Impact of off-bottom seaweed cultivation on turbulent variation in the hydrodynamic environment: A flume experiment study with mimic and natural Saccharina latissima thalli.

The seaweed industry is growing worldwide to meet future resource needs in terms of food and fuel. In the meantime, the impact of expanding off-bottom seaweed cultivation on its environment is unclear. For example, it remains poorly understood how off-bottom seaweeds affect the local hydrodynamic environment, especially concerning turbulence that is more important for nutrient transport and availability than the mean flow velocity. Here, we carried out well-controlled flume experiments with mimic seaweed thalli, which are available, controllable, and stable, to investigate the impact of off-bottom seaweed canopies on whole-depth flow velocities in terms of both mean flow and turbulence velocity profiles. A careful comparison of behavior in the flow between natural and mimic seaweed thalli was made before these experiments. The results show that the floating seaweed thalli generate a surface boundary layer and have a profound impact on the velocity structure in the bottom boundary layer. More importantly, the generation, growth and dissipation of turbulence in the seaweed thalli area deeply affect the downstream distribution of near-bed turbulent strength and associated bed shear stress. Ignoring this turbulent variation would cause inaccurate predictions of morphological changes of the seabed. Our findings suggest that expanding the seaweed cultivation area may cause high risks of bed degradation and low diffusion in the downstream cultivation area. These findings provide novel insights into the environmental influence of off-bottom seaweed cultivation, with important implications for optimizing management strategies to promote seaweed productivity while minimizing seabed destabilization.

Phosphate and Nitrate Uptake Dynamics in Palmaria palmata (Rhodophyceae): Ecological and Physiological Aspects of Nutrient Availability.

Uptake dynamics of dissolved inorganic phosphate (DIP) and dissolved inorganic nitrate (DIN) in young Palmaria palmata (n = 49), cultivated in a range of DIP concentrations (0.0-6.0 µmol · L-1 ) and nonlimiting DIN concentration (50 µmol · L-1 ) under fully controlled laboratory conditions, were quantified in a 'pulse-and-chase' approach over 5 weeks. Two different uptake rates were specified: (1) surge uptake (VS ) after starvation and (2) maintenance uptake with filled nutrient pools (VM ). VS for DIP of 1.57 ± 0.29 µmol · cm-2  · d-1 and DIN of 15.6 ± 4.3 µmol · cm-2  · d-1 , as well as VM for DIP of 0.57 ± 0.22 µmol · cm-2  · d-1 and DIN of 5.6 ± 2.1 µmol · cm-2  · d-1 were calculated. In addition, an absolute size of the internal storage capacity (ISC) for DIP of 22 µmol · cm2 and DIN of 222 µmol · cm2 was determined. A DIP-to-DIN uptake ratio of 1:10 under VM showed a weekly rhythmic uptake pattern, highlighted by a high correlation between DIP and DIN uptake (R = 0.943). VS for DIN did not occur under DIP depletion, but uptake rates increased with increasing DIP availability. Hence, DIP availability limited access to DIN, which was also reflected by total dissolvable protein concentrations in sporophytes, which ranged from 10.2 ± 2.5% to 24.6 ± 8.0% dry weight depending on DIP availability. Similarly, total dissolvable carbohydrate concentration ranged from 22.1 ± 3.6% to 54.3 ± 12.3% dry weight. The data presented in this study open further insight into ecological and physiological aspects of nutrient availability in P. palmata and allow for an optimization in cultivation.

Uptake kinetics and storage capacity of dissolved inorganic phosphorus and corresponding dissolved inorganic nitrate uptake in Saccharina latissima and Laminaria digitata (Phaeophyceae).

