Methionine synthase interreplacement in diatom cultures and communities: Implications for the persistence of B12 use by eukaryotic phytoplankton.

Three proteins related to vitamin B₁₂ metabolism in diatoms were quantified via selected reaction monitoring mass spectrometry: B₁₂ -dependent and B₁₂-independent methionine synthase (MetH, MetE) and a B₁₂ acquisition protein (CBA1). B₁₂ -mediated interreplacement of MetE and MetH metalloenzymes was observed in 21 Phaeodactylum tricornutum where MetH abundance was highest (0.06 fmol µg^-1 protein) under high B₁₂ and MetE abundance increased to 3.25 fmol µg^-1 protein under low B₁₂ availability. Maximal MetE abundance was 60-fold greater than MetH, consistent with the expected ~50-100-fold larger turnover number for MetH. MetE expression resulted in 30-fold increase in nitrogen and 40-fold increase in zinc allocated to methionine synthase activity under low B₁₂ . CBA1 abundance was 6-fold higher under low-B₁₂ conditions and increased upon B₁₂ resupply to starved cultures. While biochemical pathways that supplant B₁₂ requirements exist and are utilized by organisms such as land plants, B₁₂ use persists in eukaryotic phytoplankton. This study suggests that retention of B₁₂ utilization by phytoplankton results in resource conservation under conditions of high B₁₂ availability. MetE and MetH abundances were also measured in diatom communities from McMurdo Sound, verifying that both these proteins are expressed in natural communities. These protein measurements are consistent with previous studies suggesting that B₁₂ availability influences Antarctic primary productivity. This study illuminates controls on expression of B₁₂ -related proteins, quantitatively assesses the metabolic consequences of B₁2 deprivation, and demonstrates that mass spectrometry--based protein measurements yield insight into the functioning of marine microbial communities. [ABSTRACT FROM AUTHOR]