rhodophyta

That theory might get some support from the people who dose their unplanted fish tanks with hydrogen peroxide to "bleach" the algae. Once the algae turns white, it rarely returns. A higher oxygen level in a planted tank might just work the same way bleaching the chloroplasts out of the algae cells.

standoyo

I think this article about BGA is the best one yet and provides insight from a time before Oxygen was abundant on earth. http://www.skepticalaquarist.com/docs/algae/cyano.shtml

apistaeasy

I think this discussion has stepped away from the original question a little too far.

Recently most posts have been discussing cyanobacteria, which is not algae, but instead a photosynthetic bacteria. Applications that relate to plant and algae competition do not necessarily apply to bacteria. I believe that a discussion about cyanobacteria and plant competition belongs in another thread.

In regards to allelopathy. It is my understanding that allelochemicals actually work through bacteria. In this way, if allelopathy were occuring in our tanks, we should actually be looking at the way bacteria and algae compete for the same nutrients.

Also, in response to a comment on hydrogen peroxide. H2O2 kills algae not by raising O2 levels, but by oxidation in the same manner hydrogen peroxide kills bacteria in a wound. Algae is more susceptible to H2O2 treatment because of thinner cell walls than plants. Plants can be killed in the same way if their cell walls are thinner, like mosses or damaged parts of a leaf.

Interestingly on my local forum we have recently been having a discussion on the same topic. Here are my thoughts:

I believe that there is a critical balance between light, CO2, nitrate, phosphate, and traces (I'm including potassium in traces here). When something happens to change the balance algae sense the balance. This tip on the balance tells, or signals to, the algae that this is their time to have the outbreak we all hate.
I don't really believe there is such thing as an algae free tank. All the algae are there, they are just in a dormant phase. When a specific type of algae finds the conditions it likes it goes nuts. Some algae like low phosphate - GSA (green spot algae), some algae like low nutrients - staghorn, and some algae like almost every kind of imbalance we can create.
For each person their is some sort of unique imbalance that they create with their dosing schedule. For me it's GSA. My home tank has plenty of light (4.5 wpg), plenty of ferts, very healthy plants, is mature, hasn't had a water change in almost two months and is doing great with the only exception that every couple of weeks I start seeing typical GSA spots on the glass. My dosing schedule in the show tank at work is conducive to hair algae (I believe this is due to a low N:P ratio, and in my case it's 'cause the P is too high).
Take an example from nature - algae blooms often happen in streams and lakes where runoff from fertilized farms contaminates the perfect balance. The imbalance usually comes from phosphates. An algae bloom results.

So, to sum things up a bit...I believe the best way to keep algae at bay is to simply keep a proper ratio of light, CO2, macros and traces.

fastang80

Nice thread. Very informative.

brenmuk

A great thread!!.

I take it that after 3 years or so of this thread running that there isn't a simple definitive answer?

I have been keeping planted tanks on and off for the last 15 years with varying degrees of success and fighting what has seemed at times a losing battle with algae.

But that is nothing, plants and algae have been slugging it out for the last few 100 millions years. Higher plants I would say have broadly won the battle for
the well lit and
well nourished niche because when we provide these conditions they thrive and algae does not but when things are 'out of balance' then algae thrive.

When ever you see this type of competition evolve however in nature you often see a multitude of adaptations and strategies by both sides, if it was just one simple mechanism
that plants or algae use to dominate an ecosystem then it would have only been a matter of time before a counter strategy evolved - examples of this are antibiotics produced by fungi and enzymes produced bacteria that deactivate the antibiotic – it’s a kind of arms race.

I think allelochemicals do play a role but I don't know how important they are. People that do large weekly water changes and use activated carbon would surely
be removing them all the time but I never see people complaining that they had an algae outbreak when they added activated carbon to filters.

In a broader sense though I think allelopathy is still the key, you seem to either get good plant or good algae growth and not both together. Even in well fertilized tanks where there are plenty of nutrients, there should be plenty of nutrients for both algae and plants but either one or the other thrives not both (by thriving I mean actively growing because you can as we all know get a well planted tank that is taken over by algae).

So if allelopathy does exist [whatever the mechanism], then this implies that even in a well lit and well nourished tank that plants and algae are still fighting over some vital conponent that is limiting.

The only thing I can think of is CO2, and even with extra CO2 injection I would say that CO2 is limiting, just not as limiting as a tank without added CO2.

Diana Walstead in her book explains that CO2 concentration is quite often many times higher in aquatic systems than surrounding air but that does not compensate for the fact that C02 diffuses thousands of times slower in water than air (I'm trying to remember the details of her CO2 chapter of the top of my head).

As far as I understand vallis is one of the aquatic plants that can use bicarbonates, in my tank that uses the el natural method, CO2 is probably limiting. I have noticed that when I removed most of the vallis I was able to get better growth from other plants and a more diverse range of plants. The vallis I assume, is encouraging the conversion of disolved CO2 to bicarbonates to replace the bicarbonates it has removed thus removing CO2 and making less available for plants do no use bicarbonates. The effect is lessened when you add extra CO2 or trim the vallis back. At least that is my observation.

My observation of vallis growth may not be an example of allelopathy but perhaps shows that a slight advantage one plant has over another plant or algae can make a big difference to its success in a fish tank. Perhaps as well a slight advantage provided by allelochemicals is enough to tip the balance for either plants or algae for the continuing battle to get enough CO2.

These are just my thoughts I’m not an expert and will probably change my mind by next month but it is a fascinating subject.

BryceM

Yeah, I think this is perhaps the single best question ever posed here at APC. Certainly "the answer" is not a simple one.

My own thoughts about allelopathy have shifted a bit since this thread started. People like Tom Barr will adamantly state that this does not play a significant role. When I see algae in my aquarium, it generally shows up on older, unhealthy portions of the plant. I doubt that waterborne allelopathy has much effect. I do think that there's a possibility that "plant defenses" might be active within a local area in healthy plant tissue. Plants don't have an adaptive immune system in the classic sense but they do employ innate systems of immunity. It's possible that they can deploy proteins or protective barriers on their surfaces that prohibit the growth of certain other organisms. Unhealthy plant tissue would therefore be unable to resist algae colonization.

Certainly there is much more to the picture. It's pretty clear that certain algae "watch" for environmental clues that tell them that it's a good time to activate, grow, and reproduce. Avoiding these activating factors is probably a big part of the soultion.

Brilliant

Another vote for that. We only have a problem with nuisance algae.
My nonscientific explanation...perhaps healthy hardworking plants sweat which makes nuisance algae unable to grow.