Effect of sunlight on prokaryotic organic carbon uptake and dynamics of pigments relevant to photoheterotrophy in the Adriatic Sea

Abstract

Marine photoheterotrophic microorganisms are capable of using light to meet their energy requirements and organic compounds as both carbon and energy sources. We still have little knowledge of the extent to which stimulation of these microorganisms by light could affect the estimates of organic carbon uptake. We evaluated the light and dark prokaryotic organic carbon uptake ( 3 H-leucine) rates in a grid of stations covering the whole Adriatic Sea during winter and late summer. Light-exposed (photosynthetically active radiation and ultraviolet radiation [UVR]) surface samples were either unaffected, photostimulated or photoinhibited without any clear geographical or seasonal pattern. Light-enhanced leucine uptake occurred only in 23% of assays, suggesting that photoheterotrophy is not a major metabolic strategy and/or it is often counterbalanced by negative effects caused by UVR. Concentrations of bacteriochlorophyll a, zeaxanthin and divinyl chlorophyll a were measured in order to relate 3 H-leucine uptake to the distribution of aerobic anoxygenic photo trophs (AAPs), total Cyanobacteria and Prochlorococcus populations, respectively, together with direct estimates of total prokaryotes and Synechococcus abundance. No relationship between light-enhanced leucine uptake and presence of picocyanobacteria or AAPs was evident. Divinyl chlorophyll a concentration was below the limit of detection in February, whereas in late summer the highest values were found around 50 m depth in the central and southern basins. In contrast, bacteriochlorophyll a concentration was correlated to total prokaryote abundance and dissolved organic carbon. Since locally leucine uptake in the light was markedly different from dark controls, in situ light incubations should be more appropriate than dark incubations when carbon budget calculations are intended.KEY WORDS: Prokaryotic production · Bacteriochlorophyll a · Divinyl chlorophyll a · Zeaxanthin · Light Resale or republication not permitted without written consent of the publisher

Picoplankton Distribution and Activity in the Deep Waters of the Southern Adriatic Sea

“…In this research, fluorescence, as a proxy of chlorophyll, nitrates, nitrites, phosphates, and total dissolved phosphates, influenced the abundance of AAPs. Besides, the correlation found between bacteria and AAPs, which has also been reported by Celussi et al [11], could be explained by the fact that both AAPs and heterotrophic bacteria rely on labile DOC, part of which is released by phytoplankton. Secondly, AAPs and phytoplankton have a similar dependence on light [90], and this relationship may reflect the same dependence on limiting nutrients, such as phosphorus or nitrogen [15].…”

“…Šantić et al [12] investigated AAPs abundances along the eastern Adriatic, in coastal and transitional waters, and found that their proportion in total prokaryotes was 7.3% (±4.3%). Celussi et al [11] pointed out that the concentration of bacteriochlorophyll a, the main pigment of AAPs, could be converted into abundance. Therefore, AAPs might represent up to 10% of total prokaryotes in the open Adriatic Sea.…”

“…In this study, we used data collected during the two ESAW cruises (Evolution and spreading of the Southern Adriatic Waters), supported by the EUROFLEETS2 program. The first one was conducted in December 2015 (10)(11)(12)(13)(14)(15). The second one was conducted in April 2016 (5)(6)(7)(8)(9)(10), when the hydrographic and environmental conditions were supposed to be changed after the wintertime occurrence of vertical convection, and consequent dense water formation.…”

Uptake‐release dynamics of the inorganic and organic carbon pool mediated by planktonic prokaryotes in the deep Mediterranean Sea

“…The Kruskal‐Wallis (K‐W) ANOVA was utilized to detect significant differences in prokaryotic activities among the western Mediterranean, the eastern Mediterranean and the Adriatic Sea. The latter was separated from the eastern Mediterranean due to its high productivity (Fonda Umani, ) and DOC concentration at depth (Celussi et al ., ). The median test post hoc was applied if K‐W showed significant statistics.…”

Aerobic anoxygenic phototrophs of the Adriatic Sea

“…The first study of AAPs in the Adriatic Sea referred to their pigment BChl a along a latitudinal transect covering the entire Adriatic Sea. By converting pigment concentration to abundance, the authors assumed that AAPs could represent up to 10% of the total prokaryotes (Celussi et al, 2015). Based on the abundance of cells using infrared epifluorescence microscopy, AAPs were described for the first time on a spatial scale in coastal and estuarine (transitional) waters along the eastern Adriatic coast in summer (Šantić et al, 2017).…”