Phytoplankton dynamics in the Mar Menor, a Mediterranean coastal lagoon strongly impacted by eutrophication.

The Mar Menor hypersaline coastal lagoon has suffered serious degradation in the last three decades attributable to nutrient pollution. In 2015, the lagoon experienced an intensive bloom of cyanobacteria that triggered a drastic change of its ecosystem. Our analyses indicate that phytoplankton in 2016-2021 did not present a seasonal variability pattern; the community was mainly dominated by diatoms and punctually reached abundance peaks above 107 cell L-1 along with chlorophyll a concentrations exceeding 20 µg L-1. The predominant diatom genera during these blooms were different as well as the nutrient conditions under which they were produced. These high diatom abundances are unprecedented in the lagoon; in fact, our data indicate that the taxonomic composition, time variation patterns and cell abundance of phytoplankton in 2016-2021 differ notably in comparison to the data published before 2015. Consequently, our results support the finding that the trophic status of the lagoon has changed profoundly.

Analyzing environmental factors that favor the growth of the invasive brown macroalga Rugulopteryx okamurae (Ochrophyta): The probable role of the nutrient excess.

Time series of temperature, salinity and nutrients in the Strait of Gibraltar (SoG) were researched to analyze which factors explain the invasive success of Rugulopteryx okamurare, which has colonized wide coastal areas at the Spanish and Moroccan coasts since 2016. Temperature and salinity were higher in the SoG compared to its native habitat, implying that the alga is active during the whole seasonal cycle and grows optimally at the high salinities occurring in the SoG. Nitrate removal experiments indicate that the alga is able to linearly increase its N uptake rates following boost in nitrate concentration. Furthermore, R. okamurae N content ranged from 1.4% to 4.5% suggesting that this species has high N storage capacity potentially usable when the external N concentration decreases. These physiological characteristics would explain sharp growth of the alga in the SoG where high N concentrations are registered occasionally.

Role of small-sized phytoplankton in triggering an ecosystem disruptive algal bloom in a Mediterranean hypersaline coastal lagoon.

Monthly samplings carried out in 2016-2019 and satellite color images from 2002 to 2019 have been combined to determine the onset and causative species of the ecosystem disruptive algal bloom (EDAB) that affects the Mar Menor coastal lagoon (Western Mediterranean Sea) since 2015. Substantial changes in satellite spectral reflectance attributable to increasing abundance of Synechococcus were registered in 2014. Furthermore, cell abundances of this species in 2016 were the largest ever obtained in the lagoon (6 106 cells mL-1), with values similar to those reported for other Mediterranean hypertrophic estuaries and coastal lagoons. These results suggest that the early changes leading to the EDAB started in 2014 and that Synechococcus played a relevant role in its development. Moreover, diatom and dinoflagellate abundances changed substantially in 2016-2019, ranging from 102 to more than 104 cells mL-1. Some of these changes were linked to flood, suggesting that EDAB has modified substantially the homeostatic capacity of the lagoon.

Early life trophodynamic influence on daily growth patterns of the Alboran Sea sardine (Sardina pilchardus) from two distinct nursery habitats (bays of málaga and almería) in the western Mediterranean Sea.

The Alborán Sea sardine (Sardina pilchardus) is the most abundant small pelagic fish in the Mediterranean Sea. Along the northern coast of the Alborán Sea, Málaga and Almería Bay are two important nursery grounds with contrasting oceanographic characteristics. Post-larval stages of S. pilchardus were collected during the late autumn and late winter along with the mesozooplankton community. We examined the influence of feeding behavior on larval growth using environmental variables, daily growth metrics, otolith biometry, stable isotope analysis and trophodynamics. Trophic positions were calculated using mesoozooplankton as potential prey for S. pilchardus. During both seasons, we observed faster growth in larger otoliths that also corresponded to higher trophic positions. Our results indicate that the isotopic signature of larvae is highly influenced by the mesozooplankton community structure. Moreover, the trophic variables showed a significant linear relationship with the estimated ages for all larvae indicating dietary changes throughout ontogenic development.

