Nutrition and metabolism

https://www.niso.org/standards/z39-96/ns/oasis-exchange/table"> Key Notes Carbon and energy metabolism The chlorophytes and photo synthetic species within the excavates and chrom alveolate s are photo synthetic organisms and obtain their carbon and energy requirements by the fixation of CO₂, using photosynthesis. In a terrestrial habitat, light levels are usually adequate to support photosynthesis, but in the aquatic habitat light energy is rapidly absorbed in the top 0.5 meter of the water column. Aquatic species have evolved three chlorophylls, a, b, and c, and a large number of accessory pigments to allow them to extend the depth to which they can grow. Photosynthetic species use aerobic respiration via glycolysis and the citric acid cycle. Nonphotosynthetic protista can obtain their carbon and energy requirements from the environment by diffusion, pinocytosis, and phagocytosis. In aerobic species, glycolysis, the citric acid cycle, and mitochondrial respiration provide the cell with energy and metabolites. Anaerobic protista may utilize fermentative pathways within the hydrogenosomes. Oxygen and carbon dioxide In the terrestrial environment, photosynthetic CO₂ and O₂ requirements are almost always satisfied by atmospheric gases. In the aquatic environment, the solubility of O₂ decreases with temperature and increasing dissolved CO₂ levels, and availability of O₂ therefore becomes limiting in warm waters. Nitrogen nutrition No eukaryotic microorganisms can fix nitrogen and therefore they all must obtain it in a fixed, inorganic or organic form. Most can utilize nitrate or ammonia; some require organic compounds. Nitrogen levels may be limiting to growth in marine environments. Macronutrients, micronutrients, and growth factors Most nutrients are available to excess in the aquatic environment, but phosphates and silicon are only poorly soluble in water and are often limiting to growth in fresh water. Some species are predominantly autotrophic, but many require an external supply of amino acids, vitamins, nucleic acids, and other growth factors; such species are described as auxotrophic. Water, pH, and temperature optima The members of the chlorophytes, excavates, and chrom alveolate s do not survive severe desiccation and are therefore found mostly in damp terrestrial habitats or in water. Phagocytic species have an absolute requirement for liquid water. Most species can tolerate a wide range of pH and temperature. Some are specialized and can inhabit extremely acidic, hot springs while others can complete their entire life cycle below 0°C. Osmolarity Freshwater species have a large difference between internal and osmotic pressure, and they must either have a rigid cell wall or a contractile vacuole to compensate for water uptake. Conversely, marine species are roughly isotonic with sea water. Related topics (D1) Measurement of microbial growth(E1) Heterotrophic pathways(E3) Autotrophic reactions(E4) Other unique microbial biochemical pathways(H2) Eukaryotic cell structure(I4) The fungi and related phyla: beneficial effects (J1) Archaeplastida, Excavata, Chromalveolata, and Amoebozoa: taxonomy and structure(J4) Archaeplastida, Excavata, Chromalveolata, and Amoebozoa: beneficial effects 229