Southern Ocean primary productivity plays a key role in global ocean biogeochemistry and climate. At the Southern Ocean sea ice edge in coastal McMurdo Sound, we observed simultaneous cobalamin and iron limitation of surface water phytoplankton communities in late Austral summer. Cobalamin is produced only by bacteria and archaea, suggesting phytoplankton-bacterial interactions must play a role in this limitation. To characterize these interactions and investigate the molecular basis of multiple nutrient limitation, we examined transitions in global gene expression over short time scales, induced by shifts in micronutrient availability. Diatoms, the dominant primary producers, exhibited transcriptional patterns indicative of co-occurring iron and cobalamin deprivation. The major contributor to cobalamin biosynthesis gene expression was a gammaproteobacterial population, Oceanospirillaceae ASP10-02a. This group also contributed significantly to metagenomic cobalamin biosynthesis gene abundance throughout Southern Ocean surface waters. Oceanospirillaceae ASP10-02a displayed elevated expression of organic matter acquisition and cell surface attachment-related genes, consistent with a mutualistic relationship in which they are dependent on phytoplankton growth to fuel cobalamin production. Separate bacterial groups, including Methylophaga, appeared to rely on phytoplankton for carbon and energy sources, but displayed gene expression patterns consistent with iron and cobalamin deprivation. This suggests they also compete with phytoplankton and are important cobalamin consumers. Expression patterns of siderophore-related genes offer evidence for bacterial influences on iron availability as well. The nature and degree of this episodic colimitation appear to be mediated by a series of phytoplankton-bacterial interactions in both positive and negative feedback loops.colimitation | Southern Ocean primary productivity | metatranscriptomics | phytoplankton-bacterial interactions | cobalamin P rimary productivity and community composition in the Southern Ocean play key roles in global change (1, 2). The coastal Southern Ocean, particularly its shelf and marginal ice zones, is highly productive, with mean rates approaching 300-450 mg C m −2 ·d −1 (3). As such, identifying factors controlling phytoplankton growth in these regions is essential for understanding the ocean's role in past, present, and future biogeochemical cycles. Although irradiance, temperature, and iron availability are often considered to be the primary drivers of Southern Ocean productivity (1, 4), cobalamin (vitamin B 12 ) availability has also been shown to play a role (5, 6). Cobalamin is produced only by select bacteria and archaea and is required by most eukaryotic phytoplankton, as well as many bacteria that do not produce the vitamin (7). Cobalamin is used for a range of functions, including methionine biosynthesis and one-carbon metabolism. Importantly, phytoplankton that are able to grow without cobalamin preferentially use it when available; growth...