Shifts in the protist community associated with an anticyclonic gyre in the Alboran Sea (Mediterranean Sea).

The diversity of protists was researched in the Alboran Sea (SW Mediterranean Sea) by means of high-throughput sequencing technologies based on the amplification of the V9 region of 18S rRNA. Samples were collected at different depths in seven stations following an environmental gradient from a coastal upwelling zone to the core of an oligotrophic anticyclonic gyre (AG). Sampling was performed during summer, when the water column was stratified. The superphyla Alveolata, Stramenopila and Rhizaria accounted for 84% of the total operational taxonomic units (OTUs). The most diverse groups were Dinophyceae (21% of OTUs), Marine Alveolates-II (MALV-II; 20%), Ciliophora (9%) and MALV-I (6%). In terms of read abundance, the predominant groups were Dinophyceae (29%), Bacillariophyta (14%), MALV-II (11%) and Ciliophora (11%). Samples were clustered into three groups according to the sampling depth and position. The shallow community in coastal stations presented distinguishable patterns of diatoms and ciliates compared with AG stations. These results indicate that there was a strong horizontal coupling between phytoplankton and ciliate communities. Abundance of Radiolaria and Syndiniales increased with depth. Our analyses demonstrate that the stratification disruption produced by the AG caused shifts in the trophic ecology of the plankton assemblages inducing a transition from bottom-up to top-down control.

Interactive Effect of UVR and Phosphorus on the Coastal Phytoplankton Community of the Western Mediterranean Sea: Unravelling Eco-Physiological Mechanisms.

Some of the most important effects of global change on coastal marine systems include increasing nutrient inputs and higher levels of ultraviolet radiation (UVR, 280-400 nm), which could affect primary producers, a key trophic link to the functioning of marine food webs. However, interactive effects of both factors on the phytoplankton community have not been assessed for the Mediterranean Sea. An in situ factorial experiment, with two levels of ultraviolet solar radiation (UVR+PAR vs. PAR) and nutrients (control vs. P-enriched), was performed to evaluate single and UVR×P effects on metabolic, enzymatic, stoichiometric and structural phytoplanktonic variables. While most phytoplankton variables were not affected by UVR, dissolved phosphatase (APAEX) and algal P content increased in the presence of UVR, which was interpreted as an acclimation mechanism of algae to oligotrophic marine waters. Synergistic UVR×P interactive effects were positive on photosynthetic variables (i.e., maximal electron transport rate, ETRmax), but negative on primary production and phytoplankton biomass because the pulse of P unmasked the inhibitory effect of UVR. This unmasking effect might be related to greater photodamage caused by an excess of electron flux after a P pulse (higher ETRmax) without an efficient release of carbon as the mechanism to dissipate the reducing power of photosynthetic electron transport.

Inorganic carbon acquisition in algal communities: are the laboratory data relevant to the natural ecosystems?

Most of the experimental work on the effects of ocean acidification on the photosynthesis of algae has been performed in the laboratory using monospecific cultures. It is frequently assumed that the information obtained from these cultures can be used to predict the acclimation response in the natural environment. CO(2) concentration is known to regulate the expression and functioning of the CCMs in the natural communities; however, ambient CO(2) can become quite variable in the marine ecosystems even in the short- to mid-term. We propose that the degree of saturation of the photosynthesis for a given algal community should be defined in relation to the particular characteristics of its habitat, and not only in relation to its taxonomic composition. The convenience of high CO(2) experiments to infer the degree of photosynthesis saturation by CO(2) in the natural algal communities under the present ocean conditions, as well as its trend in a coming future is discussed taking into account other factors such as the availability of light and nutrients, and seasonality.

