Aerobic anoxygenic phototrophic bacteria in the Mid-Atlantic Bight and the North Pacific Gyre

Abstract

The abundance of aerobic anoxygenic phototrophic (AAP) bacteria, cyanobacteria, and heterotrophs was examined in the Mid-Atlantic Bight and the central North Pacific Gyre using infrared fluorescence microscopy coupled with image analysis and flow cytometry. AAP bacteria comprised 5% to 16% of total prokaryotes in the Atlantic Ocean but only 5% or less in the Pacific Ocean. In the Atlantic, AAP bacterial abundance was as much as 2-fold higher than that of Prochlorococcus spp. and 10-fold higher than that of Synechococcus spp. In contrast, Prochlorococcus spp. outnumbered AAP bacteria 5- to 50-fold in the Pacific. In both oceans, subsurface abundance maxima occurred within the photic zone, and AAP bacteria were least abundant below the 1% light depth. The abundance of AAP bacteria rivaled some groups of strictly heterotrophic bacteria and was often higher than the abundance of known AAP bacterial genera (Erythrobacter and Roseobacter spp.). Concentrations of bacteriochlorophyll a (BChl a) were low ( approximately 1%) compared to those of chlorophyll a in the North Atlantic. Although the BChl a content of AAP bacteria per cell was typically 20- to 250-fold lower than the divinyl-chlorophyll a content of Prochlorococcus, the pigment content of AAP bacteria approached that of Prochlorococcus in shelf break water. Our results suggest that AAP bacteria can be quite abundant in some oceanic regimes and that their distribution in the water column is consistent with phototrophy.

FIG. 1.

Abundance of Prochlorococcus determined by microscopy and flow cytometry in the central North Pacific Gyre. Error bars indicate standard errors (SE).

FIG. 2.

Abundance of AAP bacteria, Prochlorococcus, and Synechococcus at Gulf Stream (A, B), coastal (C), and shelf break (D) sampling sites in the Mid-Atlantic Bight in August 2003. Error bars indicate SE.

FIG. 3.

Abundance of AAP bacteria, Prochlorococcus, Synechococcus, and total prokaryotes in the central North Pacific Gyre in February 2004. The error bars indicate the variation (standard deviation) in abundance among sampling sites located near Oahu.

FIG. 4.

Contribution of AAP bacteria to community structure in the Mid-Atlantic Bight in August 2003 (filled symbols) and the central North Pacific Gyre in February 2004 (open symbols). The dashed lines were drawn by hand. Error bars indicate SE.