Biogenic selenium (Se) emissions play a major role in the biogeochemical cycle of
this essential micronutrient. Microalgae may be responsible for a large portion of
these emissions via production of methylated Se compounds that volatilize into the
atmosphere. However, the biochemical mechanisms underlying Se methylation in microalgae
are poorly understood. Here, we study Se methylation by Chlamydomonas reinhardtii,
a model freshwater alga, as a function of uptake and intracellular Se concentrations
and present a biochemical model that quantitatively describes Se uptake and methylation.
Both selenite and selenate, two major inorganic forms of Se, are readily internalized
by C. reinhardtii, but selenite is accumulated around ten times more efficiently than
selenate due to different membrane transporters. With either selenite or selenate
as substrates, Se methylation was highly efficient (up to 89% of intracellular Se)
and directly coupled to intracellular Se levels (R(2) > 0.92) over an intracellular
concentration range exceeding an order of magnitude. At intracellular concentrations
exceeding 10 mM, intracellular zerovalent Se was formed. The relationship between
uptake, intracellular accumulation, and methylation was used by the biochemical model
to successfully predict measured concentrations of methylated Se in natural waters.
Therefore, biological Se methylation by microalgae could significantly contribute
to environmental Se cycling.