| Abyssalbenthic zone (4000 - 6000m) [Bathymetric zone] |
| acetogen (CO2->CH3COOH) [Physiological group] |
| acetogenesis (CO2->acetate) [Metabolic process] |
| Acores (=Azores; Portugal) [Geographic region] |
| aerobic respiration (O2->H2O) [Metabolic process] |
| Afghanistan [Geographic region] |
| Africa [Geographic region] |
| Agricultural land [Habitat] |
| Alabama (United States) [Geographic region] |
| Alaska (United States) [Geographic region] |
| Alaskan North Slope Coastal Tundra [Global 200 ecoregion] |
| Albania [Geographic region] |
| Alberta (Canada) [Geographic region] |
| Albertine Rift Montane Forests [Global 200 ecoregion] |
| Albian (112 - 99.6 Mya) [Period] |
| Algeria [Geographic region] |
| Algivore (algae) [Nutritional group] |
| Alpine [Altitudinal zone] |
| Alpine tundra [Habitat] |
| Alsace (France) [Geographic region] |
| Altai-Sayan Montane Forests [Global 200 ecoregion] |
| Alto Paraguay (Paraguay) [Geographic region] |
| Ambush predation [Feeding strategy] |
| American Samoa [Geographic region] |
| Americas [Geographic region] |
| ammonium oxidation (NH4{+]->NO2[-]) [Metabolic process] |
| ammonium oxidizer (NH4{+]->NO2[-]) [Physiological group] |
| anaerobe [Physiological group] |
| anaerobic respiration [Metabolic process] |
| Analgesic (reduces pain) [Therapeutic property] |
| Ancash (Peru) [Geographic region] |
| Andaman and Nicobar Islands (India) [Geographic region] |
| Angola [Geographic region] |
| Annamite Range Moist Forests [Global 200 ecoregion] |
| Antarctic Region [Geographic region] |
| Antarctica [Geographic region] |
| anthelmintic (worms) [Therapeutic property] |
| anti-HIV (AIDS) [Therapeutic property] |
| antibacterial [Therapeutic property] |
| Antibiotic (inhibits germ growth) [Functional class] |
| Antigua and Barbuda [Geographic region] |
| antiinflammatory [Therapeutic property] |
| antineoplastic (cancer, tumor) [Therapeutic property] |
| Antioquia (Colombia) [Geographic region] |
| Antioxidant [Functional class] |
| Appalachian and Mixed Mesophytic Forests [Global 200 ecoregion] |
| Aquatic (water) [Lifestyle] |
| Arabian Highland Woodlands and Shrublands [Global 200 ecoregion] |
| Arable land [Habitat] |
| AraucanÃa (Chile) [Geographic region] |
| Arboreal (trees) [Lifestyle] |
| Archipel des Marquises [Geographic region] |
| Arctic Region [Geographic region] |
| Arctic tundra [Habitat] |
| Argentina [Geographic region] |
| Arizona (United States) [Geographic region] |
| Arkansas (United States) [Geographic region] |
| Armenia [Geographic region] |
| Arthropod-feeder (arthropods) [Nutritional group] |
| Artificial pond [Habitat] |
| Ascension (administrative area) [Geographic region] |
| Asia [Geographic region] |
| Astrakhan' (Russia) [Geographic region] |
| Astringent (dry) [Taste] |
| Atacama-Sechura Deserts [Global 200 ecoregion] |
| Atlantic Dry Forests [Global 200 ecoregion] |
| Atlantic Forests [Global 200 ecoregion] |
| Atlantic Ocean [Geographic region] |
| Attached [Lifestyle] |
| Australasia [Geographic region] |
| Australia [Geographic region] |
| Australian Capital Territory (Australia) [Geographic region] |
| Australian Region [Geographic region] |
| Austria [Geographic region] |
| autotroph (CO2) [Nutritional group] |
| Azerbaijan [Geographic region] |