Why Do Some Plants Need Extra CO2?

[Ken]

How can a plant need more CO2 than is normally available in water in a natural setting? 🤔

[Zenzo]

I think the answer is in your question. While pretty much most aquarium plants will grow with proper lighting and nutrients without co2 injected, some plants will do much better with injected co2. This may because our aquariums are really not "natural settings". Most of the plants that we have in the hobby are not naturally full aquatic plants, but have been conditioned to be able to live submersed. Many of the plants that we buy are grown immersed, because it is much faster for the farms to grow them this way, and the co2 in the atmosphere is readily available without injection needed. 

In their "natural setting", they would be out of the water and getting much more co2 from the air. 

[Odd Duck]

On 9/23/2021 at 9:18 PM, Ken said:

How can a plant need more CO2 than is normally available in water in a natural setting? 🤔

I’m going to jump in with a pure speculation theory since I’ve not read any research on this topic.

I would speculate that aquatic plants have evolved to use the limited amount of CO2 that’s normally available in water.  When offered increased CO2, their potential is maximized (up to certain limits) since their most limited resource is no longer as limited.  That said, I don’t know of any aquatic plants that truly require CO2 injection to survive.  There certainly are some that do better with added CO2 or display better colors (red, pink, etc) with extra CO2.

Some of the plants we grow only as aquatics can actually grow emerse, too, and will tend to display their best colors when grown emerse because they have more access to CO2 and brighter light without the water dispersing the light.

I have to wonder if that isn’t where the idea came from to do CO2 injection from some smart cookie that saw the same plant growing submerse and emerse and noticed the difference in color and growth rates.

So I don’t think any NEED injected CO2, but some certainly appear to grow more aggressively or show more desired colors with the bonus nutrient of injected CO2.  Am I going to start adding CO2 injection to any of my tanks?  Nope.  Too much hassle for me.  I’d rather hassle with algae overgrowths.  😆 

But I can’t argue with the color difference and growth rates of certain plants when offered additional CO2.

Edit to add that I just read @Zenzo and I’m glad to see our comments overlap so much!

Edited September 24, 2021 by Odd Duck

[Streetwise]

“When aquatic plants break the water surface, they not only obtain more CO2, but they appear to be released from their own internal handicaps. Figure IX-1 compares the photosynthetic response of aerial leaves and submerged leaves of Pomamogeton amplifolius to increased CO2 fertilization. The floating leaves responded much better to increased CO2 than the submerged.”

Excerpt From
Ecology of the Planted Aquarium
Diana Louise Walstad
https://books.apple.com/us/book/ecology-of-the-planted-aquarium/id661029773
This material may be protected by copyright.

[Ken]

On 9/23/2021 at 8:23 PM, Zenzo said:

I think the answer is in your question. While pretty much most aquarium plants will grow with proper lighting and nutrients without co2 injected, some plants will do much better with injected co2. This may because our aquariums are really not "natural settings". Most of the plants that we have in the hobby are not naturally full aquatic plants, but have been conditioned to be able to live submersed. Many of the plants that we buy are grown immersed, because it is much faster for the farms to grow them this way, and the co2 in the atmosphere is readily available without injection needed. 

In their "natural setting", they would be out of the water and getting much more co2 from the air. 

That makes sense. So if you wanted plants that were really well suited to life in an aquarium you would populate it with plants that were naturally fully aquatic vs plants that can be grown under water. I just spent a minute trying to find the right single words for those conditions and gave up, if anyone knows chime in. I thought it was immersed vs emmersed, that ain't it.

[OnlyGenusCaps]

So, I'm a plant physiologist by training.  Perhaps I could take a stab at an answer...

Plants use CO2 as food.  It's what they build their bodies with.  But there are a number of inefficiencies.  One of the big ones is the enzyme they are stuck with grab the C from CO2 and turn it into sugar (delicious and literally the building block of life on Earth!), RuBisCO.  I tell my students that if they were majoring in BioEngineering at this point in history and designed RuBisCO they'd get a C+ at best.  But, it is the most abundant enzyme on Earth and it's what much of life on the planet depends upon.  The thing is, it's terrible at grabbing CO2, and CO2 is in low concentration in air, and can be even lower in water.  This means plants are often CO2 limited.  So, most plants benefit from additional CO2.  On top of that, a number of plants that are grown as submerged aquatic plants in aquaria are actually plants that tolerate the condition, but naturally grow emersed.  So they don't have the ability to grow submerged long-term, unless they have greater access to CO2 than they would in nature.

