Phycology

Biochar-based catalysts have emerged as sustainable alternatives for biodiesel production, achieving high yields (up to 99%) from various feedstocks. This study aimed to utilize Spirulina-derived biochar as a bifunctional green catalyst for biodiesel synthesis from waste cooking oil (WCO) through transesterification and assess its green performance metrics. Biochar synthesized by carbonization (324 °C) was modified with calcium and sulfuric acid, featuring dual acid-base sites. Energy dispersive spectra revealed impregnation of calcium (11.11%) compared to the raw biomass (2.34%), followed by peaks of methoxy group and methylene group, and with methylene and β-carbonyl protons shown by nuclear magnetic spectroscopy. Thus, the biochar catalyst tested on WCO achieved a 93.27% yield under optimized conditions (65 °C, 1:15 methanol-to-oil ratio, 3% catalyst, 3.5 h) via central composite design. Catalyst reusability was maintained over four cycles with an average biodiesel yield (90%). Further, green metrics validate their eco-friendliness with a single-cycle reaction mass efficiency (RME) of 60.8%. When the initial catalyst mass is amortized over four cycles, the cumulative biodiesel yield per initial catalyst input reaches the equivalent of 243% of a single-batch theoretical yield (catalyst productivity = 3.12 g FAME/g catalyst). E-Factor at 0.67 (reduced to 0.17) and mass intensity at 1.68 (down to 0.42), contrasting with business-as-usual scenarios such as sulfuric acid catalysis (RME 70.0%, E-Factor 0.25) using 8.85 g H₂SO₄ vs. ~5 g H₂SO₄/kg biochar. Our results demonstrate that bio-based catalysts minimize non-benign inputs, supporting a circular economy from algal waste.

The Albufera of Valencia is a shallow, hypertrophic Mediterranean coastal lagoon. Since the 1970s, the lagoon has undergone substantial ecological deterioration, marked by the decline of macrophyte beds and the predominance of phytoplankton. The objective of this study was to monitor key water quality variables over a 10-year period (2015–2025) to assess the persistence of eutrophication and the current ecological status of the lagoon. For this purpose, a remote sensing approach was applied using the Sentinel-2 constellation, complemented by newly developed algorithms specifically calibrated with ten years of in situ field data (2016–2025). This approach was employed to estimate variables such as the chlorophyll-a (Chl-a) concentration as an indicator of phytoplankton biomass, suspended solids (S.S.), and Secchi disk depth (Z_SD). An analysis of temporal trends from 2017 to 2025 revealed a progressive system deterioration. The concentrations of both chlorophyll-a and suspended solids exhibited a statistically significant increasing trend (p < 0.01). Moreover, in line with these findings, water transparency (Z_SD) decreased significantly (p < 0.001). Thus, there has been a progressive deterioration in the trophic status and ecological quality of the lagoon over the last decade, despite prior management interventions. The results from this research highlight the need to implement more effective conservation strategies, such as regulating nutrient inputs and increasing the water renewal time in the lagoon.

Microalgae are emerging as a key biological platform for the production of important metabolites, environmental monitoring, and water treatment. However, despite their significant potential for a variety of industrial applications, several challenges associated with the efficiency of their cultivation hinder their widespread use. Here, focus was placed on the freshwater organism, Micractinium inermum strain EE-M2, to study the growth and accumulation of pigments, proteins, lipids, and starch under various strategies of increased inorganic carbon supply and ammonium nutrition conditions. NaOH and NaHCO₃ were tested as pH control agents. Combinations of constant sparging with atmospheric air enriched with CO₂ (finally 2.0% CO₂, v/v) and NaHCO₃ addition showed a slight increase in algal biomass productivity, but the metabolic profiles were indistinguishable from those obtained with pH regulation using NaOH. Decreasing the CO₂ concentration from 2.0% to 0.5% significantly reduced the final biomass yield and productivity of this strain (in a batch process). Also, the present study showed the feasibility of continuous cultivation of M. inermum to produce marketable biomass and metabolites. Under two cultivation strategies, batch and continuous, the alga effectively accumulated pigments (up to 2.7% of dry weight), proteins (up to 37.3%), lipids (up to 23.3%), and starch (up to 22.5%), indicating its biotechnological value. Overall, the obtained results demonstrate that M. inermum strain EE-M2 is a robust and fast-growing microalgal strain suitable for both laboratory and industrial cultivation.