The harmful process caused by blue-green algae is .........

An organism that performs photosynthesis but does not release any molecular oxygen during the process is:

Which category of bacteria is most abundant in nature and is helpful in making curd from milk and in the production of antibiotics?

Atmospheric nitrogen-fixation is carried on by

True sexual reproduction is absent in........

The given figure shows the structure of filamentous blue-green algae,Nostoc,with a structure marked as $X$. Select the option which shows the correct identification of $X$ with its feature.