Great question!  Hopefully that is useful. 

[Ken]

On 9/23/2021 at 8:40 PM, OnlyGenusCaps said:

So, I'm a plant physiologist by training.  Perhaps I could take a stab at an answer...

Plants use CO2 as food.  It's what they build their bodies with.  But there are a number of inefficiencies.  One of the big ones is the enzyme they are stuck with grab the C from CO2 and turn it into sugar (delicious and literally the building block of life on Earth!), RuBisCO.  I tell my students that if they were majoring in BioEngineering at this point in history and designed RuBisCO they'd get a C+ at best.  But, it is the most abundant enzyme on Earth and it's what much of life on the planet depends upon.  The thing is, it's terrible at grabbing CO2, and CO2 is in low concentration in air, and can be even lower in water.  This means plants are often CO2 limited.  So, most plants benefit from additional CO2.  On top of that, a number of plants that are grown as submerged aquatic plants in aquaria are actually plants that tolerate the condition, but naturally grow emersed.  So they don't have the ability to grow submerged long-term, unless they have greater access to CO2 than they would in nature.

Great question!  Hopefully that is useful. 

First, what in the heck are the proper words for plants that grow under or out of the water? We're all over the map on here. Second, are there some plants that are commonly available in the freshwater aquarium trade that are naturally fully under water their whole life?

BTW thanks for responding.

[Patrick_G]

On 9/23/2021 at 7:49 PM, Ken said:

First, what in the heck are the proper words for plants that grow under or out of the water?

Hydrophytes 

[Streetwise]

Immersed v emersed would refer to the sections of plants below or above the water, or growth that occurred in each condition.

[Ken]

On 9/23/2021 at 9:06 PM, Patrick_G said:

Hydrophytes 

I wasn't clear in my question. A plant that only grows out of water is called ______. A plant that only grows under water is called ______. Plants that do both are called ______.

I have seen immersed (under water) and emersed (above water by our own expert above). And maybe my ignorance is making it hard, There are plenty of plants that will do both, so they could be called either depending what they were doing today. For our uses I think it would nice to know if there are, and which ones, plants that naturally do best living under water their whole lives.

[Guppysnail]

On 9/23/2021 at 11:15 PM, Ken said:

I wasn't clear in my question. A plant that only grows out of water is called ______. A plant that only grows under water is called ______. Plants that do both are called ______.

I have seen immersed (under water) and emersed (above water by our own expert above). And maybe my ignorance is making it hard, There are plenty of plants that will do both, so they could be called either depending what they were doing today. For our uses I think it would nice to know if there are, and which ones, plants that naturally do best living under water their whole lives.

Anacharis elodea (my favorite) is an aquatic plant. 

[Patrick_G]

@Ken, I’m not sure. The vernacular of aquarium horticulture might be different from that of botany. I bet @OnlyGenusCaps can give us a scientific explanation.

On 9/23/2021 at 8:21 PM, Guppysnail said:

Anacharis elodea (my favorite) is an aquatic plant. 

I think Val is too.

[OnlyGenusCaps]

On 9/23/2021 at 10:15 PM, Ken said:

I have seen immersed (under water) and emersed (above water by our own expert above). And maybe my ignorance is making it hard, There are plenty of plants that will do both, so they could be called either depending what they were doing today. For our uses I think it would nice to know if there are, and which ones, plants that naturally do best living under water their whole lives.

So, there are a number of classification systems that have been proposed to bin plants by growth form.  This leads to many similar but distinct as well as distinct terms with overlapping meaning.  So for example, from the Raunkiær plant life-form scheme you get hydrophyte, which would includes floating leaved water lilies and submerged hornwort.  Several attempts to break aquatic plants down further have been made.  People just accept that there are a few terms that can be used.  But rather than getting hung up on the specific terms, I'll try to address the functional aspect of your curiosity, the plants that are actually adapted to growing under water.

Here I'll make a new classification system (hopefully this won't add to the confusion, but you see how these things can get out of hand).  Essentially, we have 4 types of plants we use in our aquariums:

Floaters - This is the easiest group to categorize because they are grouped as such in nature and the hobby by their growth form.  These are the plants that survive on the water surface and float there with roots dangling into the water column.  These would include duckweed, salvinia, water lettuce, etc.

True submerged aquatics - These are the plants that naturally grow entirely underwater, with the exception of their flowers.  These plants can't survive growing out of water.  This group, like the floaters, are distinguished in their habitats.  Here you'd have anacharis, val, hornwort, etc.

