A time series assessment of planktonic archaeal variability in the Santa Barbara Channel

Abstract

Although detailed temporal studies of marine planktonic cyanobacteria have been reported, relatively little is known about variation of other marine picoplankton groups on time scales on the order of months to years. In this study, we followed variation in relative abundance of planktonic archaeal rRNA at 5 depths in the upper 300 m of the Santa Barbara Channel (SBC) over 32 mo. Small subunit rRNA targeted oligonucleotide probes were used to quantify archaeal, bacterial, and eucaryal rRNA relative abundance. Archaea subgroups were quantified using probes specific to 2 planktonic archaeal groups, termed GI and GII. The archaeal rRNA signal in the upper 20 m was characterized by intermittent 'blooms' that coincided with increases in the relative abundance of GII rRNA as well as decreases in chlorophyll a (chl a). At greater depths, archaeal rRNA abundance was consistently elevated, approaching bacterial rRNA abundance. The GI rRNA accounted for the majority of the deeper archaeal rRNA signal throughout the time series. Consistent with a previous report, the 2 groups of archaea had maximal rRNA abundance at different depths. The majority of the variability in the GI archaeal signal in the upper 75 m (77%) could be attributed to a positive relationship with nutrients and negative relationships with prokaryotic abundance, chl a, and temperature; although, at most, 41% of the variability at each depth could be explained. Only 21% of the variability in the GII rRNA signal could be explained by the variables included in the model. Leucine incorporation rates and, to a lesser extent, prokaryote abundance, were highly correlated with chl a and POC levels. Only weak relationships were observed between prokaryote abundance or leucine incorporation and increases in temperature. The results of linear regressions between prokaryote abundance and leucine incorporation suggest that top-down ecological controls may regulate prokaryotic assemblages in this dynamic coastal system. The distribution of the 2 archaeal groups suggested that they responded independently to environmental conditions, are physiologically different, and likely participate in different environmental processes.