Rates of extracellular amino acid oxidase activity in natural phytoplankton, cyanobacterial, and bacterial assemblages were measured using a fluorescent analog of the amino acid lysine. Activity was measured in a variety of ecosystems with different levels of nutrient enrichment and diverse community composition. Sites included a station in Shinnecock Bay, Long Island Sound, New York (USA); the Chesapeake Bay, Maryland (USA); the NW Atlantic Ocean near the Bahamas and the Caribbean Sea; Brazilian coastal waters; and 2 estuarine mesocosms. Highest rates of amino acid oxidase activity (25 to 30 nM h-') were found in the summer mesocosm experiments when NH,+ concentrations were near the limit of detection, and biomass levels were indicative of an algal bloom. Lower rates of amino acid oxidase activity were found during a bloom of Aureococcus anophagefferens and in oligotrophic oceanic waters. High rates of amino acid oxidase activity (up to 20 nM h-') were also found in oceanic samples enriched with colonies of the diazotrophic cyanobacteria Tnchodesmiurn. No activity was observed in samples from oligotrophic environments that were prefractionated through 1.0 pm filters; however, when amended with glucose or an amino acid mixture, oxidation rates of up to 8 nM h-' were observed. No activity was found during a diatom-dominated, autumnal bloom in Chesapeake Bay. Overall, amino acid oxidation represented a higher percentage of NH,' uptake in the oligotrophic waters (up to 10%) than in the coastal waters studied. In oligotrophic waters, where ambient inorganic nitrogen concentrations are low and consequently uptake rates are low, this pathway appears to represent a potentially important source of nitrogen for phytoplankton and the diazotrophic cyanobacteria Tnchodesmium.

Expression of 3 gene families involved with photoacclimation -psbA (encoding the photosystem II reaction center protein D1), hli (encoding the high-light inducible proteins), and ptox (encoding the plastid terminal oxidase) -was compared in the marine cyanobacteria Synechococcus WH8102 and Prochlorococcus MED4 acclimated to either low or high light. These 2 strains, adapted for growth in oligotrophic marine environments, have distinct light-harvesting systems and respond differently to changes in irradiance. In response to growth at higher irradiance, Synechococcus WH8102 increased expression of the psbA multigene family ( psbA1-4) 5-fold. Within this gene family, the expression of psbA2 increased 60-fold. Expression of 4 hli genes increased 2-to 5-fold, whereas expression of the ptox gene decreased 3-fold. In comparison, expression of the psbA gene increased 2-fold in Prochlorococcus MED4 cultures grown at higher irradiances. Expression of the Prochlorococcus MED4 hli6-9 and hli16-19 operons increased 11-to 14-fold, while ptox expression increased 3-fold. Using psbA induction as a standard for acclimation to changes in irradiance, we observed that the induction ratio of ptox:psbA1 and hli:psbA1 was 144 and 70 times greater, respectively, in Prochlorococcus MED4 compared with Synechococcus WH8102. These observations suggest that induction of ptox and hli may play a key role in the phototolerance of Prochlorococcus MED4. Conversely, the induction of psbA, and the synthesis of the PSII reaction center protein D1, may be critical for the acclimation of Synechococcus WH8102 to high irradiances.KEY WORDS: Synechococcus WH8102 · Prochlorococcus MED4 · Photoacclimation · Photoinhibition · Gene expression · psbA · hli · ptox · Fluorescence characterization · Carbon fixation Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 65: [1][2][3][4][5][6][7][8][9][10][11][12][13][14] 2011 mophores PCB and phycoerythrobilin (PEB), and PE binds the chromophores PEB and phycourobilin (PUB) to harvest light. In contrast, Prochlorococcus relies on integral thylakoid membrane proteins (Pcbs) that bind divinyl chlorophyll a and b to harvest light energy. The Pcbs of Prochlorococcus MED4 form a ring around PSII (Bibby et al. 2003), their Fig. 1.High irradiance in the uppermost layer of the ocean can lead to photoinhibition and loss of function of the photosynthetic apparatus. During photosynthesis, an electron from an excited chlorophyll molecule is transferred to the primary electron acceptor pheophytin and then on to the quinone molecules (Q A and Q B ) of the plastoquinone (PQ) pool. From the PQ pool, electrons are transferred to cytochrome b 6f , plastocyanin, and finally, through photosystem I (PSI), to reduce NADP + in the classical z scheme (Fig. 1). Photoinhibition results when the PQ pool becomes over-reduced (Vass et al. 1992) or during charge recombination between PSII acceptor and donor sides (Keren et al. 1997 The phycobilisome (PBS) light-harvesting system is compo...

ABSTRACT. We compared the effects of substrate C:N ratio and macrozooplankton activity on nutrient and chlorophyll dynamics by amending the substrate C:N ratios in carboys contain~ng natural estuarine microplankton [<200 pm) with additions of glucose (High C:N), arginine (Low C:N), or nothing (Control). Adult copepods (Acartla tonsa, 10 ind. 1.' ) were added to 1 carboy of each substrate treatment. Water and copepods were collected from the Choptank River, a subestuary of the Chesapeake Bay (USA) in August 1993 Ambient concentrations of NH,', NO,-, and o -P O ,~-(orthophosphate) and dissolved primary amlnes (DPA) were all < 2 . 0 pg-at. I-' Dissolved and particulate nutrients and pigments were monitored over a 2 d period. In all carboys, plankton shifted from being net consumers of nutrients during the first 17 to 23 h (Phase 1) to being net regenerators afterwards (Phase 2). Chlorophyll concentrations declined and phaeopigments increased throughout the experiment. Both substrates stimulated microbial activity, as indicated by decreased accumulation of o-Pod3-during Phase 2. increased accumulation of particulate carbon (PC) and nitrogen (PN) and increased chlorophyll loss during n~ghttime In the High C:N and Low C:N carboys relative to the Control carboy. In addition, the low C:N substrate resulted in increased accumulations of NH,', NO; and NO,'; increased chlorophyll concentration, and a d a y h i g h t pattern in chlorophyll concentration. Copepod additions resulted in greater PC, PN and chlorophyll losses and day/night patterns in chlorophyll concentration. The additions of copepods and substrates together resulted in several interactive effects. most notably, increased accumulations of NH,', o-PO,"-, and, in the High C:N treatment, NO2-+NO3-, and greater chlorophyll, PC, and PN losses. Estimated rates of ingestion and excretion by the added copepods could not account for the observed changes in chlorophyll and nutrients, especially in the carboys with copepod and substrate additions, suggesting that the copepods increased nutrient regeneration and phytoplankton removal by microzooplankton.