# Effect of fluctuating Light Levels on Plants & Algae



## niru (28 Sep 2011)

Hi there

I have a question re lighting..

Known known: If one is unlimited in ferts & CO2, controlling light is the best way to avoid algae & get a healthy plant growth.

So if one stays in the low/mid WPG light limits, but gets fluctuations in lighting (mostly the ON period -- my 3 year old manages to reset the digital light timer even after hiding it deep inside the cabinet     ), what/how are plants and algae affected by it? If the fluctuations are not routine, perhaps its more like a few dark cloudy days with occasional sunshine weather. Plants have got into a daily routine of shop opening hours, but occasional strikes/riots/traffic jams impose a blackout of few hours...


-niru


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## niru (29 Sep 2011)

Anyone?


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## ceg4048 (30 Sep 2011)

Hi niru,
          The best way of looking at this question is to compare what effect it would have on you if for example you missed a meal here and there. The photosynthetic process is a means to an end. The photoelectric energy conversion is used to produce an end product which is sugar, so any deviation from the normal system energy input has an ultimate effect on the final output of sugar.

Under stable conditions the plant maximizes it's efficiency of energy conversion i.e. the "quantum yield" which is defined as something like;

(Number of CO2 molecules stripped) / (Number of photons absorbed by pigments)

Maximizing this efficiency therefore maximizes the food production while minimizing the amount of energy consumed to produce pigments. If the energy input is stable the plant can figure out how many pigments to produce in each leaf. If the CO2 availability is stable then the plant can figure out how an allocation plan for the Rubisco to pull as much CO2 from the surroundings as possible without spending too much energy in the production of that Rubisco (which is a very expensive molecule to fabricate).

So, minor variations in energy inputs or in CO2 availability have very little effect as measured over the long term, but chronic loss of energy or CO2 forces the plant to change it's configuration to maintain efficiency in order to maximize food production.

Less light long term therefore drives a need to increase the chlorophyll density within the leaf in order to improve the probability that more photons will strike the pigments. Less CO2 drives a need to increase the density of Rubisco in order to improve the probability of encountering and of "fixing" Carbon.

The difference between higher plants and algae is that plants take longer to assess the changed conditions and to reconfigure, but they have large food storage capacity to help them deal with the time it takes to reconfigure. They are like lumbering mammoths, whereas algae have a very simple photoelectric systems with very little storage capacity and can take advantage of changing conditions very rapidly. They are like mice. That's why dynamic conditions tend to favor algae while long term stability tend to favor plants. 

Simple variations in when the light gets turned on/off is not a severe enough dynamic to affect either of their systems very much. Minor variations in CO2 won't do too much either, but as the severity of the variations increases then the effects become more pronounced.

Cheers,


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## gmartins (30 Sep 2011)

Hi,

This conversation reminds me something I read some time ago on a scientific paper. In science, most people work around mean effect size. That is, they use mean light intensity, mean wind speed, etc. to infer the species' response to environmental or biological factors. However, variability is intrinsic to natural systems and if we are to push predictive ecology forward we ought to examine how spatial and temporal variability in processes shape the response of organisms.
To do this, a team of italian people manipulated both the mean size effect and the temporal variability of that factor (in this case a disturbance) and the results were surprising. High levels of variability in disturbance reversed the mean effect size. 

source: http://www.jstor.org/pss/20069260

How may this apply to your question? Well it cannot without proper experimentation but it suggests that variable light intensities such as the ones suggested might have adverserve effects on photosynthesis.

cheers,

GM


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