Availability of phosphate for phytoplankton and bacteria and of glucose for bacteria at different <i>p</i>CO<sub>2</sub> levels in a mesocosm study

Abstract

Abstract. Availability of phosphate for phytoplankton and bacteria and of glucose for bacteria at different pCO 2 levels were studied in a mesocosm experiment (PeECE III). Using nutrient-depleted SW Norwegian fjord waters, three different levels of pCO 2 (350 µatm: 1×CO 2 ; 700 µatm: 2×CO 2 ; 1050 µatm: 3×CO 2 ) were set up, and nitrate and phosphate were added at the start of the experiment in order to induce a phytoplankton bloom. Despite similar responses of total particulate P concentration and phosphate turnover time at the three different pCO 2 levels, the size distribution of particulate P and 33 PO 4 uptake suggested that phosphate transferred to the >10 µm fraction was greater in the 3×CO 2 mesocosm during the first 6-10 days when phosphate concentration was high. During the period of phosphate depletion (after Day 12), specific phosphate affinity and specific alkaline phosphatase activity (APA) suggested a P-deficiency (i.e. suboptimal phosphate supply) rather than a P-limitation for the phytoplankton and bacterial community at the three different pCO 2 levels. Specific phosphate affinity and specific APA tended to be higher in the 3×CO 2 than in the 2×CO 2 and 1×CO 2 mesocosms during the phosphate depletion period, although no statistical differences were found. Glucose turnover time was correlated significantly and negatively with bacterial abundance and production but not with the bulk DOC concentration. This suggests that even though constituting a small fraction of the bulk DOC, glucose was an important component of labile DOC for bacteria. SpecificCorrespondence to: T. Tanaka (tsuneo.tanaka@univmed.fr) glucose affinity of bacteria behaved similarly at the three different pCO 2 levels with measured specific glucose affinities being consistently much lower than the theoretical maximum predicted from the diffusion-limited model. This suggests that bacterial growth was not severely limited by the glucose availability. Hence, it seems that the lower availability of inorganic nutrients after the phytoplankton bloom reduced the bacterial capacity to consume labile DOC in the upper mixed layer of the stratified mesocosms.

Interactive network configuration maintains bacterioplankton community structure under elevated CO<sub>2</sub> in a eutrophic coastal mesocosm experiment

“…Compared to the effects of the phytoplankton bloom, ocean acidification did not strongly influence the bacterioplankton community structure. The results indicate that bacterial abundance and community structure at different taxonomic levels were generally similar between the HC and LC treatments at the different diatom bloom stages, in line with many previous ocean acidification mesocosm bacterioplankton community studies (Tanaka et al, 2008;Wang et al, 2016;Zhang et al, 2013;Ray et al, 2012Ray et al, , 2013Baltar et al, 2015). Differences in bacterioplankton community diversity between the HC and LC treatments were also not remarkable.…”

“…Although effects of elevated CO 2 on bacterioplankton communities have been reported Tanaka et al, 2008;Wang et al, 2016;Zhang et al, 2013;Ray et al, 2012;Roy et al, 2013;Baltar et al, 2015;reviewed in Hutchins and Fu, 2017), how marine bacteria communities react to the occurrence of elevated CO 2 in eutrophic seawater is still uncertain. This mesocosm study comprehensively investigated the effects of elevated CO 2 on bacterioplankton community structure and networks using Illumina sequencing and ecological network analysis in the context of eutrophication.…”

Mesocosm CO<sub>2</sub> perturbation studies: from organism to community level

“…It can not be ruled out that the pervasive response of the plankton community to the nutrient addition has masked possible effects caused by the CO 2 perturbations. In fact, no significant differences between CO 2 treatments were observed for PeECE II+III -concentrations of POM and DOM (Engel et al, 2004;Rochelle-Newall et al, 2004;Riebesell et al, 2007) PeECE III -phytoplankton composition and cell cycle during bloom development -inorganic nutrient utilization, nutrient stoichiometry Bellerby et al, 2007; and nutrient turnover (Tanaka et al, 2008) -biogenic calcification -bacterial abundance, diversity of attached bacteria, 14 C-leucine based bacterial production, bacteriaphytoplankton coupling -micro-zooplankton grazing -calcite loss due to microzooplankton grazing Fig. 2.…”

“…The observed biological responses were largely dominated by the nutrient pulses added at the start of the experiment. As described by Tanaka et al (2008) for the PeECE III experiment, five phases can be distinguished during the course of the plankton development ( Fig. 4): phase I -the initial period when all nutrients were replete, lasting until silicate was the first nutrient to become exhausted (day 6); phase II -extending until phosphate depletion (day 9); phase III -terminated by levelling off of nitrate drawdown (day 12); phase IVcharacterized by more or less stable concentrations of all inorganic nutrients close to exhaustion levels with limited nutrient regeneration (day 20), phase V -marked by increased nutrient turnover.…”

“…4. Development of the plankton community during the experiment: Based on nutrient availability and turnover, 5 phases can be distinguished: phase I -start of the experiment until the onset of silicate limitation (day 6), all nutrients replete; phase II -nitrate and phosphate replete, terminated by phosphate limitation (day 9); phase III -ends with onset of nitrate limitation (day 12); phase IVcharacterized by more or less stable concentrations of all inorganic nutrients close to exhaustion levels with limited nutrient regeneration (day 20), phase V -marked by increased nutrient turnover (see Tanaka et al, 2008).…”