Query: Reefs need carbon dioxide; it’s their basic food.

Relevant passages:
Breathing <=SEP=> Breathing is the process that moves air in and out of the lungs , to allow the diffusion of oxygen and carbon dioxide to and from the external environment into and out of the blood . `` Breathing '' sometimes also refers to the equivalent process using other respiratory organs such as gills in fish and spiracles in certain arthropods . For organisms with lungs , breathing is also called ventilation , which consists of inhalation ( breathing in ) and exhalation ( breathing out ) . Breathing is one part of physiological respiration required to sustain life . Aerobic organisms ( all animals , most plants and many micro-organisms ) require oxygen at cellular level to release energy by metabolizing energy-rich molecules such as fatty acids and glucose . This is often referred to as cellular respiration . Breathing is only one of the processes that delivers oxygen to where it is needed in the body and removes excess carbon dioxide . After breathing , the next process in this chain of events is the transport of these gases throughout the body by the circulatory system , and then their uptake or release from the respiring cells . Breathing fulfills another vital function : that of regulating the pH of the extracellular fluids of the body . It is , in fact , this homeostatic function which determines the rate and depth of breathing . The medical term for normal relaxed breathing is eupnea .   At the end of each exhalation the adult human lungs still contain 2.5 -- 3.0 liters of air , termed the functional residual capacity ( FRC ) . Breathing replaces only about 15 % of this volume of gas with moistened ambient air with each breath . This ensures that the composition of the FRC changes very little during the breathing cycle , and remains significantly different from the composition of the ambient air . The partial pressures of the gases in the blood flowing through the alveolar capillaries equilibrate with the partial pressures of the gases in the FRC , ensuring that the partial pressures of carbon dioxide , and oxygen of the arterial blood , and therefore its pH , remain constant . The equilibration of the gases in the alveolar blood with those in the alveolar air ( i.e. the gas exchange between the two ) occurs by passive diffusion .   Breathing is used for a number of subsidiary functions , such as speech , expression of the emotions ( e.g. laughing . yawning etc. ) , self-maintenance activities ( coughing and sneezing etc. ) and , in animals that can not sweat through the skin , panting .

Gas exchange <=SEP=> Gas exchange is the biological process by which gases move passively by diffusion across a surface . Typically , this surface is - or contains - a biological membrane that forms the boundary between an organism and its extracellular environment .   Gases are constantly consumed and produced by cellular and metabolic reactions in most living things , so an efficient system for their exchange is required . Smaller organisms , such as bacteria , have a high surface-area to volume ratio . In these unicellular organisms , the external boundary of the organism - typically the cell membrane - is usually sufficient for the necessary gas exchange . Some small multicellular organisms , such as flatworms , are also able to perform sufficient gas exchange across the skin or cuticle that surrounds their bodies . However , in most larger organisms , which have a small surface-area to volume ratio , specialised structures with convoluted surfaces such as gills , pulmonary alveoli and spongy mesophyll provide the additional area needed for effective gas exchange . These convoluted surfaces may sometimes be internalised into the body of the organism : this is the case for alveoli , which form the inner surface of the mammalian lung ; spongy mesophyll , which is found inside the leaves of some kinds of plant ; and the gills of those molluscs that have them , which may be found in the mantle cavity .   In aerobic organisms , gas exchange is particularly important for respiration , which involves the uptake of oxygen and release of carbon dioxide . Conversely , in oxygenic photosynthetic organisms such as most land plants , uptake of carbon dioxide and release of both oxygen and water vapour are the main gas-exchange processes occurring during the day . Other gas-exchange processes are important in less familiar organisms : e.g. carbon dioxide , methane and hydrogen are exchanged across the cell membrane of methanogenic archaea ; nitrogen is exchanged in denitrifying and nitrogen fixing bacteria ; and hydrogen sulfide and other gases are exchanged in giant tube worms .

Ocean acidification <=SEP=> Ocean acidification is the ongoing decrease in the pH of the Earth 's oceans , caused by the uptake of carbon dioxide from the atmosphere . Seawater is slightly basic ( meaning pH > 7 ) , and the process in question is a shift towards pH-neutral conditions rather than a transition to acidic conditions ( pH < 7 ) . Ocean alkalinity is not changed by the process , or may increase over long time periods due to carbonate dissolution . An estimated 30 -- 40 % of the carbon dioxide from human activity released into the atmosphere dissolves into oceans , rivers and lakes . To achieve chemical equilibrium , some of it reacts with the water to form carbonic acid . Some of these extra carbonic acid molecules react with a water molecule to give a bicarbonate ion and a hydronium ion , thus increasing ocean acidity ( H + ion concentration ) . Between 1751 and 1996 surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14 , representing an increase of almost 35 % in H + ion concentration in the world 's oceans . Earth System Models project that within the last decade ocean acidity exceeded historical analogues and in combination with other ocean biogeochemical changes could undermine the functioning of marine ecosystems and disrupt the provision of many goods and services associated with the ocean .   Increasing acidity is thought to have a range of potentially harmful consequences for marine organisms , such as depressing metabolic rates and immune responses in some organisms , and causing coral bleaching . By increasing the presence of free hydrogen ions , each molecule of carbonic acid that forms in the oceans ultimately results in the conversion of two carbonate ions into bicarbonate ions . This net decrease in the amount of carbonate ions available makes it more difficult for marine calcifying organisms , such as coral and some plankton , to form biogenic calcium carbonate , and such structures become vulnerable to dissolution . Ongoing acidification of the oceans threatens food chains connected with the oceans . As members of the InterAcademy Panel , 105 science academies have issued a statement on ocean acidification recommending that by 2050 , global emissions be reduced by at least 50 % compared to the 1990 level .   While ongoing ocean acidification is anthropogenic in origin , it has occurred previously in Earth 's history . The most notable example is the Paleocene-Eocene Thermal Maximum ( PETM ) , which occurred approximately 56 million years ago . For reasons that are currently uncertain , massive amounts of carbon entered the ocean and atmosphere , and led to the dissolution of carbonate sediments in all ocean basins .   Ocean acidification has been called the `` evil twin of global warming '' and `` the other problem '' .

