Energetics and growth kinetics of a deep Prochlorococcus spp. population in the Arabian Sea

During the US JGOFS process studies in the Arabian Sea (1995), secondary fl uorescence maxima (SFM) were observed frequently at the oxic-anoxic interfa ce at the extreme base of the euphotic zone. These secondary peaks were mos t prominent during the early NE monsoon in the central oligotrophic portion of the Arabian Sea, although they were spatially and temporally variable. Based on high performance liquid chromatography (HPLC) and flow cytometry a nalyses, SFM were determined to be populated almost exclusively by the mari ne cyanobacterium Prochlorococcus spp. While SFM were about half the magnit ude of primary fluorescence peaks, chlorophyll a biomass was typically an o rder of magnitude less than at the primary maxima (although total chlorophy ll (a + b) differed only by a factor of two). Photosynthesis versus irradia nce response curves revealed an efficient population adapted to extremely l ow light (similar to 0.02-0.05% surface irradiance) largely through increas ed light absorption capabilities. A theoretical spectral irradiance absorpt ion efficiency model based on available spectral irradiance, individual cel l properties, and bulk particulate spectral absorption also supports a well -adapted low-light population. Deck-incubated C-14 uptake as well as diluti on growth experiments revealed instantaneous growth rates on the order of m u = 0.01 d(-1). However, additional in situ observations suggest SFM popula tions may be more dynamic than the growth rates estimates from shipboard bo ttle incubations predict. We advance four hypotheses for the regulation of SFM populations including: (1) reduced loss rates, (2) discontinuous enviro nmental conditions. (3) enhanced sub-oxic growth, and (4) physical mechanis ms. (C) 1999 Elsevier Science Ltd. All rights reserved.