Phosphorus (P) is a critical driver of phytoplankton growth and ecosystem function in the ocean. Diatoms are an abundant class of marine phytoplankton that are responsible for significant amounts of primary production. With the control they exert on the oceanic carbon cycle, there have been a number of studies focused on how diatoms respond to limiting macro and micronutrients such as iron and nitrogen. However, diatom physiological responses to P deficiency are poorly understood. Here, we couple deep sequencing of transcript tags and quantitative proteomics to analyze the diatom Thalassiosira pseudonana grown under P-replete and P-deficient conditions. A total of 318 transcripts were differentially regulated with a false discovery rate of <0.05, and a total of 136 proteins were differentially abundant ( p <0.05). Significant changes in the abundance of transcripts and proteins were observed and coordinated for multiple biochemical pathways, including glycolysis and translation. Patterns in transcript and protein abundance were also linked to physiological changes in cellular P distributions, and enzyme activities. These data demonstrate that diatom P deficiency results in changes in cellular P allocation through polyphosphate production, increased P transport, a switch to utilization of dissolved organic P through increased production of metalloenzymes, and a remodeling of the cell surface through production of sulfolipids. Together, these findings reveal that T. pseudonana has evolved a sophisticated response to P deficiency involving multiple biochemical strategies that are likely critical to its ability to respond to variations in environmental P availability.

In low-phosphorus (P) marine systems, phytoplankton replace membrane phospholipids with nonphosphorus lipids, but it is not known how rapidly this substitution occurs. Here, when cells of the model diatom Thalassiosira pseudonana were transferred from P-replete medium to P-free medium, the phospholipid content of the cells rapidly declined within 48 h from 45±0.9 to 21±4.5% of the total membrane lipids; the difference was made up by non-phosphorus lipids. Conversely, when P-limited T. pseudonana were resupplied with P, cells reduced the percentage of their total membrane lipids contributed by a non-phosphorus lipid from 43 ± 1.5 to 7.3 ± 0.9% within 24 h, whereas the contribution by phospholipids rose from 2.2±0.1 to 44±3%. This dynamic phospholipid reservoir contained sufficient P to synthesize multiple haploid genomes, suggesting that phospholipid turnover could be an important P source for cells. Field observations of phytoplankton lipid content may thus reflect short-term changes in P supply and cellular physiology, rather than simply long-term adjustment to the environment. The ISME Journal ( Subject Category: geomicrobiology and microbial contributions to geochemical cycles Keywords: betaine lipids; DGCC; lipid substitution; non-phosphorus lipids; phosphatidylcholine; diatoms Phosphate is chronically scarce in oligotrophic oceans (Krom et al., 1991;Karl et al., 1997;Wu et al., 2000), and phytoplankton appear to posses highly effective physiological mechanisms to reduce their phosphorus (P) quota in these environments (Twining et al., 2010). For example, all phytoplankton groups examined to date, both in culture and in situ, substitute non-phosphorus (sulfur-and nitrogen-containing) lipids for phospholipids in their membranes under low-P conditions (Van Mooy et al., 2009). Such lipid substitution in bacteria (Minnikin et al., 1974;Benning et al., 1995) can take place within hours (Zavaleta-Pastor et al., 2010). However, it is not known how rapidly phytoplankton remodel their membranes, and whether field observations (Van Mooy et al., 2009) reflect longterm adjustment to the environment or an immediate cellular response to low-P conditions.The diatom Thalassiosira pseudonana and other eukaryotic phytoplankton contain the phospholipids phosphatidylcholine (PC), phosphatidylglycerol (PG) and phosphatidylethanolamine (PE). Under low-P conditions, T. pseudonana substitutes PC with the betaine lipid diacylglycerylcarboxyhydroxymethylcholine (DGCC), and PG with the sulfolipid sulfoquinovosyldiacylglycerol (SQDG) (Van Mooy et al., 2009).Two culture experiments were conducted to test how rapidly lipid substitution occurs in T. pseudonana upon changes in phosphate concentration. In the first experiment, T. pseudonana CCMP1335 was grown in P-replete medium (in triplicate, Supplementary Methods) until early log-phase. Cells were then gently filtered onto 0.2 mm polycarbonate membranes and resuspended in either phosphatefree (ÀP) or phosphate-replete ( þ P, 36 mmol l À1 phosphate) medium. However, as the ÀP cultures con...

Colonies of the cyanobacterium Trichodesmium are abundant in the oligotrophic ocean, and through their ability to fix both CO 2 and N 2 , have pivotal roles in the cycling of carbon and nitrogen in these highly nutrient-depleted environments. Trichodesmium colonies host complex consortia of epibiotic heterotrophic bacteria, and yet, the regulation of nutrient acquisition by these epibionts is poorly understood. We present evidence that epibiotic bacteria in Trichodesmium consortia use quorum sensing (QS) to regulate the activity of alkaline phosphatases (APases), enzymes used by epibionts in the acquisition of phosphate from dissolved-organic phosphorus molecules. A class of QS molecules, acylated homoserine lactones (AHLs), were produced by cultivated epibionts, and adding these AHLs to wild Trichodesmium colonies collected at sea led to a consistent doubling of APase activity. By contrast, amendments of (S)-4,5-dihydroxy-2,3-pentanedione (DPD)-the precursor to the autoinducer-2 (AI-2) family of universal interspecies signaling molecules-led to the attenuation of APase activity. In addition, colonies collected at sea were found by high performance liquid chromatography/mass spectrometry to contain both AHLs and AI-2. Both types of molecules turned over rapidly, an observation we ascribe to quorum quenching. Our results reveal a complex chemical interplay among epibionts using AHLs and AI-2 to control access to phosphate in dissolved-organic phosphorus.

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