The next two groups are where it gets complicated because they are only distinguished by their success in remaining underwater in aquariums long term.  That is not known a priori in habitat, but through experimentation in aquariums.

Submerged survivors (or something, I just made that up) - These are the plants that grow partly, or occasionally submerged in habitat, but can persist in this state long term.  This is not a group of plants you would distinguish in nature, it's only known because of things like the aquarium hobby.  Here you have things like the aroids (anubias, crypts, buce, etc.), the water lilies we maintain with only their juvenile leaves, java moss, etc.

Partially submerged - These are plants that grow part of their lives under water, either as juveniles or seasonally that can't persist under water throughout their lives without help.  these plants are often kept in a juvenile state in the aquarium and you won't see them flower because of that.  Note though, that many species we keep, e.g. water wisteria, are typically plants that grow only when young under the surface and then emerge to become adults, but they remain as young plants under the surface just find for aquarists.  Here are the plants that often need a little coaxing to grow underwater long-term with things like CO2 addition and high light.  Here would be things like cardinal lobelia, many of the tricky stem plants, etc.

No hard and fast rules separate the last two, and botanists wouldn't separate them as they are similar in habitat in many ways.  Aquarists are the ones who care about these functional differences.  On top of that there are plants that don't fit into any of these categories.  Plants that are truly aquatic, but for some reason need CO2 to succeed in aquariums.  I suspect the reason there is not a comprehensive categorization system is that it would need to be specifically for aquariums and would be changing all the time as new species are explored

[Ken]

On 9/24/2021 at 7:28 AM, OnlyGenusCaps said:

So, there are a number of classification systems that have been proposed to bin plants by growth form.  This leads to many similar but distinct as well as distinct terms with overlapping meaning.  So for example, from the Raunkiær plant life-form scheme you get hydrophyte, which would includes floating leaved water lilies and submerged hornwort.  Several attempts to break aquatic plants down further have been made.  People just accept that there are a few terms that can be used.  But rather than getting hung up on the specific terms, I'll try to address the functional aspect of your curiosity, the plants that are actually adapted to growing under water.

Here I'll make a new classification system (hopefully this won't add to the confusion, but you see how these things can get out of hand).  Essentially, we have 4 types of plants we use in our aquariums:

Floaters - This is the easiest group to categorize because they are grouped as such in nature and the hobby by their growth form.  These are the plants that survive on the water surface and float there with roots dangling into the water column.  These would include duckweed, salvinia, water lettuce, etc.

True submerged aquatics - These are the plants that naturally grow entirely underwater, with the exception of their flowers.  These plants can't survive growing out of water.  This group, like the floaters, are distinguished in their habitats.  Here you'd have anacharis, val, hornwort, etc.

The next two groups are where it gets complicated because they are only distinguished by their success in remaining underwater in aquariums long term.  That is not known a priori in habitat, but through experimentation in aquariums.

Submerged survivors (or something, I just made that up) - These are the plants that grow partly, or occasionally submerged in habitat, but can persist in this state long term.  This is not a group of plants you would distinguish in nature, it's only known because of things like the aquarium hobby.  Here you have things like the aroids (anubias, crypts, buce, etc.), the water lilies we maintain with only their juvenile leaves, java moss, etc.

Partially submerged - These are plants that grow part of their lives under water, either as juveniles or seasonally that can't persist under water throughout their lives without help.  these plants are often kept in a juvenile state in the aquarium and you won't see them flower because of that.  Note though, that many species we keep, e.g. water wisteria, are typically plants that grow only when young under the surface and then emerge to become adults, but they remain as young plants under the surface just find for aquarists.  Here are the plants that often need a little coaxing to grow underwater long-term with things like CO2 addition and high light.  Here would be things like cardinal lobelia, many of the tricky stem plants, etc.

No hard and fast rules separate the last two, and botanists wouldn't separate them as they are similar in habitat in many ways.  Aquarists are the ones who care about these functional differences.  On top of that there are plants that don't fit into any of these categories.  Plants that are truly aquatic, but for some reason need CO2 to succeed in aquariums.  I suspect the reason there is not a comprehensive categorization system is that it would need to be specifically for aquariums and would be changing all the time as new species are explored

Thank you. I have now read that post at least a dozen times. It answers exactly what I was asking. I hope others read this post, there is a bunch to be learned here.

You, my friend, are a great asset to all of us on this forum.