is it possible for epiphytes to uptake organic compounds without the intervention of bacteria or other microbes?
Good and useful question, grateful you asked because it's an interesting subject.
The
opinion of one researcher is rather interesting summary on stomates:
"It is assumed that all liquid uptake of water and dissolved substances occurs exclusively through the leaf cuticle, and not through the stomates. There are two pathways by which exogenous chemicals may traverse the distance from the leaf surface into the symplast; a lipoidal route and an aqueous pathway. Compounds that penetrate the cuticle in the lipoidal-soluble form do so principally in the non-polar, undissociated form, whereas compounds that enter via the aqueous route move in slowly, and their penetration is greatly benefitted by a saturated atmosphere."
Obviously the stomata gets a lot of attention, but not a lot of people know that there are stomata on the leaf surface as well as the underside and that this can be an environmental adaptation too. The two pathways that exist in stomatal pores indicate that they can and do adsorb a range of polar an non-polar organic molecules very effectively and include many organic molecules with considerable molecular weights.
However, in addition to the stomates there are aqueous pores with average pore radii ranged from between 0.45 to 1.18 nm that allow organic ions with molecular weights of up to 800 g mol−1 to penetrate plant cuticles (
Schönherr, 2006). In hot arid environments, plants use these pathways to emit hydrophilic carbohydrates, and the volatile compounds water and ethanol. This helps to protect their leaves from photoinhibition and to reduce water loss. When you smell lavender, you are experiencing that process.
Aside from the two major pore types, there is lipid-soluble adsorption of organic molecules across the unbridged leaf cuticle, the enormous efficiency of roots to both allow passive and pumped pathways into both cellular and vascular plant tissue, and a surprising range of newly-discovered pathways across the leaf cuticle that are far smaller than known types of pores and have been identified in varying plant species. The function of the latter is interesting because we do not know very much about the polarity or molecular weight of the organic molecules that can bridge, but there is a growing body of evidence from radiocarbon isotopic analysis and microscopy that shows they exist and can be quite abundant. My opinion is that it is rather hard to find an organic molecule that could not be absorbed rapidly by aquarium marcophytes.
The other side of the equation is a lot more interesting and could explain why algal blooms and various other biological community changes occur, and that is that aquarium plants could emit organic molecules passively (in addition to when they are cut or damaged). That fits well with the belief that macrophyte growth has some biochemical mechanism for inhibiting algal growth, and this would be especially relevant to both emergent and aquatic plants that may well have evolved such competitive adaptations. For a rheophyte, it is fairly useless because the biochemical inhibitor would get washed away in the water current, but for still water plants it could be more advantageous, especially as a community response happening across a wide population. This is one of the great unknowns about aquatic communities. Why should they not have evolved chemical defence mechanisms when we know that they do promote root horizons of microbial growth to benefit nutritional uptake.
On a side note, ever wondered why
Rotala Wallichii seems to stop growing sometimes, or why
Ludwigia Palustris var. Super Red Mini seems to turn brown and die in hi-tech. Could there be phytohormones in the water causing their stomata to close and the plants to suffocate I wonder. Abscisic acid would be a likely candidate for this mechanism if the plants were generating it to cope with stress. This explains both why established tanks are more stable and conducive to aquatic growth and newly planted tanks can take off quickly. Advanced filter media like Purigen could have the ability to mitigate any undesirable organic molecules if they were present, but then what about the desirable ones. Many aquarists find themselves in a juxtaposition over which approach to take, and I guess that I would suggest that expensive filtration media could be ideal for new tanks and short-term applications, but in reality organic molecules add considerable unpredictability. Future research could look at how we measure them in aquarium water, but if we did discover a naturally-occurring algal growth inhibitor that is safer than glutaraldehyde then we would be laughing. That would be my second
nomination for the UKAPS Nobel Prize award in addition to lean dosing.
@jaypeecee - sorry that I have not answered you question relating to epiphytes. I guess that microbial symbionts (like but probably not
including Trichoderma T-22 ) have an inherited role in feeding epiphytes like
@aquanoobie suggests, and perhaps some are more parasitic (about 1% of angiosperms, found in almost every biome) but we don't generally keep them. I cannot think of any parasitic epiphytes in aquariums, and I am not sure they would need to behave in that way unless they were targeting certain organic nutrients like carbohydrates. Non-parasitic ones may well utilise microbial symbionts, but many also grow in-vitro in sterile conditions.
The definitions of epiphyte we see in many dictionaries or encyclopaedias are incorrect. Epiphytes are plants living on other living plants. Algae can be epiphytic to aquatic plants. You are right, epiphytes do not need to be parasitic. A quick check online confirms that one of our sponsors Tropica are not using the term epiphyte to describe the rhizome plants that you mention, whereas Aquasabi and Buceplant are two online websites where the term is used incorrectly to describe them. When it is plant living in fast moving water then it is a rheophyte, and as you point out, if it is living on rocks then it is a lithophyte. A plant living on dead wood is certainly a saprophyte. I can see two papers defining woody terrestrial shrubs as xylophytes (
