Elin M. Haugen

Overestimation of heterotrophic bacteria in the Sargasso Sea: direct evidence by flow and imaging cytometry

Abstract Accurate measurements of bacterial biomass in the ocean are needed for modeling marine microbial food webs and global biogeochemical cycling. We present direct evidence that previous estimates of heterotrophic bacteria biomass in the oligotrophic ocean are confounded by the presence of the abundant photosynthetic procaryote, Prochlorococcus . The chlorophyll autofluorescence of these photosynthetic bacterial cells is very faint and fades rapidly under epifluorescence microscopy. Detection and enumeration of these cells thus far has almost exclusively been by flow cytometry. Using a cooled, charge-coupled device (CCD) camera we were able to image these cells for direct biovolume measurements. A double-exposed image of DAPI-stained Prochlorococcus cells shows that they are indistinguishable from heterotrophic bacteria in standard slide preparations. At two Sargasso Sea stations Prochlorococcus could cause an overestimation of surface (top 150 m) integrated heterotrophic bacterial biovolume (biomass) of 18 and 22% determined by standard microscope methods. At the subsurface chlorophyll maximum Prochlorococcus was 33 and 43% of the heterotrophic bacterial biovolume (biomass) at these stations. Prochlorococcus cell size increased from 0.05 μm 3 in the surface mixed layer to about 0.2 μm 3 below 100 m, confirming previous interpretations of flow cytometric light scatter measurements. Shifting biomass from the heterotrophic bacteria pool to the primary producer compartment has significant implications for ecosystem structure and trophic transfer in marine food webs.

Nanoplankton And Protozoan Microzooplankton During The JGOFS North-Atlantic Bloom Experiment - 1989 And 1990

Complex mesoscale eddy interactions are characteristic of the North Atlantic, resulting in a mosaic of water masses with different physical, chemical and biological properties. Observations of protist assemblages during spring 1989 and 1990 in the vicinity of 47°N 18°W indicate that timing, composition, and further development of the spring bloom community are highly variable between years. During 1989 a microbial community, dominated by small photosynthetic nanoplankton and protist grazers, was observed after the main diatom bloom in the transition zone between two cyclonic eddies. This community was characterized by a high ratio of ‘protozoan’ to ‘phytoplankton’ carbon, and dominance of the microzooplankton by mixotrophic ciliates. A nanodiatom/prymnesiophyte bloom was observed to replace the typical ‘microdiatom’ bloom in a front between a cyclonic and anticyclonic eddy during 1990. After the demise of the diatoms, high standing stocks of nanophytoplankton persisted until early June. In this post-diatom-bloom period, the ‘protozoan’ biomass was lower and the ‘nanophytoplankton’ stocks higher than in 1989. Very high stocks of heterotrophic nanodinoflagellates were observed in 1990. The factors responsible for the development of these quite different microbial food-webs in two consecutive years and the consequences thereof for ecosystem function remain to be more fully explored.

Phagotrophy of fluorescently labeled bacteria by an oceanic phytoplankter

Using fluorescently-labeled bacteria and detection by flow cytometry and epifluorescence microscopy, we demonstrate inducible mixotrophy in a marine photosynthetic flagellate, Ochromonas sp. (class Chrysophyceae). Phagotrophic uptake of bacteria increases under conditions of low or limiting light and nutrients, but deceases in periods of prolonged darkness; sustained phagotrophy may require light. In addition, this alga appears to discriminate between and preferentially ingest different types of bacteria. Although this clone is primarily photosynthetic, phagotrophy contributes to its nutrition, especially when light or nutrients limit photosynthesis.

Seasonal distribution and temperature tolerance of Synechococcus in Boothbay Harbor, Maine

Abstract We report on the distribution of coccoid cyanobacteria ( Synechococcus spp.) and other components of the phytoplankton community, determined during a seasonal study in Boothbay Harbor, Maine. Synechococcus abundance peaked in late summer and in mid-winter. Blooms of centric diatoms occurred in spring and fall. These two groups, but no other identified phytoplankton type, showed a pattern of reciprocal dominance. Synechococcus abundance did not correlate with salinity and no significant correlation with temperature was found; it was abundant even in 2°C waters. Among the factors that may permit abundance of Synechococcus at cold water temperatures are: occurrence of a northern race or cline that does not extend to the south of Cape Cod, or control by a temperature-sensitive predator that is limited by local winter water temperatures.

OCCURRENCE AND ABUNDANCE OF GREEN-FLUORESCING DINOFLAGELLATES IN SURFACE WATERS OF THE NORTHWEST ATLANTIC AND NORTHEAST PACIFIC OCEANS1

We have cultured green fluorescing heterotrophic dinoflagellates whose continuous green fluorescence is due to an unidentified compound, probably a flavin, that excites with blue (∼460 nm) light and emits green (∼535 nm) light. No evidence of bioluminescence was found, but we note that compounds with similar fluorescence characteristics have been associated with bioluminescence in other taxa. These cells, all naked gymnodinoids, are widespread and abundant in the Northwest Atlantic and Northeast Pacific Oceans (103–105 L−1). They comprise 4–100% of the total heterotrophic dinoflagellate component which, in turn, is usually equivalent magnitude to the phototrophic naked dinoflagellate component of the phytoplankton community.

TAXONOMIC AFFINITIES OF MARINE COCCOID ULTRAPHYTOPLANKTON: A COMPARISON OF IMMUNOCHEMICAL SURFACE ANTIGEN CROSS-REACTIONS AND HPLC CHLOROPLAST PIGMENT SIGNATURES1

Immunological grouping, determined by affinities of polyclonal antibodies to surface antigens of intact cells, was used to characterize 19 clones of marine coccoid ultraplankton. The resulting cross‐reactive antigen groups corresponded to pigment‐groups as defined by HPLC analysis of chloroplast pigments (carotenoids and chlorophylls). Because immunological cross reactions are specific at the species level in groups of algae having well defined morphological criteria, we suggest that immunological methods can be used to recognize algae presently indistinguishable by standard morphological criteria, and especially in oceanographic applications involving qualitative cell enumerations of the ultraplankton.