Thierry Moutin

Phytoplankton in the ocean use non-phosphorus lipids in response to phosphorus scarcity

Phosphorus is an obligate requirement for the growth of all organisms; major biochemical reservoirs of phosphorus in marine plankton include nucleic acids and phospholipids. However, eukaryotic phytoplankton and cyanobacteria (that is, ‘phytoplankton’ collectively) have the ability to decrease their cellular phosphorus content when phosphorus in their environment is scarce. The biochemical mechanisms that allow phytoplankton to limit their phosphorus demand and still maintain growth are largely unknown. Here we show that phytoplankton, in regions of oligotrophic ocean where phosphate is scarce, reduce their cellular phosphorus requirements by substituting non-phosphorus membrane lipids for phospholipids. In the Sargasso Sea, where phosphate concentrations were less than 10 nmol l-1, we found that only 1.3 ± 0.6% of phosphate uptake was used for phospholipid synthesis; in contrast, in the South Pacific subtropical gyre, where phosphate was greater than 100 nmol l-1, plankton used 17 ± 6% (ref. 6). Examination of the planktonic membrane lipids at these two locations showed that classes of sulphur- and nitrogen-containing membrane lipids, which are devoid of phosphorus, were more abundant in the Sargasso Sea than in the South Pacific. Furthermore, these non-phosphorus, ‘substitute lipids’ were dominant in phosphorus-limited cultures of all of the phytoplankton species we examined. In contrast, the marine heterotrophic bacteria we examined contained no substitute lipids and only phospholipids. Thus heterotrophic bacteria, which compete with phytoplankton for nutrients in oligotrophic regions like the Sargasso Sea, appear to have a biochemical phosphorus requirement that phytoplankton avoid by using substitute lipids. Our results suggest that phospholipid substitutions are fundamental biochemical mechanisms that allow phytoplankton to maintain growth in the face of phosphorus limitation.

Longitudinal and vertical trends of bacterial limitation by phosphorus and carbon in the Mediterranean Sea.

The effect of phosphate (P), nitrate (N), and organic carbon (C, glucose) enrichment on heterotrophic bacterial production was examined along two longitudinal transects covering the whole Mediterranean Sea during June and September 1999. During these cruises, integrated bacterial production ranged from 11 to 349 mgC m(-2) d(-1) for the 0-150 m layer. P was found to stimulate bacterial production (BP) in 13 out of 18 experiments, in the eastern and in the western Mediterranean Sea. Organic carbon stimulation of bacterial production was observed at two stations in the Alboran Sea, where the highest bacterial production was recorded (216 and 349 mg C m(-2) d(-1)) and in the Sicily Strait. Maximum rates of alkaline phosphatase (AP) increased from the Alboran to the Levantine Sea whereas AP turnover time decreased. Moreover, alkaline phosphatase activity was not systematically reduced following additions of P. In cases of P limitation, however, the alkaline phosphatase activity to bacterial production ratio was severely reduced in the P and NPC enrichments. Generally, the addition of the limiting factor--whether P or C--had a synchronous stimulating effect on bacterial production and ectoaminopeptidase activity and induced a decline in the amino acid respiration percentage. At two selected stations in the eastern and northwestern Mediterranean, response to enrichment was tested on vertical profiles. Bacteria shifted from P to C limitation at a depth where soluble reactive phosphorus was still undetectable, but corresponding to a strong increase in alkaline phosphatase turnover time. Our results showed that values of AP turnover time lower than 100 h corresponded to situations of P limitation of bacterial production.

