Diel patterns of carbon and nitrogen assimilation were investigated in 5 taxonomically diverse species of unicellular algae grown in NH:-limited cyclostats (12.12 L: D). Calculated 24 h average rates of N-assimilation based on C-14 incorporation into protein overestimated the actual rate by a n average of 24 ? 27 %. Overeshmation of carbon assimilation by the C-14 method (16 i-19 "h) accounted for most of the observed discrepancy in the estimation of N-assimilation rates. Estimates of actual N-assimilation rates were more accurate at high relative growth rates (104 f 20 %) than at low relative growth rates (143 * 22 o4,). The N/C assimilation ratio provided an accurate estimate (104 * 16 %) of the N/C composibon ratio after a 1 2 : 12 L: D cycle. The percentage decrease in particulate C-14 acbvity at night due to respiration was 105 ".I greater than the predicted decrease estimated from the loss in particulate carbon concentration. Dark respiration of carbon with an average specific activity equal to that of the inorganic carbon could not entirely account for thls phenomenon. Excretion of dissolved organic carbon (DOC) dunng the photopenod and uptake of low specific activity DOC at mght may have been responsible for the decrease m specific activity of particulate C-14 at night.

Changes in chlorophyll a (chl a) and phaeopigment concentrations during 24 h incubations in water prefiltered through 2.0 pm Nuclepore filters were determined on a weekly basis over a period of 13 mo using water from Kaneohe Bay, a subtropical inlet in the Hawaiian Islands, USA. In bottles illuminated at a constant irradiance of 4.0 E m-' h-', both chl a and phaeopigment concentrations were consistently lower than initial values at the end of the incubations. Chl a concentrations declined at a lower rate in dark bottles than in light bottles. There was no evidence of a change in phaeopigment concentrations in dark bottles. There was no temporal pattern in the exponential decay rates of phaeopigments in light bottles over the course of the 13 mo study, the median value being 0.016 m2 E-' There was, however, evidence of a nonrandom temporal pattern in the chl a decay constants. Winter values were about twice as large as summer values, a result presumably reflecting changes in the physiology and/or species composition of the phytoplankton community. In about 30% of the incubations phaeopigment concentrations were higher than initial values at intermediate time points, in some cases by as much as a factor of 2 to 3 during the first 4 to 8 h of the incubations. These results are beheved to have been caused by stress associated with the effort to remove grazers by filtration. Inclusion of nanoplankton ( 2 to 10 pm) in the incubation bottles consistently resulted in a higher concentration of phaeopigments in the picoplankton fraction after 24 h than was the case in control bottles containing only picoplankton. In this study as in other work, prescreening through 10 pm filters appeared to be insufficient to eliminate grazing artifacts from phaeopigment photodegradation experiments. Dilution rather than filtration may be a more practical way to account for grazing effects in such studies.

A single-cell isolation technique was used to study the assimilation of 14C0, by natural populations of the silicoflagellate Dictyocha perlaevis. The method uses a concentrated natural sample and a high activity of radioisotope. The technique facil~tates physiological studies of certain species which cannot be conveniently studied in the laboratory due to difficulty in maintaining laboratory cultures. Data obtained from a subtropical embayment are used to illustrate the application of the method, and include in particular a study of carbon partitioning patterns into major polymers over the course of a die1 cycle.

word count: 250 (Abstract), 167 (Importance) 20 Text word count: 21 22 23In this study, a strain of SAR11 subgroup IIIa (termed HIMB114) isolated from the 24 tropical Pacific Ocean was grown in seawater-based batch and continuous culture in order to 25 quantify cellular features and metabolism relevant to SAR11 ecology. We report the first direct 26 measurements of cellular elemental quotas for nitrogen (N) and phosphorus (P) for SAR11: 1.4 ± 27 0.9 fg N and 0.44 ± 0.01 fg P, respectively, that were consistent with the small size of HIMB114 28 cells (average volume of 0.09 µm 3 ). However, the mean carbon (C) cellular quota of 50 ± 47 fg 29 C was anomalously high, but variable. Rates of phosphate (PO 4 3-) uptake measured from both 30 batch and continuous cultures were exceptionally slow: in chemostats growing at 0.3 d -1 , 31HIMB114 took up 1.1 ± 0.3 amol P cell -1 d −1 , suggesting that <30% of the cellular P requirement 32 of HIMB114 was met by PO 4 3assimilation. The mean rate of leucine incorporation, a measure 33 of bacterial production, during late log phase growth of batch HIMB114 cultures was 0.042 ± 34 0.02 amol Leu cell −1 h −1 . While only weakly correlated with changes in specific growth rates, the 35 onset of stationary phase resulted in decreases in cell-specific leucine incorporation that were 36 proportional to changes in growth rate. Rates of cellular production, respiratory oxygen 37 consumption, and changes in total organic C concentrations constrained cellular growth 38 efficiencies to 13 ± 4%. Hence, despite the small, streamlined genome and diminutively sized 39 cells, SAR11 strain HIMB114 appears to grow at efficiencies similar to naturally occurring 40 bacterioplankton communities. 41 42 Importance 43While SAR11 bacteria contribute a significant fraction to the total picoplankton biomass in the 44 ocean and likely are major players in organic C and nutrient cycling, the cellular characteristics 45 3 and metabolic features of most lineages have either been only hypothesized from genomes or 46 otherwise not measured in controlled laboratory experimentation. The dearth of data on even the 47 most basic characteristics for what is arguably the most abundant heterotroph in seawater has 48 limited the specific consideration of SAR11 in ocean ecosystem modeling efforts. In this study, 49 we provide measures of cellular P, N, C, aerobic respiration and bacterial production for a 50 SAR11 strain growing in natural seawater media that can be used to directly relate these features 51 of SAR11 to biogeochemical cycling in the oceans. Through the development of a chemostat 52 system to measure nutrient uptake during steady-state growth, we have also documented 53 inorganic P uptake rates that allude to the importance of organic phosphorous to meet cellular P 54 demands, even in the presence of non-limiting PO 4 3concentrations. 55 56 57 58The SAR11 bacterial lineage is a genetically diverse clade of aquatic, free-living cells 59 with compact, streamlined genomes, found broadly distribute...