The factors that control the growth and nitrogen fixation rates of marine diazotrophs such as Trichodesmium have been intensively studied because of the role that these processes have in the global cycling of carbon and nitrogen, and in the sequestration of carbon to the deep sea. Because the phosphate concentrations of many ocean gyres are low, the bioavailability of the larger, chemically heterogeneous pool of dissolved organic phosphorus could markedly influence Trichodesmium physiology. Here we describe the induction, by phosphorus stress, of genes from the Trichodesmium erythraeum IMS101 genome that are predicted to encode proteins associated with the high-affinity transport and hydrolysis of phosphonate compounds by a carbon-phosphorus lyase pathway. We show the importance of these genes through expression analyses with T. erythraeum from the Sargasso Sea. Phosphonates are known to be present in oligotrophic marine systems, but have not previously been considered to be bioavailable to marine diazotrophs. The apparent absence of genes encoding a carbon-phosphorus lyase pathway in the other marine cyanobacterial genomes suggests that, relative to other phytoplankton, Trichodesmium is uniquely adapted for scavenging phosphorus from organic sources. This adaptation may help to explain the prevalence of Trichodesmium in low phosphate, oligotrophic systems.

Lipid production in the industrial microalga Nannochloropsis gaditana exceeds that of model algal species and can be maximized by nutrient starvation in batch culture. However, starvation halts growth, thereby decreasing productivity. Efforts to engineer N. gaditana strains that can accumulate biomass and overproduce lipids have previously met with little success. We identified 20 transcription factors as putative negative regulators of lipid production by using RNA-seq analysis of N. gaditana during nitrogen deprivation. Application of a CRISPR-Cas9 reverse-genetics pipeline enabled insertional mutagenesis of 18 of these 20 transcription factors. Knocking out a homolog of fungal Zn(II)Cys-encoding genes improved partitioning of total carbon to lipids from 20% (wild type) to 40-55% (mutant) in nutrient-replete conditions. Knockout mutants grew poorly, but attenuation of Zn(II)Cys expression yielded strains producing twice as much lipid (∼5.0 g m d) as that in the wild type (∼2.5 g m d) under semicontinuous growth conditions and had little effect on growth.

Utilization rates of inorganic and organic phosphorus by different picophytoplankton in the oligotrophic ocean are not well quantified. We used radioisotope tracers of orthophosphate (Pi) and the nucleic acid adenosine 5'triphosphate (ATP) to quantify P utilization into flow cytometrically sorted groups of picophytoplankton during the summer and fall of 2007 in the western Sargasso Sea. Dissolved organic phosphorus (DOP) dominated the dissolved P pool (mean ± SD 71 ± 56%), while soluble reactive phosphorus (SRP) concentrations were consistently < 5 nmol l -1 . All of the groups studied assimilated Pi (ρ Pi ) and ATP (ρ* ATP ) at significant rates. In addition, ρ Pi increased with ambient SRP concentrations, while ambient DOP concentrations had no apparent effect on either ρ Pi or ρ* ATP . Consistent with community composition and contributions to autotrophic biovolume, prokaryotes were primarily responsible for Pi and ATP turnover. In regions where SRP was depleted to < 3 nmol l -1 , ATP accounted for > 70% of the total P utilized. Among the individual taxa, ρ Pi and ρ* ATP increased in the order Prochlorococcus, Synechococcus, pico-, and nanoeukaryotes, when uptake was normalized to cell number, but the opposite relationship was observed when normalized to cell volume. This suggests that cyanobacteria are physiologically superior to the larger eukaryotes with respect to scavenging both Pi and ATP in the oligotrophic Sargasso Sea. A comparison of estimated C:P utilization rates with particulate C:P ratios suggests that different phytoplankton groups may be experiencing different degrees of P stress in the same ambient nutrient environment. Collectively, these data suggest that the labile DOP pool, assuming that ATP is a reasonable proxy for the labile DOP pool, in the Sargasso Sea may constrain primary productivity in the absence of sufficient SRP, and that cyanobacteria have a physiological advantage for P utilization under these conditions. KEY WORDS: Flow cytometry · Phosphate utilization · DOP utilization · Sargasso Sea · PicophytoplanktonResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 58: 31-44, 2009 2001). These low concentrations have been suggested to limit microbial metabolism in the subtropical North Atlantic (Ammerman et al. 2003). Throughout most of the year in the Sargasso Sea, the majority (> 80%) of total dissolved phosphorus (TDP) exists as dissolved organic phosphorus (DOP) (Ammerman et al. 2003, M. W. Lomas et al. unpubl. data). Thus, in the absence of sufficient SRP, phytoplankton may assimilate substantial quantities of biologically labile DOP to satisfy cellular P quotas and support primary production. Indeed, Mather et al. (2008) inferred enhanced DOP utilization in the subtropical North Atlantic, relative to the subtropical South Atlantic (where SRP concentrations are 1 to 2 orders of magnitude higher), based upon significantly lower DOP concentrations and elevated alkaline phosphatase activities (APA), the enzyme that hydrolyz...

The marine diazotroph Trichodesmium is a major contributor to primary production and nitrogen fixation in the tropical and subtropical oceans. These regions are often characterized by low phosphorus (P) concentrations, and P starvation of Trichodesmium could limit growth, and potentially constrain nitrogen fixation. To better understand how this genus responds to P starvation we examined four genes involved in P acquisition: two copies of a high-affinity phosphate binding protein (pstS and sphX) and two putative alkaline phosphatases (phoA and phoX). Sequence analysis of these genes among cultured species of Trichodesmium (T. tenue, T. erythraeum, T. thiebautii and T. spiralis) showed that they all are present and conserved within the genus. In T. erythraeum IMS101, the expression of sphX, phoA and phoX were sensitive to P supply whereas pstS was not. The induction of alkaline phosphatase activity corresponded with phoA and phoX expression, but enzyme activity persisted after the expression of these genes returned to basal levels. Additionally, nifH (nitrogenase reductase; involved in nitrogen fixation) expression was downregulated under P starvation conditions. These data highlight molecular level responses to low P and lay a foundation for better understanding the dynamics of Trichodesmium P physiology in low-P environments.

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