Nutrients that limit growth in the ocean

Abstract

Phytoplankton form the basis of the marine food web and are responsible for approximately half of global carbon dioxide (CO2) fixation (∼ 50 Pg of carbon per year). Thus, these microscopic, photosynthetic organisms are vital in controlling the atmospheric CO2 concentration and Earth's climate. Phytoplankton are dependent on sunlight and their CO2-fixation activity is therefore restricted to the upper, sunlit surface ocean (that is, the euphotic zone). CO2 usually does not limit phytoplankton growth due to its high concentration in seawater. However, the vast majority of oceanic surface waters are depleted in inorganic nitrogen, phosphorus, iron and/or silica; nutrients that limit primary production in the ocean (Figure 1). Phytoplankton growth is mainly supported by either the recycling of nutrients or by reintroduction of nutrients from deeper waters by mixing. A small percentage of primary production, though, is fueled by 'external' or 'new' nutrients and it is these nutrients that determine the amount of carbon that can be sequestered long term in the deep ocean. For most nutrients such as phosphorus, iron, and silica, the external supply is limited to atmospheric deposition and/or coastal and riverine inputs, whereas their main sink is the sedimentation of particulate matter. Nitrogen, however, has an additional, biological source, the fixation of N2 gas, as well as biological sinks via the processes of denitrification and anammox. Despite the comparatively small contributions to the overall turnover of nutrients in the ocean, it is these biological processes that determine the ocean's capacity to sequester CO2 from the atmosphere on time scales of ocean circulation (∼ 1000 years). This primer will highlight shifts in the traditional paradigms of nutrient limitation in the ocean, with a focus on the uniqueness of the nitrogen cycling and its biological sources and sinks.

Prevalence of viral photosynthesis genes along a freshwater to saltwater transect in Southeast USA

“…This characteristic is probably due to shorter latent periods of Podoviridae that are likely too short for complete production and activation of the photosynthesis genes compared to Myoviridae cyanophages (Mann, ; Sullivan et al ., ). Consistent with this interpretation, the lower abundance of freshwater AMGs compared with the marine counterparts may reflect the higher nutrient load (and shorter latent periods) of riverine environments than the ocean (Bristow et al ., ). Indeed, Chattahoochee River samples showed higher nutrient content (∼0.07 mg/l NO 2 + NO 3 and ∼0.13 mg/l PO 4 , [Supporting Information Table S6]) than those typically observed in the Gulf of Mexico surface waters (below detection for NO 2 + NO 3 and ∼0.014 mg/l PO 4 ) (Tsementzi et al ., ).…”

Microbiome of the deep Lake Baikal, a unique oxic bathypelagic habitat

“…5 mg L −1 ), several orders of magnitude lower than in the ocean, hence one could expect that sulfur metabolism was not very widespread in dLB microbes. Nitrogen, on the other hand, is present in different compounds (such as cyanate, urea, nitrate, nitrite, or ammonia) in freshwater, and some of them can be limited in marine ecosystems (Bristow et al 2017). We have explored the number of N and S related metabolic pathways comparing bathypelagic marine and dLB to detect their relative importance (Fig.…”

Surface and Temporal Biosignatures

“…However, the evolutionary drive to maximize photon capture may not necessarily cause a commensurate shift of 'edge' wavelengths with the spectrum of the host star. Photosynthesizers (at least on Earth) are not typically photon limited at the surface and are instead limited by the availability of water (e.g., in deserts or arid regions) or nutrients such as fixed nitrogen, dissolved iron, or phosphorous (Moore et al 2013;Bristow et al 2017).…”