Can algal uptake stop NO₃⁻ pollution?

Abstract

Arising from Cardinale, B. J. Nature 472, 86–89 (2011)10.1038/nature09904

The influence of biodiversity on ecosystem function has been of interest to community ecologists for decades. Recently, Cardinale¹ reported that biodiversity affects nitrate (NO₃⁻) uptake in algal communities and that, as a result, biodiversity may help mitigate nutrient pollution. Although Cardinale’s conclusions about niche partitioning are interesting (figure 2 in ref. 1), his extension of these findings to problems of nutrient pollution is premature. Algal uptake is only a short-term nitrogen sink; control of NO₃⁻ pollution requires long-term solutions.

Main

Nitrate removal in streams typically occurs via two pathways. Denitrification is the microbial conversion of NO₃⁻ to N₂ gas (Fig. 1). This is the most important means of permanent NO₃⁻ removal from streams. However, it is not considered in Cardinale’s study.

Figure 1: Nitrogen cycling in streams.

Red arrows indicate processes not measured by Cardinale¹. Storage in groundwater and sediments, N₂ fixation and fluxes to other ecosystems (for example, floodplains) may also occur.

PowerPoint slide

The second NO₃⁻ uptake pathway is assimilation by primary producers (in this case, algae). Algae acquire nutrients to fulfil physiological demands and support growth, but these same nutrients are later released after cell death or disruption (such as the brushing treatment applied to some flumes to create heterogeneity). Algal uptake leads to temporary storage of nitrogen in algal tissues (turnover time ranges from ∼17.5–47 days^2,3,4), and can have significant short-term effects on nitrogen export to downstream ecosystems. But, because benthic algae do not accumulate over long (annual or interannual) periods, this uptake mechanism represents only a transient storage pool and thus has no enduring effect on downstream water quality (Fig. 1).

Algae could potentially mediate permanent nitrogen removal in streams by facilitating increased rates of denitrification^5,6. Indeed, Cardinale’s results raise the intriguing possibility that algal diversity could affect denitrification rates via changes in the availability of nitrogen, carbon or oxygen to denitrifiers. However, until this potential effect is demonstrated, conclusions about biodiversity effects on the control of aquatic nitrogen pollution are premature.

We agree with Cardinale that nitrogen enrichment threatens the integrity of aquatic ecosystems and that nitrogen removal is a critical ecosystem service provided by streams⁷. And, although Cardinale has elucidated a potential mechanism for biodiversity to influence stream nutrient cycling, for this work to be extended to pollution control in natural ecosystems it must be tested in environments incorporating critical biogeochemical pathways—namely, denitrification and nitrogen fixation—and it must consider long-term nitrogen dynamics.