Microzooplankton diversity: relationships of tintinnid ciliates with resources, competitors and predators from the Atlantic Coast of Morocco to the Eastern Mediterranean

We examined tintinnid (loricate ciliate microzooplankton) diversity using data from 11 stations between the Moroccan upwelling system and the oligotrophic Eastern Mediterranean. Taxonomic and morphological diversity of tintinnids was compared to phytoplankton distribution and size-structure, to the abundance of competitors in the form of oligotrich ciliates, and predators as copepods. Tintinnid taxonomic diversity was estimated as numbers of species and the Shannon Index, H 0 ; morphological diversity was quantified by substituting size classes of lorica dimensions for species. Total chlorophyll was partitioned into micro-, nano- and pico-fractions using pigment data and a size-diversity was estimated by considering the 3 size classes as 3 species. Along a west-to-east gradient, average water column concentrations of most organism groups declined approximately an order of magnitude yielding tight correlations. However, tintinnid diversity, both taxonomic and morphological, increased from the Atlantic upwelling station into the western basin of the Mediterranean, and declined slightly towards the Eastern Mediterranean, paralleling shifts in the chlorophyll size-diversity estimate. Diversity varied with absolute or relative abundance of oligotrich or copepods, but different diversity metrics were significantly correlated only with phytoplankton size-diversity. We conclude that tintinnid diversity more closely reflects resource diversity than competitive interactions or predation. r 2002 Elsevier Science Ltd. All rights reserved.

The input of nutrients by the Rhône river into the Mediterranean Sea: Recent observations and comparison with earlier data

The nutrient input by the Rhone river into the Mediterranean Sea was measured from June 1994 to May 1995. The various fractions of N and P (dissolved and particulate, organic and inorganic) as well as chlorophyll and dissolved silicate concentrations were measured twice a month at Arles, 40 km upstream of the river mouth, in this period. In addition, some samples were taken when the river was in high flood.The study shows that nitrogen is mainly present as nitrate (76%). Dissolved and particulate organic nitrogen represent 8 and 9%, ammonium 5% and nitrite 1% of total nitrogen respectively. Almost half of the phosphate is particulate phosphate, the largest part of which is calcium-bound phosphate. Orthophosphate represents 31% of total phosphate.We estimated the total input of nitrogen, phosphate and silicate as 115–127 kt y-1 (N), 6.5–12.2 kt y-1 (P) and 135–139 kt y-1 (Si) by different methods. For nitrogen an annual input of 92.3 to 96.1 kt of nitrate is found, 1.3 to 1.5 kt of nitrite, 6.3 to 6.7 kt of ammonium, 9.7 to 9.8 kt of dissolved organic nitrogen and 5.3 to 12.7 kt of particulate nitrogen; for phosphate the annual imput was 2.7 to 3.0 kt of orthophosphate, 1.3 to 1.7 kt of dissolved organic phosphate & polyphosphate and 2.5 to 7.5 kt of particulate phosphate. While the N-input was mainly in the form of nitrate, a large part of the phosphate input was particulate-P. Comparisons to previous estimations show that the mean annual nitrate concentration in the Rhone has increased by about 50% during the last two decades.

The coupled physical‐new production system in the equatorial Pacific during the 1992–1995 El Niño

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high-resolution Ocean General Circulation Model forced with satellite-derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September-October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate-depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the eastern edge of the warm pool marked by a sharp salinity front, also coincides with a “new production front”. Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh pool and oligotrophic region has an average new production of 0.9 mmol NO3 m−2 d−1, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3 m−2 d−1 and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink.

Decrease of phosphate concentration in a high rate pond by precipitation of calcium phosphate: Theoretical and experimental results

Abstract The mechanism involved in the decrease of phosphate in a high rate pond (HRP) is investigated. The calcium concentration in the water (1.25–3.75 mM) and the high pH obtained in the reactors (8–10), result in the precipitation of calcium phosphate minerals. Although calcium hydroxyapatite [Ca 5 (PO 4 ) 3 OH, p K 5 = 57] is the thermodynamical stable state, the phosphate concentration is determined by the solubility of the amorphous tricalcium phosphate [Ca 3 (PO 4 ) 2 , p K 5 = 25.2]. This is confirmed by the calculation of the theoretical predicted solubility as well as various experiments. The relationship between pH and phosphate concentration may be used to optimize the efficiency of this type of reactor to produce a minimum concentration of phosphate in the effluent.

