteria are normally located on the roots of legumes. Their association means that legumes are the only plants that can use nitrogen in the air as fertilizer (Zahran, 1999).
Nitrogen is abundant in the air. However, without rhizobia, it would be useless to plants. Nitrogen fixation does not benefit only legumes but all other plants around them. Therefore, their intervention of the bacteria is highly beneficial to the plant kingdom. In return for their chemical conversion capabilities, rhizobia benefit from the shelter provided by the plants’ nodules. Furthermore, they also absorb some of the micronutrients and carbon substrates found in the roots. These help them to generate energy and vital metabolites that are essential for life sustenance. None of these two organisms is harmed by this relationship.
Apart from rhizobia, many other bacteria fix nitrogen for different plants. For instance, Frankia, Azospirillum and Azotobacter ectosymbiotically fix nitrogen for trees, grasses and crop plants respectively. Furthermore, many plants in a terrestrial ecosystem depend on actions of bacteria to produce nitrogen compounds. Azolla, an aquatic fern, depends on cyanobacteria Anabena spp to convert atmospheric nitrogen to usable forms. In turn, Anabena gets shelter, nutrients and oxygen from the plant (Bashan et al., 2008).
In addition to nitrogen fixation, bacteria have several other benefits to plants. For example, Micrococcus, Pseudomonas, Sarcina, Bacillus, Actinomyces and Proteus can convert insoluble phosphate compounds into utilizable forms such as zinc and sequester iron, all of which are crucial for plant growth. The bacteria benefit through the shelter provided by the plants. Their own growth is enhanced since they exploit the nutritive exudates from the roots. Other types of bacteria such as rhizobacteria that defend plant hosts against potentially harmful soil microbes. They also subjugate recalcitrant pollutants like pesticide residues, hydrocarbons and heavy metals (Pongslip, 2012). This rhizobacteria-plant association is valuable since it forms the basis of biofertilization and phytoremediation applications.
Bacteria are common ecto and endo symbiotic microorganisms of human beings, animals and insects. When a person is born, he/she acquires specific bacteria as normal flora. This association lasts the person’s entire life span. Some animals such as cattle have bacteria as resident microflora. Flora bacteria append and inhabit the epithelial mucus lining in the genitourinary, gastrointestinal and respiratory organs of humans and animals. There, they receive a protective environment and nutrition from the host. Meanwhile, they help in the manufacture of B vitamins and activation and development of immune system against pathogenic microbes. For instance, Lactobacilli, which resides in the vagina, produces lactic acid that protect the female genitourinary tract against contagious yeasts such as Candida and other microbes (Ljungh & Wadström, 2009).
In herbivorous animals, bacteria play a vital role in digestion. For example, Propionibacterium spp. that resides in the intestines of ruminants such as cattle helps in the digestion of cellulose. Considering that these animals lack alternative ways of cellulose utilization, they would have missed the benefits of cellulosic content found in vegetative matter if they did not have bacteria in their rumen. The bacteria benefit from this association since they are provided with shelter and nutrition.
Bacteria have also established beneficial relationships with insects. The most popular such association is between bacteria Buchnera aphidicola and aphids. This bacterium provides all enzymes essential for the manufacture of amino acids, which cannot be found in plant sap (aphid food). In return, the insects synthesize enzymes needed to build a bacterial cell wall. Another symbiotic relationship is observed between bacteria and gutless worms that reside in subterranean hydrothermal vents. These worms contain sulfur-oxidizing bacteria within their bodies. Since they do not have a digestive tract, they completely rely on nutrition offered by the bacteria. In return, they provide shelter for the bacteria. Another remarkable symbiotic relationship can be observed between bacteria and aquatic animals. Jellyfish, squids and some fishes contain luminescent bacteria (such as Vibrio fischeri) in their vision organs. This enables them to locate prey, find paths in deep seas, attract mates and warn off potential predators. For their bioluminescence benefits, the bacteria are offered protective shelter (Wilson & Hastings, 1998).
The bacteria-protozoa association has been studied for quite some time. One of the most widely studied associations involves bacteria, protozoa Triconympha and termites. The principle food for termites is cellulose, which is found in plant fiber. However, termites cannot digest cellulose without help from Triconypha. Meanwhile, Triconympha rely on the bacteria located on its surface to produce lytic enzymes that can digest cellulosic material. This deep association means that the three organisms cannot live in the absence of any other (Solomon et al., 2011). Another protozoan-bacterial association is observed in Mixotricha. Mixotricha lacks mitochondria. However, it shelters bacteria that play the essential functions of mitochondria.
Symbiotic relationships can also be observed between bacteria and some higher algae. These algae lack the capability of producing cobalamin on their own. As a result, they rely on bacteria that can synthesize vitamin B12 for exogenous provision of cobalamin. In turn, the bacteria obtain nutrition offered by the algae. Another algal-bacterial relationship that has since been exploited to sewage treatment is the carbon dioxide and oxygen exchange. As plants, algae produce oxygen during photosynthesis. Bacteria need this oxygen for survival. In return, it produces carbon dioxide for the algae.
Bacteria also have symbiotic relations with fungi. Many scholars have found that the fungal-algal mutual association in lichens is enabled by alpha-proteobacteria. In the recent past, a mutual relationship has been observed between Burkholderia bacteria and fungus Rhizopus microsporus. These two microbes combine to break down developing rice crops for their nutrients, thereby causing a disease referred to as rice seedling blight. The Burkholderia bacteria facilitate the virulence of Rhizopus microsporus against rice seedlings. It also produces rhizoxin, a plant poison, which causes the disease. Both microbes benefit from the nutrients they derive from the crops (Partida‐Martinez & Hertweck, 2007).
Bacteria are noted for their deleterious effects. However, they are not always harmful. In many instances, they are extremely beneficial. For instance, their nitrogen fixation property enables plants to grow healthily, thereby benefitting humans. Some plant ecosystems largely depend on the actions of bacteria to thrive. These organisms are also beneficial to humans and animals directly. For instance, by lining themselves in some vital organs within our bodies, they prevent pathogenic microbes from attacking us. Animals such as cattle and horses require bacteria in order to digest cellulose, an essential component of their diet. Insects such as aphids and gutless worms also depend on these microorganisms for nutrition. Their association with other many animals shows how important they are to the ecosystem.
Bashan, Y., Puente, M. E., de-Bashan, L. E., & Hernandez, J. P. (2008). Environmental uses of plant growth-promoting bacteria. Plant-Microbe Interactions, 69-93.
Ljungh, A., & Wadström, T. (2009). Lactobacillus molecular biology: From genomics to probiotics. Norfolk, UK: Caister Academic.
Partida‐Martinez, L. P., & Hertweck, C. (2007). A gene cluster encoding rhizoxin biosynthesis in “Burkholderia rhizoxina”, the bacterial endosymbiont of the fungus Rhizopus microsporus. ChemBioChem, 8(1), 41-45.
Pongslip, N. (2012). Phenotypic and Genotypic Diversity of Rhizobia. Sharjah: Bentham Science Publishers.
Solomon, E. P., Berg, L. R., & Martin, D. W. (2011). Biology. Belmont, CA: Brooks/Cole.
Wilson, T., & Hastings, J. W. (1998). Bioluminescence. Annual review of cell and developmental biology, 14(1), 197-230.
Zahran, H. H. (1999). Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an arid climate. Microbiology and molecular biology reviews, 63(4), 968-989.