User:Robertinventor/Simple animals could live in Martian brines - Extended Interview with planetary scientist Vlada Stamenković: Difference between revisions

▲A historical 101 on multicellular life on Mars may be of interest here. First, of course back in the early twentieth century there was much speculation about multicellular life there, with Lowell even speculating that intelligent Martians built the canals that he thought he could see in his telescope. By the time of early spaceflight it was already clear that the atmosphere was far too thin for terrestrial animals, but there was some hope for plant life. But the early observations from space showed a barren crater covered land and since then the idea of life on Mars has focused mainly on anaerobic microbes and photosynthetic life.

▲<!-- For next para: Vera et al in the background information is an example of papers on lichens such as Pleopsidium chlorophanum for Mars and the DLR source gives an overview of their experiments into lichens and blue-green algae-->

▲In 2014, however, Vera et al surprised many astrobiologists with their experimental result that multicellular life could be possible on present day Mars. But only photosynthetic life able to produce its own oxygen. Some lichens, such as {{w|Pleopsidium chlorophanum}} are able to survive in close to Mars-like conditions high up on Antarctic mountain ranges, and show promise in Mars simulation chamber experiments. They can do this because the algal component is able to make the oxygen needed by its fungal component. They can also do this without needing any extra water as intermediary, in the experiments they are able to grow in partial shade, using only the night time humidity of the atmosphere itself.

▲In principle you could also have microscopic (sub millimeter sized) multicellular animals in anoxic conditions. There are only a three species of such creatures known on Earth, however, and they are not candidates for life on Mars. They are three species of {{W|Loricifera#In_anoxic_environment| Loricifera}}, tiny animals about the size of a large amoeba, are able to survive without oxygen in deep extremely salty mud sediments in the Mediterranean.<!--See sources in linked Wikipedia article and: Animals thrive without oxygen at sea bottom in background sources -->

▲==Background information - why oxygen is so significant for multicellular life==

▲<!--Note to reviewer - optional section on background information which I thought might help some readers.-->

▲<!-- summarizes Catling et al from 2014 in background section -->

▲There is much more energy available to an organism from the same amount of food with oxygen. Without oxygen, the {{w|Glucose|glucose}} that's the source of energy in Earth organisms can only be broken up into two {{w|Pyruvic acid|pyruvate}} molecules (each of 3 carbon atoms). This process produces two molecules of {{w|Adenosine triphosphate|ATP}}, the energy molecule that powers our cells. This works quickly but the amount of energy is limited; it's the energy production method used in sprinter's "fast twitch" white muscle fibers.

▲With oxygen, glucose can be reduced all the way to carbon dioxide, and in the process {{w|Cellular respiration#Efficiency of ATP production|30 or 32 ATP molecules}} can be produced from a single molecule of glucose. This is the type of energy production used in the "slow twitch" red muscle fibers of a {{w|Marathon|marathon runner}}, or {{w|Ultramarathon|ultramarathon}} runner. It lets them run for many miles using little by way of glucose from their food (about 100 calories of excess calories over the resting metabolic calories, depending on body mass, or around 25 grams of glucose per mile, which in turn is derived from {{w|Glycogen|glycogen}} and then fat as the race progresses). In this way a large organism can sustain itself with little by way of food if it relies on oxygen.

▲[[File:Spinoloricus.png|thumb|This tiny animal, which never grows larger than a millimeter in size, is a species of {{w|Loricifera#In_anoxic_environment|loricifera}}, a rare multicellular anaerobe. Shown this colour because it is stained with Rose Bengal. Scale bar is 50 μm. In 2014, Carling et al. showed from general energetic considerations that should also apply to extraterrestrial carbon based life, that it is not easy for a food chain of anaerobes to sustain complex lifeforms even as large as 10 cms in diameter {{Image source|Danovaro et al.}}]]

▲Without oxygen a lifeform would need to eat more than ten times as much food to get the same amount of energy. It is hard for it to grow large because so much food is needed just to sustain itself and for reproduction. On Earth we don't know of any {{w|Anaerobic organism|anaerobes}} (organisms that can manage without oxygen) larger than those three species of {{w|Loricifera#In_anoxic_environment|loricifera}}, which are less than a millimeter in size. Anaerobes can buld large structures, as with the {{w|Stromatolite|stromatolites}}, which build up layer by layer but only the outermost layer, a thin biofilm, is actually alive.

▲What about extraterrestrial life though? Could it use something else in place of oxygen? Well, perhaps, but there is some earlier research that may suggest that it couldn't get very large. In 2005, Catling et al investigated the amount of energy available to carbon based organisms if with and without the use of oxygen in their metabolism. Oxygen produces the largest amount of free energy per electron transfer apart from fluorine and bromine, which are too reactive to build up in quantities useful to life. They found on general energetic principles that it still needs oxygen to sustain large complex lifeforms, from around 10 cms in size or larger. This is for any extraterrestrial food web for carbon based life.

▲They worked it out by looking at how many smaller organisms are needed to support larger ones in the food web. For aerobic life the number of organisms in a food web is inversely proportional to the mass, if you are ten times heavier then there are ten times fewer of you in the food web. These numbers are much less for {{w|Anaerobic organism|anaerobes}}.

▲According to Catling et al's modeling, if an organism is a hundred times larger (and so a million times more massive), then organisms of that size are a million times less numerous in an aerobic food web, but the same size of organism is a billion times less numerous in an anaerobic food web. So for instance, if you have a 200 micron organism preying on 20 micron organisms and they in turn on 2 micron organisms then there will be only a thousandth of the aerobic numbers in an anaerobic food chain. By the time you get to large organisms of 10 cm scale or larger they should be almost non existent in an anaerobic food web.

▲So, if Catling et al are correct in their inference here, then on general energetic principles that an extraterrestrial biosphere with large carbon based animals, at least of 10 cms scale or larger, is going to need oxygen. It seems to apply to Earth anyway. In anoxic environments, Earth animals have found it a challenge to get as large as 1 mm in size without oxygen, though it is possible that larger creatures lived on Earth when the seas were all anoxic.