Coherence of particulate beam attenuation and backscattering coefficients in diverse open ocean environments

We present an extensive data set of particle attenuation (c(p)), backscattering (b(bp)), and chlorophyll concentration (Chl) from a diverse set of open ocean environments. A consistent observation in the data set is the strong coherence between c(p) and b(bp) and the resulting constancy of the backscattering ratio (0.010 +/- 0.002). The strong covariability between c(p) and b(bp) must be rooted in one or both of two explanations, 1) the size distribution of particles in the ocean is remarkably conserved and particle types responsible for c(p) and b(bp) covary, 2) the same particle types exert influence on both quantities. Therefore, existing relationships between c(p) or Chl:c(p) and phytoplankton biomass and physiological indices can be conceptually extended to the use of b(bp). This finding lends support to use of satellite-derived Chl and b(bp) for investigation of phytoplankton biomass and physiology and broadens the applications of existing ocean color retrievals.

Aphotic N2 Fixation in the Eastern Tropical South Pacific Ocean

We examined rates of N2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niño (2010) and La Niña (2011). Replicated vertical profiles performed under oxygen-free conditions show that N2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 µmol N m−2 d−1 in 2010 and 24±14 to 118±87 µmol N m−2 d−1 in 2011. In the aphotic layers, volumetric N2 fixation rates were relatively low (<1.00 nmol N L−1 d−1), but when integrated over the whole aphotic layer, they accounted for 87–90% of total rates (euphotic+aphotic) for the two cruises. Phylogenetic studies performed in microcosms experiments confirm the presence of diazotrophs in the deep waters of the Oxygen Minimum Zone (OMZ), which were comprised of non-cyanobacterial diazotrophs affiliated with nifH clusters 1K (predominantly comprised of α-proteobacteria), 1G (predominantly comprised of γ-proteobacteria), and 3 (sulfate reducing genera of the δ-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N2 fixation in the core of the OMZ at all stations tested and as did simple carbohydrates at stations located nearest the coast of Peru/Chile. The episodic supply of these substrates from upper layers are hypothesized to explain the observed variability of N2 fixation in the ETSP.

Seasonal variations of P compounds and their concentrations in two coastal lagoons (Herault, France)

This article concerns seasonal variations in the phosphate concentrations in two coastal lagoons near Montpellier (Mediterranean coast, France). The o-P concentration in the overlying water is highest during summer. The role of the sediment, particularly that of the different P fractions in the sediment, is discussed. Significant variations, especially in the FeOOH ≈ P fraction, occur. For both Tot-Psed and the Fe00H≈P fraction a gradient from surface to bottom is observed, as well as a distinct decrease in the FeOOH≈P fraction in the surface sediments during summer and autumn. Variations in the FeOOH≈P fraction appear to be compensated by variations in the CaC03≈P fraction. These variations appear to be determined by the ferric hydroxide concentration. This compound represents only a small part (maximally 15%) of the total iron in the sediments and is related to the dissolved oxygen content of the immediately overlying water. Besides the fractions o-P, Fe(OOH)≈P, a large part of the CaC03≈P fraction is potentially bioavailable. A large proportion of the Tot-Psed is therefore bioavailable.[/p]

Hot spot of N-2 fixation in the western tropical South Pacific pleads for a spatial decoupling between N-2 fixation and denitrification

Nitrogen (N) is the building block of life. Quantifying the sources and sinks of N to the ocean is essential for predicting its productivity and potential carbon sequestration. In his paper, Gruber (1) seeks for “elusive marine nitrogen fixation” following results from Knapp et al. (2), who measured unexpectedly low N input through N2 fixation in the eastern tropical South Pacific (ETSP), seriously bringing into question the proposed close spatial coupling between N input (through N2 fixation) and loss (through denitrification) (3). Here, we compile data from recently published and unpublished studies revealing a hot spot of N2 fixation in the western tropical South Pacific (WTSP) arguing for a spatial decoupling between N sources and sinks in the South Pacific. Based on four cruises performed between 2012 and 2015 during austral winter and summer conditions, with a total of more than 600 15N2 incubations-based measurements, and particularly a 4,000-km zonal transect at ∼20°S in … [↵][1]1To whom correspondence should be addressed. Email: sophie.bonnet{at}univ-amu.fr. [1]: #xref-corresp-1-1