Horizontal Gene Transfer and Major Biogeochemical Cycles
Mechanisms of Asexual Reproduction and Gene Transfer
Reproduction is asexual. Haploid organisms possess mechanisms that facilitate parasexual genetic transference (Horizontal Gene Transfer).
Transformation
Transformation occurs when a recipient cell captures free DNA from the surrounding medium, often through a complex protein present in the cell wall. This DNA originates from another cell. The transformed cell incorporates these new genes. Cells capable of undergoing transformation are considered competent. We find prokaryotes, such as Archaea, that are naturally transformable.
Conjugation
Conjugation involves physical contact between the donor and recipient cell. This contact is established via specialized structures, often called sex pili, through which small, independent DNA molecules (plasmids) are transferred. DNA synthesis is required for conjugation to occur. Replication is performed simultaneously with the transfer process.
Transduction
The genetic transfer agent in transduction is a bacteriophage virus. The virus initiates a lytic cycle during which it accidentally incorporates host DNA into its own genome. In this way, the virus transfers this genetic material to another cell upon subsequent infection. Transduction can transfer whole plasmids and portions of chromosomes.
These three mechanisms represent horizontal gene transfer, which must be differentiated from vertical gene transfer (from parent to child).
Biological Function and Classification
Bacterial function is evidenced by their modes of response to stimuli perceived from their environment. These responses often involve metabolic modifications or motility, which may utilize flagella.
Classification of Organisms Mentioned
The text references the following groups:
- Prokaryotes: Bacteria, Archaea.
- Eukaryotes (Heterotrophic Protists): Sporozoa, Amoebae, Flagellates, Ciliates.
- Fungi: Filamentous fungi, Slime molds, Yeasts. (Often saprophytic heterotrophs.)
- Symbiotic Relationships:
- Lichens (Fungi + Algae/Photosynthetic Bacteria)
- Mycorrhizae (Fungi + Roots)
- Other Groups: Parasites, Algae (e.g., Dinoflagellates).
Essential Biogeochemical Cycles
The Carbon Cycle
The carbon cycle describes how carbon moves from the atmosphere into organic biomolecules, forming the fundamental skeleton of life. It is fundamentally linked to CO₂.
The main active exchange reservoir is the atmosphere. CO₂ is absorbed during photosynthesis to form initial carbohydrates (sugars) and is released back into the atmosphere by aerobic respiration in organisms that utilize oxygen for energy.
Carbon, in the form of organic matter, passes from one organism to others. Carbon is stored in reserves, such as mineral carbonates (limestone rocks) and fossil fuel deposits.
Carbonates are very abundant, forming the Earth’s crust (e.g., limestone). Carbonate rocks form via the precipitation of the carbonate ion dissolved in water. This ion, in turn, results from the reaction of water with dissolved CO₂ (often associated with karst processes).
Carbonates are not the only way active carbon is stored outside the cycle; fossil fuel deposits also serve as major reservoirs.
The Nitrogen Cycle
Nitrogen gas (N₂) is the most abundant component of the atmosphere. It is fundamental for life, being a component of all proteins and nucleic acids.
A key particularity of nitrogen cycling in the biosphere is that photosynthetic organisms, heterotrophs, and eukaryotic cells cannot directly utilize N₂ gas from the air. Only certain bacteria and cyanobacteria are capable of fixing atmospheric nitrogen.
Plants and most other photosynthetic organisms must absorb nitrogen dissolved in water, typically as nitrate (NO₃⁻).
Nitrogen is often low in soil, explaining the need to fertilize crops with soluble nitrogen compounds. Some plants, such as legumes (pulses), associate symbiotically with nitrogen-fixing bacteria to ensure access to this element.
While some nitrogen reaches living organisms thanks to atmospheric nitrogen-fixing bacteria and cyanophytes, more than 90% of usable nitrogen comes from the decomposition of organic matter. Organic remains, along with the processes of fixation, nitrification, and denitrification, are crucial components of this cycle.
The Phosphorus Cycle
Phosphorus exists primarily in the form of mineral phosphates. Although it is a relatively abundant element (making up about 1% of the human body), phosphorus, as phosphate (PO₄³⁻), often limits plant growth in ecosystems.
Inside organisms, the majority of phosphorus remains as phosphate, forming part of the skeleton (in vertebrates) or dissolved as ions that help maintain pH stability in bodily fluids.
The remainder of the phosphorus is incorporated into organic molecules, such as phosphoric acid derivatives (e.g., ATP and nucleic acids).