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. Phaeocystis antarctica is an important phytoplankter of the Ross Sea where it dominates the earlyseason bloom after sea ice retreat and is a major contributor to carbonexport. The factors that influence Phaeocystis colony formation and the resultant RossSea bloom initiation have been of great scientific interest, yet there islittle known about the underlying mechanisms responsible for thesephenomena. Here, we present laboratory and field studies on Phaeocystis antarctica grown undermultiple iron conditions using a coupled proteomic and transcriptomicapproach. P. antarctica had a lower iron limitation threshold than a Ross Sea diatomChaetoceros sp., and at increased iron nutrition (> 120 pM Fe') a shift fromflagellate cells to a majority of colonial cells in P. antarctica was observed, implyinga role for iron as a trigger for colony formation. Proteome analysisrevealed an extensive and coordinated shift in proteome structure linked toiron availability and life cycle transitions with 327 and 436 proteinsmeasured as significantly different between low and high iron in strains1871 and 1374, respectively. The enzymes flavodoxin and plastocyanin thatcan functionally replace iron metalloenzymes were observed at low irontreatments consistent with cellular iron-sparing strategies, withplastocyanin having a larger dynamic range. The numerous isoforms of theputative iron-starvation-induced protein (ISIP) group (ISIP2A and ISIP3) hadabundance patterns coinciding with that of either low or high iron (andcoincident flagellate or the colonial cell types in strain 1871), implyingthat there may be specific iron acquisition systems for each life cycletype. The proteome analysis also revealed numerous structural proteinsassociated with each cell type: within flagellate cells actin and tubulinfrom flagella and haptonema structures as well as a suite of calcium-bindingproteins with EF domains were observed. In the colony-dominated samples avariety of structural proteins were observed that are also often found inmulticellular organisms including spondins, lectins, fibrillins, andglycoproteins with von Willebrand domains. A large number of proteins ofunknown function were identified that became abundant at either high or lowiron availability. These results were compared to the first metaproteomicanalysis of a Ross Sea Phaeocystis bloom to connect the mechanistic information to thein situ ecology and biogeochemistry. Proteins associated with both flagellate andcolonial cells were observed in the bloom sample consistent with the needfor both cell types within a growing bloom. Bacterial iron storage and B12 biosynthesis proteins were also observed consistent with chemicalsynergies within the colony microbiome to cope with the biogeochemicalconditions. Together these responses reveal a complex, highly coordinatedeffort by P. antarctica to regulate its phenotype at the molecular level in response toiron and provide a window into the biology, ecology, and biogeochemistry ofthis group.

Colony formation in &lt;i&gt;Phaeocystis antarctica&lt;/i&gt;: connecting molecular mechanisms with iron biogeochemistry

Colony formation in <i>Phaeocystis antarctica</i>: connecting molecular mechanisms with iron biogeochemistry