Spatial and temporal variability in the response of phytoplankton and 2 bacterioplankton to B-vitamin amendments in an upwelling system

Abstract. We evaluated the temporal (inter-day and inter-season) and spatial variability in microbial plankton responses to vitamins B12 and B1 supply in coastal and oceanic waters. Inter-day variability in microbial plankton responses to B-vitamins was not of great concern, suggesting that B-vitamins availability was controlled by factors operating at larger temporal scales, such as those driving microbial community seasonal succession. Most positive responses were produced by treatments containing either B12 alone or B12 combined with B1 in oceanic waters, which was consistent with the significantly lower average vitamin B12 ambient concentrations compared to that in the coastal station. Growth stimulation by B1 addition was more frequent on bacteria, which is coherent with their widespread dependence on exogenous sources for this growth factor. Negative responses to B-vitamins were generalized in coastal waters in summer, and were associated to a high contribution of Flavobacteriales to the prokaryote community. This observation suggests that the external supply of B12 and/or B1 may promote negative interactions between microbial components when B-vitamins auxotrophs are abundant. The microbial response patterns to B12 and/or B1 amendments were significantly correlated with changes in the prokaryotic community composition, highlighting the pivotal role of prokaryotes in B-vitamins cycling in marine ecosystems.



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Phytoplankton accounts for almost half of the global net primary production (Field et al.,31 1998) and may eventually cause toxic episodes entailing human health problems and large 32 economic losses (Hallegraeff, 1993; van Dolah et al., 2001). Recent emerging evidence    Within this context, the aim of our study was to explore spatial (horizontal and vertical) 91 and temporal (seasonal and short-term) variability patterns in B12 and B1 vitamin 92 limitation in relation to the prevailing initial abiotic (e.g., nutrient and B12 93 concentrations) and biotic (eukaryote and prokaryote community composition) 94 conditions in this productive ecosystem. We conducted a total of 36 microcosm bioassays 95 in February, April, and August 2016 to evaluate the response of heterotrophic bacteria 96 and phytoplankton to the addition of B12 and/or B1. 97 Considering that a large fraction of eukaryotic phytoplankton and bacterial taxa require     Different hydrographic conditions were found during each cruise ( Fig. 1 and 2). In

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February, heavy rainfall combined with relaxed winds (Fig. 1) caused a halocline at 10 258 meters depth (Fig. 2). High levels of Chl-a (as derived from the calibrated CTD 259 fluorescence sensor) were observed at the coastal station, being maximum (4.97 µg l -1 ) 260 by the end of the cruise. At the oceanic station, Chl-a levels remained low (less than 3 µg 261 l -1 ) throughout the cruise, being slightly higher in the subsurface layer. Archaea were most abundant in February at both sampling locations.

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B12 concentration was low, ranging from 0.06 to 0.55 pmol l -1 (Table S1 in the 311 Supplement) Mean B12 concentration was significantly higher in the coast (0.30±0.13 312 pmol l -1 ) than in the ocean (0.15±0.12 pmol l -1 ) (t-test, p = 0.001), and showed less 313 variability at the coastal than at the oceanic station (Fig. 4c). vitamins also caused negative responses of phytoplankton and bacterial biomass (Fig. 5). as compared to the inorganic nutrient addition alone (Fig. 5).

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In order to quantify the relevance of inter-day variability, we calculated the mean describe spatial and temporal patterns in the response to B vitamin amendments (Fig. 6). When averaging the responses within each sampling point (Fig. 6), some general patterns  (Table 1).

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The positive responses of phytoplankton in surface oceanic waters in February were 473 associated with high abundance of Synechococcus and SAR11 (Fig. 4, 7). Synechococcus to B12 when its synthesis is likely reduced, due to the low abundance of B12 producers.

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The higher abundance of Synechococcus in oceanic compared to coastal waters may 481 explain the low concentration of B12 (Fig. 4).

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There were positive effects of B1 addition on phytoplankton and bacteria in subsurface 483 oceanic waters in winter, also associated to high abundance of Synechococcus and, to 484 some extent, of Actinobacteria ( Fig. 6 and 7). While Synechococcus is capable of B1 Even though B1 caused a significant effect on phytoplankton only in subsurface waters 498 in winter, half of the positive responses of bacteria were associated to B1 supply (Fig. 6).

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This pattern is consistent with the recently described widespread dependence of supply in surface coastal water during summer (Fig. 6), associated to high abundance of 529 Flavobacteriales (Fig. 7). All isolates of Bacteroidetes sequenced so far are predicted to summer is most likely associated to increased predation rather than to competition with 535 phytoplankton. By contrast, the negative responses observed in subsurface coastal waters 536 in summer were mostly associated to high abundances of Planktomarina and 537 Cellvibrionales (Fig. 7). Both bacterial groups showed a significantly negative correlation 538 with the phytoplankton response to B1 and/or B12 (Table 1)