Sea <=SEP=> A sea is a large body of salt water that is surrounded in whole or in part by land . More broadly , `` the sea '' is the interconnected system of Earth 's salty , oceanic waters -- considered as one global ocean or as several principal oceanic divisions . The sea moderates Earth 's climate and has important roles in the water cycle , carbon cycle , and nitrogen cycle . Although the sea has been traveled and explored since prehistory , the modern scientific study of the sea -- oceanography -- dates broadly to the British Challenger expedition of the 1870s . The sea is conventionally divided into up to five large oceanic sections -- including the International Hydrographic Organization 's four named oceans ( the Atlantic , Pacific , Indian , and Arctic ) and the Southern Ocean ; smaller , second-order sections , such as the Mediterranean , are known as seas .   Owing to the present state of continental drift , the Northern Hemisphere is now fairly equally divided between land and sea ( a ratio of about 2:3 ) but the South is overwhelmingly oceanic ( 1:4.7 ) . Salinity in the open ocean is generally in a narrow band around 3.5 % by mass , although this can vary in more landlocked waters , near the mouths of large rivers , or at great depths . About 85 % of the solids in the open sea are sodium chloride . Deep-sea currents are produced by differences in salinity and temperature . Surface currents are formed by the friction of waves produced by the wind and by tides , the changes in local sea level produced by the gravity of the Moon and Sun . The direction of all of these is governed by surface and submarine land masses and by the rotation of the Earth ( the Coriolis effect ) .   Former changes in sea levels have left continental shelves , shallow areas in the sea close to land . These nutrient-rich waters teem with life , which provide humans with substantial supplies of food -- mainly fish , but also shellfish , mammals , and seaweed -- which are both harvested in the wild and farmed . The most diverse areas surround great tropical coral reefs . Whaling in the deep sea was once common but whales ' dwindling numbers prompted international conservation efforts and finally a moratorium on most commercial hunting . Oceanography has established that not all life is restricted to the sunlit surface waters : even under enormous depths and pressures , nutrients streaming from hydrothermal vents support their own unique ecosystem . Life may have started there and aquatic microbial mats are generally credited with the oxygenation of Earth 's atmosphere ; both plants and animals first evolved in the sea .   The sea is an essential aspect of human trade , travel , mineral extraction , and power generation . This has also made it essential to warfare and left major cities exposed to earthquakes and volcanoes from nearby faults ; powerful tsunami waves ; and hurricanes , typhoons , and cyclones produced in the tropics . This importance and duality has affected human culture , from early sea gods to the epic poetry of Homer to the changes induced by the Columbian Exchange , from burial at sea to Basho 's haikus to hyperrealist marine art , and inspiring music ranging from the shanties in The Complaynt of Scotland to Rimsky-Korsakov 's `` The Sea and Sinbad 's Ship '' to A-mei 's `` Listen to the Sea '' . It is the scene of leisure activities including swimming , diving , surfing , and sailing . However , population growth , industrialization , and intensive farming have all contributed to present-day marine pollution . Atmospheric carbon dioxide is being absorbed in increasing amounts , lowering its pH in a process known as ocean acidification . The shared nature of the sea has made overfishing an increasing problem .

Anthozoa <=SEP=> Anthozoa is a class within the phylum Cnidaria . Unlike other cnidarians , anthozoans do not have a medusa stage in their development . Instead , they release sperm and eggs that form a planula , which attaches to some substrate on which the cnidarian grows . Some anthozoans can also reproduce asexually through budding . More than 6,100 species have been described .   The name comes from the Greek words -LSB- : wikt : άνθος , άνθος -RSB- ( ánthos ; `` flower '' ) and -LSB- : wikt : ζῷον , ζώα -RSB- ( zóa ; `` animals '' ) , hence ανθόζωα ( anthozoa ) = `` flower animals '' , a reference to the floral appearance of their perennial polyp stage .