FIU OCB3043 Lecture Notes

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Biogeochemical Mineral Cycles

Oxic mineralization: heterotrophic bacteria decompose organic material and produce CO₂ and mineral nutrients
Anoxic mineralization: occurs in anoxic water (Black, Baltic Sea) and sediments; anaerobic bacteria utilize oxygen in SO₄ and NO₃ ions and produce highly reduced products: methane (CH₄), hydrogen (H₂), H₂S, NH₄
Chemosynthesis: anaerobic, chemoautotrophic bacteria can utilize these reduced but energy-rich substrates to reduce CO₂ and form organic biomass
Importance: Rate at which essential, limiting nutrients are recycled
Limiting nutrients: can be nitrate (nitrogen), phosphate, iron (bioavailable iron!), silicate (for diatoms, silicoflagellates, radiolaria)
Si-Cycle: simplest, only inorganic forms; organism include Si in their skeletons, which is dissolved after their death; silica frustle of diatoms is covered by organic matrix, which first has to be degraded by bacteria

The Phosphorus Cycle:

Relatively simple and fast cycle: at the usual alkaline pH of seawater, organic phosphates are easily hydrolyzed to inorganic phosphate
Alkaline phosphatase: enzyme of algae and bacteria that helps converting organic phosphate into inorganic phosphate, which is subsequently taken up; can occur intra- or extracellularly
Limitation: most marine systems are less P, more N limited; freshwater and coastal areas can be severely P-limited, though.

The Nitrogen Cycle

Complex because nitrogen occurs in different inorganic (NH₄, NO₃, NO₂, N₂) and organic (PON, DON) forms that are mostly biologically transformed
Dominant forms: nitrate, taken up by phytoplankton; ammonia, produced by bacteria, protozoa, zooplankton and consumed by bacteria and phytoplankton
Limiting nutrient in most marine systems for phytoplankton production
Nitrification: conversion of NH₄ to NO₂ to NO₃ by nitrifying bacteria; uses oxygen!
Denitrification: conversion of NO₃ to NO₂ to N₂ by bacteria; releases oxygen but net loss of nitrogen to water column!
Nitrogen fixation: conversion of atmospheric N₂ into organic nitrogen by bacteria and cyanobacteria

The concept of „new“ and „regenerated“ primary production

Concept: Refers to the origin of nitrogen used by phytoplankton, basically NO₃ vs. NH₄; some nitrogen originates from recycling by bacteria, the microbial food web or zooplankton (NH₄), other nitrogen originates from inputs from deeper water layers, from land/river, or the atmosphere
„New Production“: relies on nitrate from upwelling, river/land, atmospheric input
„Regenerated Production“: relies on NH₄ recycled within the euphotic zone
Nitrogen fixation: fixation of atmospheric N₂ is new production!
f-ratio: ratio of new production to total production:

f-ratio ranges from 0.1 in oligotrophic, open ocean waters to 0.8 in upwelling areas

The Marine Carbonate System

Carbon dioxide: enters the ocean from atmosphere; steady exchange at surface
Solubility: in water is low! But CO₂ reacts with water to form ions: carbonate (CO₃^2-) and bicarbonate (HCO₃^-)
Most of marine CO₂ is stored as bicarbonate Therefore, CO₂ is never limiting photosynthesis in seawater, but it is in freshwater, because the lower pH of freshwater prevents transformation into bicarbonate
Respiration: CO₂ is added, reacts with water:

H₂O + CO₂ = HCO₃^- + H⁺; [H⁺] rises, which means pH sinks!

Primary production: CO₂ is removed, HCO₃^- converts to free CO₂ to keep the chemical balance; [H⁺] falls, which means pH rises!

The Carbon Cycle

Carbon: The basic currency unit in ecology; it is tranferred from inorganic to organic forms and back by biological processes
Photosynthesis and Respiration are the major pathways totransform forms of C
Calcification: Some marineorganisms combine calciumwith bicarbonate ions to makecalcareous shells or skeletons
CO₂ balance of calcification: Calcification produces CO₂ !!!
Ca²⁺ + 2 HCO₃^- = CaCO₃ + H₂O + CO₂
Oceanic blooms of coccolithophorids and production of coral reefs DO NOT help decreasing the atmospheric increase in CO₂