EFFECT OF CARBONIC ANHYDRASE INHIBITORS ON THE INORGANIC CARBON UPTAKE BY PHYTOPLANKTON NATURAL ASSEMBLAGES(1).

The role of carbonic anhydrase (CA) in inorganic carbon acquisition (dissolved inorganic carbon, DIC) was examined in Alboran Sea phytoplankton assemblages. The study area was characterized by a relatively high variability in nutrient concentration and in abundance and taxonomic composition of phytoplankton. Therefore, the relationship between environmental variability and capacity for using HCO3 (-) via external CA (eCA) was examined. Acetazolamide (AZ, an inhibitor of eCA) inhibited the primary productivity (PP) in 50% of the samples, with inhibition percentages ranging from 13% to 60%. The AZ effect was more prominent in the samples that exhibited PP >1 mg C · m(-3) · h(-1) , indicating that the contribution of eCA to the DIC photosynthetic flux was irrelevant at low PP. The inhibition of primary productivity by AZ was significantly correlated to the abundance of diatoms. However, there was no a relationship between AZ effect and CO2 partial pressure (pCO2 ) or nutrient concentration, indicating that the variability in the PP percentage supported by eCA was mainly due to differences in taxonomic composition of the phytoplankton assemblages. Ethoxyzolamide (EZ, an inhibitor of both external and internal CA) affected 13 of 14 analyzed samples, with PP inhibition percentages varying from 50% to 95%. The effects of AZ and EZ were partially reversed by doubling DIC concentration. These results imply that CA activity (external and/or internal) was involved in inorganic carbon acquisition in most the samples. However, EZ effect was not correlated with pCO2 or taxonomic composition of the phytoplankton.

Diversity of inorganic carbon acquisition mechanisms by intact microbial mats of Microcoleus chthonoplastes (Cyanobacteriae, Oscillatoriaceae).

The dissolved inorganic carbon (DIC) acquisition mechanisms were researched in intact microbial mats dominated by the cyanobacteria Microcoleus chthonoplastes Thuret, by determining the effect on photosynthesis of different inhibitors. The microbial mats exhibited high affinity for DIC at alkaline pH, with K(m(DIC)) values similar to the ones described for pure cultures of cyanobacteria and algae in which carbon concentrating mechanisms have been researched. Besides, the photosynthesis was non-sensitive to pH changes within the range of 5.6-9.6, indicating that HCO(3)(-) was the main DIC source used for photosynthesis. The M. chthonoplastes mats featured external and internal carbonic anhydrase (CA) activity as measured in intact cells and cell extracts, respectively. Acetazolamide (AZ, which slowly enters the cell and then inhibits mainly the external CA) and ethoxyzolamide (EZ, which inhibits both external and internal CA) reduced significantly the oxygen evolution rates, demonstrating that the CA was implied in the DIC acquisition. Vanadate inhibited photosynthesis by 60% although its application, when CA being inhibited (i.e. after applying AZ + EZ), did not produce any additional effect. It could indicate that ATPase-dependent HCO(3)(-) use occurred and also that this putative mechanism was coupled with CA-like activity at the plasma membrane. The involvement of Na(+)-dependent HCO(3)(-) transporters in DIC acquisition was also inferred as monensin and 4-4'-diisothiocyanatostibilene-2,2'-disulfonate (DIDS) reduced photosynthesis by 70%. DIDS produced a strong inhibitory effect even after application of AZ + EZ + vanadate, indicating that this mechanism was not related to CA activity. The microbial mats become subject to very unfavourable conditions for Rubisco carboxylation at their natural habitats (e.g. external pH of 10.5 and O(2) concentration doubled with respect to saturation concentration); therefore, this putative diversity of DIC acquisition mechanisms could ensure their growth under these extreme conditions.

Evidence for a plasmalemma-based CO2 concentrating mechanism in Laminaria saccharina.

