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

Abstract

Abstract. Phaeocystis antarctica is an important phytoplankter of the Ross Sea where it dominates the early season bloom after sea ice retreat and is a major contributor to carbon export. The factors that influence Phaeocystis colony formation and the resultant Ross Sea bloom initiation have been of great scientific interest, yet there is little known about the underlying mechanisms responsible for these phenomena. Here, we present laboratory and field studies on Phaeocystis antarctica grown under multiple iron conditions using a coupled proteomic and transcriptomic approach. P. antarctica had a lower iron limitation threshold than a Ross Sea diatom Chaetoceros sp., and at increased iron nutrition (> 120 pM Fe') a shift from flagellate cells to a majority of colonial cells in P. antarctica was observed, implying a role for iron as a trigger for colony formation. Proteome analysis revealed an extensive and coordinated shift in proteome structure linked to iron availability and life cycle transitions with 327 and 436 proteins measured as significantly different between low and high iron in strains 1871 and 1374, respectively. The enzymes flavodoxin and plastocyanin that can functionally replace iron metalloenzymes were observed at low iron treatments consistent with cellular iron-sparing strategies, with plastocyanin having a larger dynamic range. The numerous isoforms of the putative ironstarvation-induced protein (ISIP) group (ISIP2A and ISIP3) had abundance patterns coinciding with that of either low or high iron (and coincident flagellate or the colonial cell types in strain 1871), implying that there may be specific iron acquisition systems for each life cycle type. The proteome analysis also revealed numerous structural proteins associated with each cell type: within flagellate cells actin and tubulin from flagella and haptonema structures as well as a suite of calcium-binding proteins with EF domains were observed. In the colony-dominated samples a variety of structural proteins were observed that are also often found in multicellular organisms including spondins, lectins, fibrillins, and glycoproteins with von Willebrand domains. A large number of proteins of unknown function were identified that became abundant at either high or low iron availability. These results were compared to the first metaproteomic analysis of a Ross Sea Phaeocystis bloom to connect the mechanistic information to the in situ ecology and biogeochemistry. Proteins associated with both flagellate and colonial cells were observed in the bloom sample consistent with the need for both cell types within a growing bloom. Bacterial iron storage and B 12 biosynthesis proteins were also observed consistent with chemical synergies within the colony microbiome to cope with the biogeochemical conditions. Together these responses reveal a complex, highly coordinated effort by P. antarctica to regulate its phenotype at the molecular level in response to iron and provide a window into the biology, ecology, and biogeochemistry of this group.

Transcriptomic and proteomic responses of the oceanic diatomPseudo‐nitzschia graniito iron limitation

“…Protein yield was determined using the BioRad DC Protein Assay. Protein digestion was performed using a modified tube gel method according to Bender et al, 2018. Briefly, protein was reduced, alkylated and digested with a 1:20 trypsin: protein ratio.…”

Deciphering Patterns of Adaptation and Acclimation in the Transcriptome of Phaeocystis antarctica to Changing Iron Conditions1

“…, Bender et al. ). A few molecular markers, suggested to play a role in the formation of the extracellular colonial matrix and in cell aggregation (Bender et al.…”

“…), whereas iron addition was reported to increase growth rates and trigger colony formation in P. antarctica (Bender et al. ). In situ iron fertilization experiments in the SO reported haptophytes ( P. antarctica) among the groups contributing to the elevation in chlorophyll a signal after iron enrichment (Gall et al.…”

Differential Gene Expression Supports a Resource‐Intensive, Defensive Role for Colony Production in the Bloom‐Forming Haptophyte, Phaeocystis globosa

“…Focal adhesion proteins connect cells to extracellular matrices both literally and figuratively, by holding cells in place and by initiating cellular responses to external conditions (Wozniak et al 2004). Likewise, Bender et al (2018) found genes for focal adhesion proteins, specifically glycoproteins, upregulated in colonial Phaeocystis antarctica. It is therefore likely that these proteins have an important function in structurally maintaining cell positions in the colonial matrix and signaling between colonial cells.…”