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is it true that UV cannot penetrate water ver well?

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There was a comment made on another thread here that UV light cannot penetrate water very well, is this true?

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Depends what you mean by well, it can get through the whole of the earths atmosphere down to the ground but when it reaches the sea only 22% of visible light makes it 10 meters deep.

 

But I don't really have a comparison to other liquids, so that could be quite far.

 

I also fail to see how this question is relevant to the forum it has been put in.

Edited by Psycho

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UV light penetrates pure water very well, but not sea water because it has lots of things in it- not least plankton, bacteria etc.

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There was a comment made on another thread here that UV light cannot penetrate water very well, is this true?

 

Psy: An evolution forum seem appropriate enough for this topic, since "UV inhibition of phytoplankton" illustrates what evolution has always had to cope with....

 

136492.jpg

 

....[Note: cDOC = chromophoric Dissolved Organic Carbon (Humus/Humic Substances).]

"The photosynthetic underwater light climate (photosynthetically active radiation, PAR) is controlled by cDOC in two ways, by determining the quality of underwater light and the maximum depth at which photosynthesis can take place. The depth to which UV light penetrates... is a function of cDOC components in water...." --p.118

 

Just small concentrations (of at least >4mg/L) "...are sufficient to act as a biogeochemical shield against UV-B radiation, allowing UV-B to penetrate only a few decimeters into the water column." -p.119

 

Anthropogenic oxidation of humus worldwide is a growing problem for various reasons; not least of which is that "...while the DOC content decreases linearly, the UV-B penetration increases exponentially. This is especially striking at lower concentrations (<3 mg/L)." -p.119

 

....[Note: "forest borders" relate to levels of cDOC]

"Aquatic ecosystems suffer from several symptoms of global climate changes, such as ozone depletion and subsequent increase in UV-B radiation, and movement of the forest border. Yet, which of these symptoms has the most adverse effect on lake or river [ecosystems]?" -p.121

 

"...a 20% reduction in cDOC has a much greater effect on UV inhibition of phytoplankton, than a similar percentage depletion in stratospheric ozone." -p.121

 

"The discovery of HS formation in the primitive atmosphere has further far reaching consequences for the understanding of ecosystem functioning. The HS are to be granted the role of an independent ecosystem component, such as atmosphere, water, or light, since they come into being simultaneously with early life. This means that living organisms have to adapt to humus or HS-like materials with which they come in contact from the very time they evolve [as a species]." ~p.36/37 (2002, Steinberg; Ecology of Humic Substances in Freshwaters)

 

In addition to contributing 30% of the global greenhouse-gas emissions, through Land-Use changes....

 

Anthropogenic oxidation of humus worldwide, which continues through Land-Use changes, is a growing problem for other reasons! [see: Sixth Mass-Extinction Event and UV-B radiation]

~ ;)

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I also fail to see how this question is relevant to the forum it has been put in.

 

Life was able to form because UV doesn't penetrate sea water completely, supposedly.

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Life was able to form because UV doesn't penetrate sea water completely, supposedly.

This appears very much to be an inaccurate misrepresentation on your part. Presence of UV light very likely helped life form, as opposed to your suggestion here that it was the absence of UV light that assisted.

 

 

http://en.wikipedia.org/wiki/Abiogenesis#Thermodynamic_origin_of_life:_ultraviolet_and_temperature-assisted_replication_.28UVTAR.29_model

It turns out that both RNA and DNA when in water solution are very strong absorbers and extremely rapid dissipaters of ultraviolet light within the 200–300 nm wavelength range, just that high energy part of the sun's spectrum that could have penetrated the dense prebiotic atmosphere.

 

Cnossen et al.[62] have shown that the amount of ultraviolet (UV) light reaching the Earth's surface in the Archean could have been up to 31 orders of magnitude larger than it is today at 260 nm where RNA and DNA absorb most strongly.

 

Absorption and dissipation of UV light by these organic molecules at the Archean ocean surface would have increased significantly the temperature of the surface skin layer leading to enhanced evaporation and thus augmenting the primitive water cycle. Since absorption and dissipation of high energy photons is an entropy producing process, Michaelian argues that non-equilbrium abiogenic synthesis of RNA and DNA utilizing UV light[63] would have been thermodynamically favored.

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This appears very much to be an inaccurate misrepresentation on your part. Presence of UV light very likely helped life form, as opposed to your suggestion here that it was the absence of UV light that assisted.

 

 

http://en.wikipedia....8UVTAR.29_model

 

 

Well if 100% of UV light could penetrate water for 100% of its depth, then bacterium couldn't survive in it. I didn't say it didn't help life, the first life was those (forgot the scientific name) algae that use photosynthesis. But do you really think you'd survive outside if there was no ozone?

Edited by questionposter

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I'm not sure exactly what is being suggested here, UV does indeed penetrate clear sea water, mostly UVA, the deeper you go in sea water the bluer the light becomes, at certain depths all that is left is violet and UVA, coral at that depth use special pigments to use UV light, it's very dim but in clear sea water like where coral reefs grow UVA does indeed penetrate and even shallow water corals use pigments that fluoresce to allow them to use UV light, are we just talking degrees here? If we are talking absolutes then UV does penetrate and at levels that allow it to be used by symbiotic algae...

