Species-specific variation in the phosphorus nutritional sources by microphytoplankton in a Mediterranean estuary

Abstract

We investigated the species-specific phosphorus (P) nutritional strategies in the microphytoplankton community in the Mahon estuary (Minorca, Western Mediterranean) in 2011, under two contrasting hydrographic scenarios. Estuarine flow, nutrient concentrations, phytoplankton community composition, and enzyme-labeled fluorescence (ELF) were measured in June and October, corresponding to the beginning and the end of summer. Dissolved inorganic nitrogen (DIN) and inorganic phosphate (P i ) exhibited enhanced concentrations in the inner estuary where N:P molar ratios suggested P-limitation in both surveys. P i was low and variable (0.09 ± 0.02 µmol L −1 in June and 0.06 ± 0.02 µmol L −1 in October), whereas organic phosphorus remained a more readily available P source. Even though ambient P i concentrations were slightly higher on June, when the microphytoplankton assemblage was dominated by dinoflagellates, the percentage of ELF labeled cells was notably higher (65 ± 9% of total cells) than in October (12 ± 10%), when the presence of diatoms characterized the microphytoplankton community. Alkaline phosphatase activity (AP) was mainly expressed by dinoflagellate taxa, whereas diatoms only displayed significant AP in the inner estuary during the June survey. A P-addition bioassay in which response of AP to P i enrichment was evaluated showed marked reduction in AP with increasing P i . However, some dinoflagellate species maintained AP even when P i was supplied in excess. We suggest that in the case of some dinoflagellate species AP is not as tightly controlled by ambient P i as previously believed. AP activity in these species could indicate selective use of organic phosphorus, or slow metabolic response to changes in P forms, rather than physiological stress to low P i availability. We emphasize the importance of identifying the species-specific phosphorus requirements and their capability for P assimilation in order to understand the contribution of microphytoplankton to the whole community response to biogeochemical perturbations.

Episodic Biogeochemical Variability in a Low-Flow Mediterranean Estuary

“…Because of the persistently high nitrate and silicate concentrations at St1, the inner sub-basin was Plimited (N:P >16) for most of the time. This is consistent with results based on alkaline phosphatase activity obtained by Martínez-Soto et al (2015) at this site. N limitation was marginal and some episodic increases in P, while reducing P limitation drove the system towards Si deficiency.…”

“…The wastewater effluents of these urban areas are collected through a sewer that discharges offshore, yet, effluents from some buildings in the port area may seep to the estuary representing a minor, but continuous, eutrophication source. These discharges enrich the water at the head of the estuary with nitrate and less so with phosphate (Martínez-Soto et al 2015). Storm water from impervious urban surfaces flows directly into the bay following the steep coastal topography.…”

“…Excess supply of these nutrients represents a fundamental difference in the biogeochemical functioning of the estuary, which is dependent on P availability, with respect to open coastal waters, where inorganic nitrogen is scarce. Phosphorus deficiency does not represent a limiting factor for overall phytoplankton growth, but it triggers the use of alternative P sources resulting in variations in the competitive advantages of the different phytoplankton species (Martínez-Soto et al 2015). Indeed, while Mahon is a net exporter of inorganic nitrogen, it either scavenges P from nearby coastal waters or relies in the P supply from human activities either in organic or inorganic forms.…”

High-Content Screening of Plankton Alkaline Phosphatase Activity in Microfluidics

“…[15][16][17][18] Alkaline phosphatase activity (APA) is mainly measured in bulk or as a presence-absence of enzyme activity using soluble substrates and enzyme labelled fluorescence (ELF) methods. 19,20 Since the pioneer study of the ELF substrate by Nedoma et al,no simple method effectively links the kinetics of activity to a single living cell. 21 The lack of a suitable method is in part due to the presence of motile cells in natural sample which are difficult to isolate over -long periods of time.…”

The global explosion of eukaryotic algae: the potential role of phosphorus?

“…Alkaline phosphatase activity was higher in the 0.2 μM P conditions signifying a greater level of nutritional stress. APA also varied between the two species and was higher for T. suecia compared to S. salina, suggesting T. suecia was experiencing a higher level of P stress [72].…”