Phosphorus from wastewater to crops: An alternative path involving microalgae

Phosphorus (P) is a non-renewable resource, a major plant nutrient that is essential for modern agriculture. Currently, global food and feed production depends on P extracted from finite phosphate rock reserves mainly confined to a small number of countries. P limitation and its potential socio-economic impact may well exceed the potential effects of fossil fuel scarcity.The efficiency of P usage today barely reaches 20%, with the remaining 80% ending up in wastewater or in surface waters as runoff from fields. When recovered from wastewater, either chemically or biologically, P is often present in a form that does not meet specifications for agricultural use. As an alternative, the potential of microalgae to accumulate large quantities of P can be a way to direct this resource back to crop plants. Algae can acquire and store P through luxury uptake, and the P enriched algal biomass can be used as bio-fertilizer.Technology of large-scale algae cultivation has made tremendous progress in the last decades, stimulated by perspectives of obtaining third generation biofuels without requiring arable land or fresh water. These new cultivation technologies can be used for solar-driven recycling of P and other nutrients from wastewater into algae-based bio-fertilizers.In this paper, we review the specifics of P uptake from nutrient-rich waste streams, paying special attention to luxury uptake by microalgal cells and the potential application of P-enriched algal biomass to fertilize crop soils

Appendix A. Partial derivatives with respect to k for the positive and negative branches of Eq. 13.

Partial derivatives with respect to k for the positive and negative branches of Eq. 13

Mean (standard error) food quality expressed in molar C∶N and C∶P ratios for stock cultures (chemostat), P-poor, P-sufficient and P- rich treatments used in the two experiments.

<p>Mean (standard error) food quality expressed in molar C∶N and C∶P ratios for stock cultures (chemostat), P-poor, P-sufficient and P- rich treatments used in the two experiments.</p

Mass-specific P-content in relation to specific growth rates of <i>D. magna</i>, <i>D. pulicaria</i> and <i>D. galeata x hyalina</i>, after four days (closed symbols) and at maturation (open symbols).

<p>Lines were fitted using ANCOVA with quadratic term for P-content.</p

Specific growth rates (GR) of 4-day-old <i>D. magna</i>, <i>D. pulicaria</i> and <i>D. galeata x hyalina</i> cultured on P-poor (C∶P 400; open symbols) and P-rich (C∶P 80, closed symbols) algal food at four food concentrations (0.08, 0.14, 0.2 and 0.3 mg C L⁻¹).

<p>Lines were fitted using the Monod model.</p

Relationships between A) specific growth rates and N∶P ratios, and B) specific growth rate and clutch size for <i>Daphnia magna</i> (circles), <i>D. pulicaria</i> (squares) and <i>D. galeata x hyalina</i> (triangles).

<p>Lines were fitted using ANCOVA.</p

Results of ANCOVA for the effects of <i>Daphnia</i> body P-content and age (4-day-old juveniles, mature adults) on specific growth rates (GR) of <i>D. magna, D. pulicaria</i> and <i>D. galeata x hyalina</i>.

<p>The explanatory variable “P – content” was linearized before analysis.</p

Functional response of Anodonta anatina feeding on a green alga and four strains of cyanobacteria, differing in shape, size, and toxicity

We studied the functional response of the freshwater unionid bivalve Anodonta anatina, feeding on five phytoplankton strains differing in food quality: the small green alga Scenedesmus obliquus, a toxic and a non-toxic strain of the filamentous cyanobacterium Planktothrix agardhii and a toxic and a non-toxic strain of the coccoid cyanobacterium Microcystis aeruginosa. On S. obliquus, A. anatina had a type II functional response with a maximum mass-specific ingestion rate (IRmax) of 5.24 mg C g DW−1 h−1 and a maximum mass-specific clearance rate (CRmax) of 492 (±38) ml g DW−1 h−1, the highest values for all the phytoplankton strains that were investigated. On toxic and non-toxic P. agardhii filaments, A. anatina also had a type II functional response, but IRmax and CRmax were considerably lower (IRmax 1.90 and 1.56 mg C g DW−1 h−1; CRmax 387 (±97) and 429 (±71) ml g DW−1 h−1, respectively) than on S. obliquus. Toxicity of P. agardhii had no effect on the filtration rate of the mussels. On the non-toxic M. aeruginosa (small coccoid cells), we also observed a type II functional response, although a type I functional response fitted almost as good to these data. For the colonial and toxic M. aeruginosa, a type I functional response fitted best to the data: IR increased linearly with food concentration and CR remained constant. CRmax and IRmax values for the (colonial) toxic M. aeruginosa (383 (±40) ml g DW−1 h−1; 3.7 mg C g DW−1 h−1) demonstrated that A. anatina filtered and ingested this cyanobacterium as good as the other cyanobacterial strains. However, on the non-toxic M. aeruginosa we observed the lowest CRmax of all phytoplankters (246 (±23) ml g DW−1 h−1, whereas IRmax was similar to that on toxic M. aeruginosa. The high maximum ingestion rates on S. obliquus and M. aeruginosa indicate a short handling time of these phytoplankton species. The high clearance rates on S. obliquus, toxic M. aeruginosa and P. agardhii reflect a high effort of the mussels to filter these particles out of the water column at low concentrations. The low clearance rates on non-toxic M. aeruginosa may be explained by the small size and coccoid form of this cyanobacterium, which may have impaired A. anatina to efficiently capture the cells. Although A. anatina had relatively high maximum clearance rates on non-toxic and toxic P. agardhii, this cyanobacterium does not seem to be a good food source, because of the observed high rates of pseudofaeces production and hence low ingestion rates.