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Well if 100% of UV light could penetrate water for 100% of its depth, then bacterium couldn't survive in it. I didn't say it didn't help life, the first life was those (forgot the scientific name) algae that use photosynthesis. But do you really think you'd survive outside if there was no ozone?

At what point in time prior to your post here was 100% penetration of UV light added to the discussion? At what point in time prior to your post here was survival with no ozone introduced to the discussion? Why do you think those points are even vaguely relevant to my feedback on your post?

 

 

It's as if I said the sky is blue, and you replied, "No, bananas are yellow." :huh:

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At what point in time prior to your post here was 100% penetration of UV light added to the discussion? At what point in time prior to your post here was survival with no ozone introduced to the discussion? Why do you think those points are even vaguely relevant to my feedback on your post?

 

 

It's as if I said the sky is blue, and you replied, "No, bananas are yellow." :huh:

 

You said I was making myself out to say UV light didn't help life, I fixed that. As you can see, what I meant was that that I knew UV does help life, but thought it was overlooked that too much of it kills things, and it surely would have killed the nascent bacterium that first came about or just inhibited their existance if water didn't block a bit of UV light.

Edited by questionposter

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I'm not sure exactly what is being suggested here, UV does indeed penetrate clear sea water, mostly UVA, the deeper you go in sea water the bluer the light becomes, at certain depths all that is left is violet and UVA, coral at that depth use special pigments to use UV light, it's very dim but in clear sea water like where coral reefs grow UVA does indeed penetrate and even shallow water corals use pigments that fluoresce to allow them to use UV light, are we just talking degrees here? If we are talking absolutes then UV does penetrate and at levels that allow it to be used by symbiotic algae...

 

The prebiotic soup may have been helped along (in very interesting ways) by UV radiation.

But clearly, once DNA became critical to the continuity of life, it needed some protection from the ionizing effect of UV radiation.

 

Note: DOC = Dissolved Organic Carbon

[2003, Steinberg; Ecology of Humic Substances in Freshwaters]

"Dissolved, colored (chromophoric) DOC (cDOC) provide a natural biogeochemical protection against UV radiation. This protection applies to a whole-lake scale as well as potentially to single cells: Campbell et al. (1997) discuss a rather speculative possibility of HS as a UV shield. The authors [in this 2003 book] demonstrate that HS can precipitate and accumulate on the surfaces of aquatic organisms such as phytoplankton. These precipitates can protect from UV radiation and, under light exposure, can simultaneously release and supply organic and inorganic nutrients, providing a further advantage to the algae (Chap. 6)...." --p.117/118

[my comment]

 

If there is something to take advantage of, life seems to finds a way....

But I can't find any info on life directly using UV light to get energy, i.e.:

http://plankt.oxford...23/12/1373.full

 

For example, Laws et al. concluded that by not correcting for spectral shifts down the water column, photosynthesis could be underestimated by >30% (Laws et al., 1990). They recommended that the standard practiceof P versus E incubations under white light should be abandoned in favour of more complex protocols that accurately simulate both the intensity and spectral characteristics of the underwater light field. More recent studies have noted that although the need to incorporate spectral dependencies in photosynthetic models is now well established, in practice this type of correction is often ignored for the sake of convenience and simplicity (Kyewalyanga et al., 1997; Figueiras et al., 1999).

 

Morel drew attention to the importance of spectral matching as a function of depth in the water column and introduced the term photosynthetically usable radiation (PUR) (Morel, 1978). He defined PUR as the amount of light absorbed by the phytoplankton (Ea) normalized to the maximum absorption coefficient (amax) for that community [see also (Sakshaug et al., 1997)]. PUR has proven to be a valuable concept for marine and freshwater studies, e.g. in analysis of the ecophysiological effects of algal photoacclimation (Culver and Perry, 1999), development of models of UV photoinhibition of photosynthesis (Arrigo, 1994; Neale et al., 1998), assessing light limitation in the ocean (Figueiras et al., 1999) and for defining the habitat requirements of macrophytes (Gallegos, 1994).

 

I just wanted to post this, about the protective value of humus; but I'm curious about algae using UV. I'll appreciate any examples.

 

Thanks

~ :)

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UV light helps lots of chemical reactions happen. As such, it would have been very useful to early pre-life and life. But modern life has enzymes for all the reactions we need, and so for the most part any extra chemistry happening in our bodies is going to be harmful. Nevertheless, a few reactions still require UV, such as formation of certain vitamins (Vitamin D for us, plenty of others for plants).

 

Because UV light is generally harmful to life, we have chemicals to block it. Since it also has a lot of energy, there's plenty of chemicals that absorb it to use it as well. So a decent amount of algae in the water will alone absorb plenty of UV. Water also absorbs UV, though not too well, so don't expect clouds nor swimming to protect you from sunburn.

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From http://en.wikipedia.org/wiki/Water_absorption

 

Water_absorption_spectrum.png

 

There is significant absorption everything outside of the visible/photosynthetic regions of the solar emf spectrum. So it would be an impediment.

 

UVA 400 nm–315 nm

UVB 315 nm–280 nm

 

But only at UVC levels

 

UVC 280 nm-100 nm

 

The NMR is around 120hz for regular tap water (perhaps related to the frequency choice of the AC power supply).

Edited by vampares

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