Diatom populations in an upwelling environment decrease silica content to avoid growth limitation

Abstract

Summary A mix of adaptive strategies enable diatoms to sustain rapid growth in dynamic ocean regions, making diatoms one of the most productive primary producers in the world. We illustrate one such strategy off coastal California that facilitates continued, high, cell division rates despite silicic acid stress. Using a fluorescent dye to measure single-cell diatom silica production rates, silicification (silica per unit area) and growth rates we show diatoms decrease silicification and maintain growth rate when silicon concentration limits silica production rates. While this physiological response to silicon stress was similar across taxa, in situ silicic acid concentration limited silica production rates by varying degrees for taxa within the same community. Despite this variability among taxa, silicon stress did not alter the contribution of specific taxa to total community silica production or to community composition. Maintenance of division rate at the expense of frustule thickness decreases cell density which could affect regional biogeochemical cycles. The reduction in frustule silicification also creates an ecological tradeoff: thinner frustules increase susceptibility to predation but reducing Si quotas maximizes cell abundance for a given pulse of silicic acid, thereby favoring a larger eventual population size which facilitates diatom persistence in habitats with pulsed resource supplies.

Arctic (Svalbard Islands) Active and Exported Diatom Stocks and Cell Health Status

“…When compared across the contrasting stages of bloom development represented in the data set analyzed here, the results presented conform to a conceptual model where diatom growth during the Arctic spring bloom proceeds until silicic acid stocks are depleted (Wassmann et al, 1997;Krause et al, 2018). Silicic acid depletion 30 leads to diatom senescent and subsequent cell death, which then results in rapid sinking of the diatom stock.…”

“…However, silicic acid concentrations [Si(OH) 4 ] are characteristically low in the European Sector of the Arctic, due to the inflow of Si-depleted Atlantic water (Rey 2012). Recently, Krause et al (2018) showed diatoms to be limited by [Si(OH) 4 ] at the spring bloom and suggested that silicon limitation could collapse a diatom bloom before nitrogen when spring conditions favor diatoms, instead of the haptophyte…”

“…Water samples were taken between the surface and the bottom (max. 500 meters) for analysis of nutrients, diatom silica, productivity, and other properties (Krause et al 2018).…”

Reply to RC2

“…Diatoms have an r-selected ecological strategy and are typically the first phytoplankton group to bloom in this region under stratified conditions (Reigstad et al, 2002). The 1.7 Si : N from Lomas and Krause (2018) for nutrient-replete polar diatoms suggests they consume 70 % more Si relative to N. However, under kinetic limitation, diatoms have long been inferred to reduce Si per cell in culture to avoid growth limitation (Paasche, 1973) -this was recently observed directly in the field for the first time (McNair et al, 2018). Given the clear kinetic limitation observed during ARCEx (Fig.…”

“…In the Edgeøya and Erik Eriksenstretet stations, diatoms accounted for a majority or all of PP, at 70 % and 180 %, respectively. Given that diatoms can reduce their cellular Si in response to kinetic limitation (Paasche, 1973;McNair et al, 2018), the Si : C ratio of 0.25 based on nutrient-replete polar diatoms in culture may systematically underestimate diatom contribution to PP using our approach. For example, if kinetic limitation reduced Si per cell by 50 % (i.e., Fig.…”