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Abstract A flow cytometry method for enumerating marine heterotrophic bacteria and phytoplankton in a living or preserved sample using a low power solid state near‐ultraviolet laser is described. The method uses Hoechst 34580 to stain DNA in microbial cells in seawater samples. This stain is optimally excited at 375 nm unlike the similar Hoechst 33342, which requires ~ 350 nm excitation only available on more expensive lasers. Phytoplankton abundances from the Hoechst 34580 method are comparable to those of unstained samples and when analyzed by the Hoechst 33342 staining method. With this new method, nonpigmented marine bacteria and phytoplankton abundances are obtained simultaneously in a single sample as the Hoechst emission wavelength (~ 450 nm) is well separated from the emission wavelengths of chlorophyll and phycoerythrin fluorescence. Bacteria abundances are similar between this new method and those obtained with established Hoechst 33342 and SybrGreen I methods. Precision estimates (coefficient of variation) on populations with abundances near ~ 10⁵cells mL⁻¹are 1–3%, increasing to 3–9% at lower cell concentrations of 10³cells mL⁻¹. The Hoechst 34580 method is simple, requiring no heating or pretreatment with RNAse, can be used on unpreserved and formaldehyde‐preserved cells, and is amenable to at‐sea use with portable, compact, low power‐requiring flow cytometers. 

Abstract We investigated competition betweenSalpa thompsoniand protistan grazers during Lagrangian experiments near the Subtropical Front in the southwest Pacific sector of the Southern Ocean. Over a month, the salp community shifted from dominance by large (> 100 mm) oozooids and small (< 20 mm) blastozooids to large (~ 60 mm) blastozooids. Phytoplankton biomass was consistently dominated by nano‐ and microphytoplankton (> 2 μm cells). Using bead‐calibrated flow‐cytometry light scatter to estimate phytoplankton size, we quantified size‐specific salp and protistan zooplankton grazing pressure. Salps were able to feed at a > 10,000 : 1 predator : prey size (linear‐dimension) ratio. Small blastozooids efficiently retained cells > 1.4μm (high end of picoplankton size, 0.6–2 μm cells) and also obtained substantial nutrition from smaller bacteria‐sized cells. Larger salps could only feed efficiently on > 5.9μm cells and were largely incapable of feeding on picoplankton. Due to the high biomass of nano‐ and microphytoplankton, however, all salps derived most of their (phytoplankton‐based) nutrition from these larger autotrophs. Phagotrophic protists were the dominant competitors for these prey items and consumed approximately 50% of the biomass of all phytoplankton size classes each day. Using a Bayesian statistical framework, we developed an allometric‐scaling equation for salp clearance rates as a function of salp and prey size:urn:x-wiley:00243590:media:lno11770:lno11770-math-0001where ESD is prey equivalent spherical diameter (µm), TL isS. thompsonitotal length,φ = 5.6 × 10⁻³ ± 3.6 × 10⁻⁴,ψ = 2.1 ± 0.13,θ = 0.58 ± 0.08, andγ = 0.46 ± 0.03 and clearance rate is L d^‐1salp^‐1. We discuss the biogeochemical and food‐web implications of competitive interactions among salps, krill, and protozoans.