Microbes in Human Welfare

Anabaena is a filamentous cyanobacterium with heterocysts, the cells in which nitrogen fixation takes place. It lives freely in water and soil.
Anabaena azollae lives symbiotically in the leaf cavities of the water fern Azolla. The Azolla–Anabaena association is used as a biofertilizer in rice fields, as it adds nitrogen to the soil.
Nostoc and Tolypothrix are also nitrogen-fixing cyanobacteria, but the partner of Azolla is Anabaena. Chlorella is a green alga, not a cyanobacterium, and does not fix nitrogen.
So the cyanobacterium is Anabaena.

Gobar gas (biogas) is produced when cattle dung and other organic waste are decomposed anaerobically by methanogenic bacteria (e.g. Methanobacterium) in a biogas plant.
Biogas consists mainly of methane ($CH_4$, about 50–70 percent), with carbon dioxide and small amounts of hydrogen sulphide and hydrogen.
Methane is inflammable and is used as a fuel for cooking and lighting. The spent slurry is used as manure.
So the major component of gobar gas is methane.

Dough for bread or idli contains yeast (Saccharomyces cerevisiae) or bacteria.
In warm conditions these microbes ferment the sugars in the dough anaerobically, producing ethyl alcohol and carbon dioxide:
$C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$
The $CO_2$ gas gets trapped in the dough as bubbles, making it rise and become soft and spongy. Warm weather speeds up the activity of the microbes.
The $CO_2$ is produced inside the dough, not absorbed from the atmosphere.
So the dough becomes soft and spongy because of fermentation.

Vitamin $B_{12}$ (cyanocobalamin, a cobalt-containing vitamin) is made only by microorganisms. Animals get it from food and store it mainly in the liver.
Liver (e.g. goat's liver) is the richest animal source of vitamin $B_{12}$.
Spirulina, a cyanobacterium used as single cell protein, is also rich in protein, minerals and vitamins including $B_{12}$ (in the form of related cobalamin compounds).
Plant foods such as chocolate, green gram, rice and carrot contain little or no vitamin $B_{12}$; egg and chicken contain only moderate amounts.
Its deficiency causes pernicious anaemia.
So the richest sources are goat's liver and Spirulina.

Spirulina, a cyanobacterium, is grown as single cell protein (SCP), a rich source of protein; correctly matched.
Rhizobium fixes nitrogen in the root nodules of legumes and is used as a biofertilizer; correctly matched.
Streptomyces species produce many antibiotics such as streptomycin, tetracycline and chloramphenicol; correctly matched.
Serratia marcescens is a bacterium known for its red pigment and as a cause of hospital infections; it has no connection with drug addiction.
So the pair that is not correctly matched is Serratia – drug addiction.