Uptake rates of dissolved inorganic phosphorus and dissolved inorganic nitrogen under unsaturated and saturated conditions were studied in young sporophytes of the seaweeds Saccharina latissima and Laminaria digitata (Phaeophyceae) using a "pulse-and-chase" assay under fully controlled laboratory conditions. In a subsequent second "pulse-and-chase" assay, internal storage capacity (ISC) was calculated based on VM and the parameter for photosynthetic efficiency Fv /Fm . Sporophytes of S. latissima showed a VS of 0.80 ± 0.03 µmol · cm-2  · d-1 and a VM of 0.30 ± 0.09 µmol · cm-2  · d-1 for dissolved inorganic phosphate (DIP), whereas VS for DIN was 11.26 ± 0.56 µmol · cm-2  · d-1 and VM was 3.94 ± 0.67 µmol · cm-2  · d-1 . In L. digitata, uptake kinetics for DIP and DIN were substantially lower: VS for DIP did not exceed 0.38 ± 0.03 µmol · cm-2  · d-1 while VM for DIP was 0.22 ± 0.01 µmol · cm-2  · d-1 . VS for DIN was 3.92 ± 0.08 µmol · cm-2  · d-1 and the VM for DIN was 1.81 ± 0.38 µmol · cm-2  · d-1 . Accordingly, S. latissima exhibited a larger ISC for DIP (27 µmol · cm-2 ) than L. digitata (10 µmol · cm-2 ), and was able to maintain high growth rates for a longer period under limiting DIP conditions. Our standardized data add to the physiological understanding of S. latissima and L. digitata, thus helping to identify potential locations for their cultivation. This could further contribute to the development and modification of applications in a bio-based economy, for example, in evaluating the potential for bioremediation in integrated multitrophic aquacultures that produce biomass simultaneously for use in the food, feed, and energy industries.

Elevated CO2 and phosphate limitation favor Micromonas pusilla through stimulated growth and reduced viral impact.

Growth and viral infection of the marine picoeukaryote Micromonas pusilla was studied under a future-ocean scenario of elevated partial CO2 (pCO2; 750 µatm versus the present-day 370 µatm) and simultaneous limitation of phosphorus (P). Independent of the pCO2 level, the ratios of M. pusilla cellular carbon (C) to nitrogen (N), C:P and N:P, increased with increasing P stress. Furthermore, in the P-limited chemostats at growth rates of 0.32 and 0.97 of the maximum growth rate (µmax), the supply of elevated pCO2 led to an additional rise in cellular C:N and C:P ratios, as well as a 1.4-fold increase in M. pusilla abundance. Viral lysis was not affected by pCO2, but P limitation led to a 150% prolongation of the latent period (6 to 12 h) and an 80% reduction in viral burst sizes (63 viruses per cell) compared to P-replete conditions (4 to 8 h latent period and burst size of 320). Growth at 0.32 µmax further prolonged the latent period by another 150% (12 to 18 h). Thus, enhanced P stress due to climate change-induced strengthened vertical stratification can be expected to lead to reduced and delayed virus production in picoeukaryotes. This effect is tempered, but likely not counteracted, by the increase in cell abundance under elevated pCO2. Although the influence of potential P-limitation-relieving factors, such as the uptake of organic P and P utilization during infection, is unclear, our current results suggest that when P limitation prevails in future oceans, picoeukaryotes and grazing will be favored over larger-sized phytoplankton and viral lysis, with increased matter and nutrient flow to higher trophic levels.

MORPHOLOGICAL AND PHYSIOLOGICAL EFFECTS IN PROBOSCIA ALATA (BACILLARIOPHYCEAE) GROWN UNDER DIFFERENT LIGHT AND CO2 CONDITIONS OF THE MODERN SOUTHERN OCEAN(1).

The combined effects of different light and aqueous CO2 conditions were assessed for the Southern Ocean diatom Proboscia alata (Brightwell) Sundström in laboratory experiments. Selected culture conditions (light and CO2(aq) ) were representative for the natural ranges in the modern Southern Ocean. Light conditions were 40 (low) and 240 (high) µmol photons · m(-2) · s(-1) . The three CO2(aq) conditions ranged from 8 to 34 µmol · kg(-1) CO2(aq) (equivalent to a pCO2 from 137 to 598 µatm, respectively). Clear morphological changes were induced by these different CO2(aq) conditions. Cells in low [CO2(aq) ] formed spirals, while many cells in high [CO2(aq) ] disintegrated. Cell size and volume were significantly affected by the different CO2(aq) concentrations. Increasing CO2(aq) concentrations led to an increase in particulate organic carbon concentrations per cell in the high light cultures, with exactly the opposite happening in the low light cultures. However, other parameters measured were not influenced by the range of CO2(aq) treatments. This included growth rates, chlorophyll a concentration and photosynthetic yield (FV /FM ). Different light treatments had a large effect on nutrient uptake. High light conditions caused an increased nutrient uptake rate compared to cells grown in low light conditions. Light and CO2 conditions co-determined in various ways the response of P. alata to changing environmental conditions. Overall P. alata appeared to be well adapted to the natural variability in light availability and CO2(aq) concentration of the modern Southern Ocean. Nevertheless, our results showed that P. alata is susceptible to future changes in inorganic carbon concentrations in the Southern Ocean.