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Light dependence of phosphorus uptake by microorganisms in the subtropical North and South Pacific Ocean

Duhamel, Solange; Björkman, Karin M.; Karl, David M.

Light and dark phosphate (PO_4^3−) uptake rates were investigated in the North Pacific Subtropical Gyre (NPSG) and along a coastal to open ocean transect in the South Pacific Ocean. PO_4^3− uptake rates were consistently higher when incubated in the light, but the ratio of uptake in the light and dark (L:D) decreased with depth. In the NPSG, the L:D ratio of euphotic-layer integrated PO_4^3− uptake was 1.58 ± 0.08 (±SE, n = 15 profiles), and the average L:D ratio was 1.60 ± 0.45 (±SE, n = 42) between 5 and 45 m and 1.25 ± 0.70 (±SE, n = 28) between 150 and 175 m. The L:D ratio was higher for the pigmented plankton-enriched size fractions (0.6 to 2 µm and >2 µm), but results were difficult to interpret at the oligotrophic stations where non-pigmented and pigmented plankton cell sizes overlapped. Group-specific measurements obtained using flow cytometric cell sorting demonstrated that Prochlorococcus PO_4^3− uptake rates were higher when the samples were incubated under ambient light. Adenosine-5′-triphosphate (ATP) uptake by Prochlorococcus was also higher in the light for the uptake of both the terminal PO_4^3− group ([γ‑^33P]ATP) and adenine moiety ([2,8-^3H]ATP). This could be the result of secondary uptake of PO_4^3− and/or adenine after ATP cleavage by non-pigmented picoplankton. There was no significant difference in P-assimilation by non-pigmented picoplankton between light and dark incubated samples. Light dependence of phytoplankton PO_4^3− uptake could thus influence the functioning of the microbial loop and the flows of matter and energy in marine environments by creating temporal patterns of resource utilization.

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Aquatic Microbial Ecology

More About This Work

Academic Units
Lamont-Doherty Earth Observatory
Published Here
May 10, 2013
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