Nitrite is converted into nitrate by ........ bacteria.

Which of the following is a limiting nutrient for both natural and agricultural ecosystems?

$NO_2^- \rightarrow NO_3^-$
The above mentioned reaction occurs in the presence of .....

Nitrifying bacteria perform which of the following processes?

Complete the nitrification process equation:
........... $+$ .......... $\rightarrow 2NO_2^- + 2H^+ + 2H_2O$

Match the organisms given in Column $I$ to their functions given in Column $II$ and choose the correct option.

Column $I$	Column $II$
$A$. Thiobacillus	$1$. Free-living nitrogen-fixing cyanobacteria
$B$. Nitrosomonas	$2$. Denitrification
$C$. Nostoc	$3$. Free-living aerobic nitrogen-fixing bacteria
$D$. Azotobacter	$4$. $NH_{3}$ to Nitrite

Codes:
$A \quad B \quad C \quad D$