The water in Lonar Lake, a flooded meteor crater in Maharashtra State, India, roughly 500 km to the east of Mumbai, turned from green to pink overnight this week, much to the surprise of local residents. The usually gets its colour from the Cyanobacterium Arthrospira spp., living in its upper layers, but apparently exceptionally warm weather in the area has caused both the temperature and the salinity of the lake to rise (the water level has gone down due to evaporation during the sustained warm spell, but the amount of salt in the lake has remained constant; the same amount of salt in a lower amount of water raises the salinity), until it eventually passed outside the tolerance level of Arthrospira spp., which died en masse. This enabled other organisms, notably the halophile (salt loving) Green Alga Dunaliella salina, which turns pink under stressful conditions, but has a higher environmental tolerance, to take over as the dominant group, blooming with the extra available resources; both an increased access to sunlight due to the the absence of Cyanobacteria, and nutrients released by the Cyanobacteria as they died.
Conditions of high salinity are harmful to most organisms, due to the osmotic stress they induce. Put simply, the membranes of most micro-organisms are impervious to salt but allow water to pass through. Under most circumstances this is not a problem, but when the outside environment becomes too salty the water moves so that it is at the same concentration on either side of the membrane, flowing out of the organism and causing it to contract and die. Dunaliella salina is able to counteract this by producing glycerol and β-carotene in response to osmotic stress, which equalises the water concentration on either side of the cell membrane without changing the internal salinity, and incidentally turns it pink, and thus can take over as the dominant micro-organism when the salinity in a body of water exceeds the tolerance of other organisms.
Dunaliella salina cells in different culture conditions. (A) Green cell from a non-stressed culture. (B) Stressed cell turning orange. (C) Orange cell from a culture exposed to nutrient stress due to β-carotene accumulation. Scale bar represents 10 μm. Munevver Aksoy in Ramos et al. (2011).
Lake Lonar has an average diameter of 1.2 km, and sits within an impact crater with an average diameter of 1.8 km. It is slightly elliptical in shape, which has led to the estimation that it was formed by an object coming from the east, which struck at an angle of 35-40°. The crater was formerly thought to be about 50 000 years old, but recent estimates based upon argon-argon dating have suggested it may be as much as 570 000 years old.
Satellite image of Lonar Lake, showing the ejecta blanket surrounding it which identifies it as an impact crater. The smaller feature to the right is Lake Ambar, which is thought to have been caused by a fragment of the same object. Jesse Allen/Earth Observatory/NASA/Wikimedia Commons.
Argon-Argon dating relies on determining the ratio of radioactive Argon⁴⁰ to non-radioactive Argon³⁹ within minerals from igneous or metamorphic rock (in this case volcanic ash) to determine how long ago the mineral cooled sufficiently to crystallise. The ratio of Argon⁴⁰ to Argon³⁹ is constant in the atmosphere, and this ratio will be preserved in a mineral at the time of crystallisation. No further Argon³⁹ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.
The approximate location of Lake Lonar. Google Maps,
The waters of Lake Lonar are both highly alkaline and highly saline, due to the chemistry of the rock on which it sits (basalt, from the Deccan Traps Flood Basalts) and the evaporitic nature of the lake; water does not flow out of Lake Loner, but is lost entirely by evaporation. The lake is fed by two small streams and the seasonal monsoon rains.
Monsoons are tropical sea breezes triggered by heating of the land during the warmer part of the year (summer). Both the land and sea are warmed by the Sun, but the land has a lower ability to absorb heat, radiating it back so that the air above landmasses becomes significantly warmer than that over the sea, causing the air above the land to rise and drawing in water from over the sea; since this has also been warmed it carries a high evaporated water content, and brings with it heavy rainfall. In the tropical dry season the situation is reversed, as the air over the land cools more rapidly with the seasons, leading to warmer air over the sea, and thus breezes moving from the shore to the sea (where air is rising more rapidly) and a drying of the climate.
Diagrammatic representation of wind and rainfall patterns in a tropical monsoon climate. Geosciences/University of Arizona.
The unique chemistry of Lonar Lake gives it a distinctive microbial community, typically dominated by Cyanobacteria, but with a broad range of Bacteria and Archaea, as well as Eukaryotic groups such as Alveolates, Fungi, Stramenopiles, Choanoflagellates, Amoebozoans and Cercozoans. Many of these Eukaryotic groups are thought to contain species endemic to the lake (i.e. not found elsewhere).
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