2. There are many kinds of cyanobacteria, about 2000 species, of which 100 species have nitrogen fixation ability. Nitrogen-fixing cyanobacteria can use free nitrogen in the air to synthesize nitrogen-containing compounds and release them continuously. After it dies and decomposes, it will release more nitrogen compounds.

Some cyanobacteria can follow the representative species of lichens, mosses, ferns, gymnosperms and even angiosperms. Anabaena (a kind of cyanobacteria) lives in the leaf cavity of Azolla, providing nitrogen nutrition for Azolla.

First of all, point out the wrong place of the problem. Autotrophication belongs to the category of metabolism, and parasitism belongs to the category of interspecific relationship. The two are not opposing events, and they may happen at the same time. Just as ethnic minorities are determined by racial factors, Beijingers are determined by geographical factors. If you ask me, are you a Uighur or a Beijinger, I really can't answer. Can't I be from Beijing because I am a Uighur? (Of course, I'm A Dai from Yunnan) So, you can't ask whether cyanobacteria are autotrophic or parasitic. There is a plant called dodder, which is an autotrophic parasite. Of course, some people think that parasitic plants and insect-eating plants are transitional types between autotrophic and heterotrophic, but there is no clear statement about this view. Metabolic types can be divided into autotrophic and heterotrophic according to assimilation types, and aerobic and anaerobic according to alienation types; The relationship between species can be divided into mutual benefit, competition, predation and parasitism, that is, if the relationship between species is * * *, it can't be parasitism. )

Whether cyanobacteria are parasitic can be explained from two angles:

First, cyanobacteria contain chlorophyll, which can be used for photosynthesis. So it is autotrophic. Yes

Second, your question is' Do cyanobacteria born with plants all fix nitrogen? Are these cyanobacteria autotrophic or parasitic? -the qualifier is' this'. What kind of, this kind of natural plants * * *, since they are all natural plants * * *, can it still be parasitic?

And here is the answer to the problem of nitrogen fixation:

Your question is: Do cyanobacteria born with plants all fix nitrogen? Since * * * was born, it means that plants born with it can use it, what to do with it, and of course, use its nitrogen-fixing ability, so this kind of cyanobacteria can fix nitrogen. Without nitrogen fixation, plants are unwilling to live with it.

Of course, careful people will ask, plants can get nitrogen from cyanobacteria, but what can cyanobacteria get from plants? Since cyanobacteria can live on their own, why should they live with other plants? At first, I couldn't figure out this problem either. In many places, no plants can provide cyanobacteria. However, according to the growth of rhizobia and leguminous plants, it seems that some answers close to the truth can be obtained: every bacterium in rhizobia has a specific correspondence with some leguminous plants, and each rhizobia only establishes relations with several leguminous plants with specific correspondence to form nodules, and does not form nodules with other plants. The reason is that the root hairs of leguminous plants can secrete a special kind of protein, and there are polysaccharides on the surface of rhizobia cells. Only protein secreted by the root hairs of leguminous plants can bind specifically with the polysaccharides on the surface of leguminous rhizobia. By analogy, it may be that cyanobacteria can grow with some plants because these plants can produce substances that cyanobacteria need but cannot synthesize by themselves. Although cyanobacteria can carry out photosynthesis, the essence of photosynthesis is only to convert inorganic carbon dioxide into sugar, which is only one of n kinds of organic compounds. It is obviously not true that only sugar is needed for the growth and reproduction of cyanobacteria, otherwise cyanobacteria can be seen everywhere in supermarkets, so the reason why cyanobacteria should be born together with other plants should be because other plants can produce substances necessary for their growth and reproduction. I don't know what substance this is.

I wonder if this answer is satisfactory:)