A kinetic analysis of the photosynthesis inhibition by buffers allowed quantification of some components of the carbon concentrating mechanism (CCM) of the brown macroalga Laminaria saccharina. The CCM was based on the presence of acid regions outside the plasma membrane that increased the CO(2) concentration available for photosynthesis by 10-20 times above that of the bulk medium at alkaline pH. Furthermore, the results suggested that the CCM is located mainly on the cell membrane and not in the chloroplast, as suggested for most macroalgae. The degree of dissipation of the acid regions by a buffer was related to the buffer anion concentration (B(-)), estimated from the titration of the buffer from bulk medium pH to a pH endpoint value close to the first pK (a) of the carbonic acid system. A kinetic model describing the relationship between inhibition of photosynthesis by a buffer and B(-) was developed suggesting that buffers act as competitive inhibitors with IC(50) (the concentration of the buffer anion which reduces the reaction velocity by half) of 5.0 mol m(-3). This model can be used to estimate the inhibitory effect of any buffer on the photosynthesis of L. saccharina. Nevertheless, some buffers tested showed a lower effect than that predicted from the hyperbolic model suggesting that their strength as inhibitors depended on: (1) the pK (a) in relation to the first pK (a) of the carbonic acid system and (2) its molecular weight (i.e. its mobility).

Limited acclimation of photosynthesis to blue light in the seaweed Gracilaria tenuistipitata.

Changes in photosynthetic capacity of the seaweed Gracilaria tenuistipitata Zhang et Xia acclimated to monochromatic blue light were studied. For this purpose, affinity for external inorganic carbon, light use efficiency, carbonic anhydrase (CA; EC 4.2.1.1) activity and content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were determined in thalli acclimated to 45 &mgr;mol m-2 s-1 of blue light. Thalli cultured in white light of the same photon fluence rate were used as a control. Lower maximal photosynthetic rates (i.e. at light and carbon saturation) were obtained in the thalli cultured in blue light. Apparently, this lower photosynthetic capacity was not due to differences in affinity and/or capacity for use of external dissolved inorganic carbon (DIC) since (1) CA activity did not change significantly and (2) similar values of photosynthetic conductance for DIC at alkaline pH were obtained (0.95 x 10-6 m s-1). In addition, the pool size of Rubisco was not modified by the blue light treatment since there were no significant differences in Rubisco content between white (12.14% of soluble proteins) and blue light (12.13% of soluble proteins) treatments. In contrast, Fv/Fm was increased by 11% and photosynthetic efficiency for oxygen production was reduced by 50% in blue light. This absence of correlation between quantum yields for maximum stable charge separation of photosystem II and oxygen evolution suggests that blue light promote changes in rates of photosynthetic electron flow.

Photosynthesis might be limited by light, not inorganic carbon availability, in three intertidal Gelidiales species.

• The interaction between incident photon fluence density (PFD) and dissolved inorganic carbon (DIC) availability on photosynthesis is reported for three species of Gelidiales (Rhodophyta) from the Canary Islands. • Photosynthetic O2 evolution in response to both DIC concentration and PFD, and external carbonic anhydrase activity were measured in thalli of Gelidium canariensis, Gelidium arbuscula and Pterocladiella capillacea maintained in a controlled-environment room. • No detectable external carbonic anhydrase activity, high sensitivity to alkaline pH and moderate values of photosynthetic conductance for DIC indicated that the three species had a low capacity for using the external pool of HCO3 - ; therefore photosynthetic rates were dependent on CO2 availability. The seawater concentration of DIC was insufficient to saturate photosynthesis at high PFD; photosynthesis vs PFD was not affected by DIC concentration at 0.07-0.04 mol O2 mol-1 photon. ANOVA revealed that incident PFD had a greater effect than DIC availability on photosynthesis rates in G. canariensis. • Photosynthesis is probably limited by incident light rather than DIC concentration despite the apparent low affinity for HCO3 - in G. canariensis, G. arbuscula and P. capillacea in their natural habitats.