Answer
$S^{0}$
Work Step by Step
In sulfur cycle, microbes degrade organic sulfur compounds such as amino acids to release $H_{2}S$ which can be oxidized by Thiobacillus to $SO_{4}^{2-} $. This ion can be assimilated into amino acids by plants and bacteria or reduced by desufolvibrio into $H_{2}S$. $H_{2}S$ is used by photoautotropic bacteria as an electron donor to synthesize Carbohydrates. The sulfur-containing-by-product of this metaboliam is $S^{0}$.
